Introduction

The agricultural industry is confronted with several challenges including increasing the production to feed world’s rising population and increasing resource use efficiency, all while limiting the environmental impact on ecosystems and human health. The agro-ecosystem is consequently disturbed by extreme climatic events and such changes might have impact on both the quality and quantity of crops. The increasing level of pollutants, as well as the negative impact of climate change on soil microbiome, are additional constraints to agriculture. Increase in atmospheric carbon dioxide (CO2), temperature, and humidity, in particularly, are predicted to exacerbate the frequency of major plant diseases, insect infestation, and the migration of major pathogens and insect pests into new geographic areas. To increase agricultural production and fulfil the food requirement of increasing population inorganic fertilizer and pesticides are used liberally and it has serious implications on both agro-ecosystem and human health. In developing countries, farmers rely on the use of inorganic pesticides, insecticides, chemical fertilizers, and herbicides and their use has become an unavoidable risk in agriculture. Moreover, agriculture World is the backbone of our economy, and extended and excessive use of these inorganic fertilizers results in massive economic losses. Though their use has led to increased crop production their improper employment has caused agro contamination which is a troublesome issue. The use of chemical fertilizer could impact the agricultural environment by changing its parameter such as eutrophication, reducing the crop yield, salinization of soil, and increasing in pH of the soil (Prakash et al. 2018). To enhance agricultural production several technological advancements have been proposed which also include the identification of new plant biostimulants and efficient methods for their application. The use of plant bio-stimulants is promising and environmental friendly (Colla and Rouphael 2015) and the alternative to chemical fertilizer would be the use of organic fertilizer such as seaweed liquid fertilizer, humic acid, vermicompost, and essential microorganisms and these are biodegradable, non-toxic, and non-polluting. Kauffman et al.(2007) reported that bio-stimulants are not fertilizers, but when applied in low quantities improve plant growth (Kauffman et al. 2007). The biostimulant sector is expected to develop at an annual pace of 11.24% and reach USD 4.9 billion by 2025 (Caradonia et al. 2018). There is a growing interest in the farming sector for novel biostimulant compounds and much research is being conducted in this emerging sector of the business. Seaweeds serve as a valuable source of several elicitors that may be used for inducing physiological development and multitude of defensive responses. This review discusses the utilisation of various seaweed extracts (SEs) as biostimulants to mitigate biotic and abiotic stress. The mechanism of action, composition and function of several seaweed are discussed elaborately.

Seaweeds

Seaweeds or marine algae are marine aquatic plants that inhabit the coasts of oceans and seas. These are macroscopic algae that are found attached to the solid bottom of rocks, shells, and other plant material. It constitutes about 6000 species with a great diversity of forms and size and only 5% of it is being used. Seaweeds are a rich source of bioactive compounds and produce secondary metabolites with a varying range of biological functions such as antibacterial (Singh and Chaudhary 2010), antifungal (De Corato et al. 2017), antiviral, anti-inflammatory, nematocidal, and anticoagulant (Caijiao et al. 2021). They are divided into three categories based on their pigmentation, such as red algae (Rhodophyta), green algae (Chlorophyta), and brown algae (Phaeophyta) (Kim 2011). They are rich in minerals (Mg, Ca, P, K, and I), proteins, vitamins, undigestable carbohydrates, and fibers. The composition of minerals varies from species to species and the source of dietary fibers varies chemically and physiochemically from those of the plants found in the land (Jiménez-Escrig and Cambrodón 1999). Seaweed active compounds are used in wide range of sector such as food, wastewater treatments, agriculture, pharmaceutical, and cosmetics (Pereira and Correia 2015). Beyond these applications, the global demand for seaweed usage has increased. Seaweeds are beneficial alternative feeds for domesticated animals, because they contain valuable source of nutrients, particularly chelated microminerals, the availability of which is greater than that of inorganic minerals; complex carbohydrates with prebiotic properties; and pigments and polyunsaturated fatty acids that are beneficial to consumer health.

Seaweed is extremely important in agriculture and human lives and the huge seaweed reserve along the world's coastal areas should be used more effectively and strategically to reduce the waste of these vital resources (Parmar et al. 2017). The seaweeds have been utilized as organic fertilizer for centuries and in coastal regions seaweeds have been used either directly or in the composted form to improve crop productivity (Craigie 2010). Seaweeds are employed as biofertilizers to compensate for soil nutrient deficiencies. Seaweed extract (SEs) includes regulators, plant growth hormones, carbohydrates, auxins, gibberellins, and vitamins which can help to improve crop productivity and to maintain soil fertility. The SEs target several pathways to improve tolerance under stress and the certain plundering caused by bacteria, fungi, insects, and parasites can be reduced in plants. SEs biochemical composition is complex and as a result, understanding seaweed mode of action is exceedingly complex. Because of the various interactions between a large number of bioactive compounds within the same extract, a multidisciplinary approach is required.

The extraction of seaweed is still the most important phase in the production of agronomically efficient products. The seaweed extract is obtained through various processes but the widely used method is alkali extraction with or without heating (Khan et al. 2009). The seaweed-based biostimulant is produced in either liquid or soluble powder from a different range of seaweeds (Michalak et al. 2016). The use of seaweed extract’s intrinsic chelating characteristics, which prevent trace metal ions from precipitating, the extract is commonly fortified with plant fertilizer and micronutrients. The raw materials used and the extraction procedure have a significant impact on the various mechanism of action of seaweed-derived biostimulants (Shukla et al. 2019).

There are many seaweeds extracts available in the market and a considerable amount of seaweed is used as nutrient supplements and as biostimulants or biofertilizers. The most commonly manufactured seaweed species include Ascophyllum nodosum, Macrocystis pyrifera, Ecklonia msaxima, Sargassum species, Laminaria species, and Fucus serratus (Sharma et al. 2013). Numerous studies have found that seaweed extract applications have a wide range of favourable impact on crops (Fig. 1) which include early seed germination and establishment, improved crop performance and yield, increased resilience to biotic and abiotic stress and increased postharvest shelf-life of perishable products. For example A. nodosum one of the most studied seaweed when used as foliar spray improved the flowering phenomenon in tomatoes (Dookie et al. 2021).

Fig. 1
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The schematic illustration of different application of seaweed extracts in plants

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The biostimulant prepared from different seaweeds elicits plant defense through a distinct mode of action. The effect of specified applications of seaweed extract (rates and timings are crucial for specific responses to be evoked) is also linked to the activation of many molecular and biochemical changes in the treated plants, demonstrating the possible impact of SEs on genetic pathways.

Effect of seaweed extract on plant growth and development

The seaweed extract has been demonstrated to positively affect seed germination and plant development at all phases including harvest and post-harvest (Table 1). Seaweed products have been demonstrated to improve germination rate and a considerable increase in seedling vigor by enhancing root length and density. Recently it was reported that Padina gymnospora, Gracilaria edulis, and Ulva fasciata aqueous extracts are used as seaweed biofertilizers for the seed germination of Capsicum annuum (Shamya Arokia rajan et al. 2020). Rengasamy et al.(2015) reported that the use of eckol and phloroglucinol isolated from the Ecklonia maxima increased the germination rate in maize seeds (Rengasamy et al. 2015). This has been linked to the activity of the enzyme α-amylase, which facilitates the conversion of starch to simple sugars in the roots of maize seedlings treated with eckol and phloroglucinol-treated. The sugar produced was transported to the embryo in order to provide the necessary energy for metabolism. The use of Kappaphycus alvarezii seaweed extract on maize during the grain-filling stage boosted yield attributes such as cob length, the number of grains per cob, and length of grain fill (Trivedi et al. 2018). Different concentration of K. alvarezii was applied together with 100% of the required amount of fertilizer on maize (Singh et al. 2015) and potato tuber (Pramanick et al. 2017) and increased growth, yield, and quality was observed in both studies. Treatment with A. nodosum has been shown to promote growth and nutrient uptake from the soil in Brassica napus (winter rapeseed) and strawberry (Jannin et al. 2012). Application of A.nodosum to wheat increased the nutrient uptake and the grain nutrient accumulation was linearly associated with grain production showing enhanced soil nutrient absorption and remobilization to the reproductive organs (Stamatiadis et al. 2018).

Table 1 The effects of different seaweed extracts on plant growth and development
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Seasol a commercially available extract prepared from the seaweeds Duvillaea potatorum and A. nodosum increased the root length density and the yield (Mattner et al. 2018). Soybean treated with A. nodosum extract showed increased photosynthetic efficiency and chlorophyll content while also promoting the growth of the root system (do Rosário Rosa et al. 2021). Seaweed extract-treated tomato and pepper showed enhanced chlorophyll content (Ali et al. 2019). The extract obtained from Macrocystis pyrifera promoted the growth of tomato plants and increased the adventitious root formation in the mung bean (Briceño-Domínguez et al. 2014).

Finnie and Staden (1985) reported that Ecklonia maxmia seaweed promoted the in-vitro growth of tomato root. Both root elongation and root extension were significantly enhanced when applied (Finnie and van Staden 1985). Liquid seaweed extracts (LSEs) derived from seaweed such as Ulva lactuca, Caulerpaa sertularioides, P. gymnospora and Sargassum liebmannii enhanced germination response and increased the physiological traits of tomato. When compared to foliar spray, soil drench application was shown to be more effective (Hernández-Herrera et al. 2014). Application of U. fasciata and S. lacerifolium to soil boosted radish growth, improved germination percentage, and improved morphological and biochemical indices (Ahmed et al. 2020). The wheat seed primed with an aqueous extract of Ulva linza showed a substantial increase in physiological characteristics and mitotic index. New proteins were expressed in seedlings treated with an extract which might attribute to the activity of a bioactive substance in the extract (Hamouda et al. 2022a). Wine grape yield of several cultivars was increased by an average of 14.7% by foliar application of Durvillaea potatorum and A. nodosum (Arioli et al. 2021).

A study using A. nodosum extract in spinach showed enhanced biomass, protein content, chlorophyll and carotenoid content, flavonoids and antioxidant activity. The increase in biomass is associated with an increase in the expression of the Glutamine synthetase 1 (GS1) which is involved in nitrogen integration (Xu and Leskovar 2015). The increased expression of glutathione reductase (GR), and choline monooxygenase (CMO) was linked to increase in chlorophyll content. The upregulation of glutathione reductase (GR), thylakoid bound peroxidase (tAPX) and monodehydroascorbate reductase (MDHAR) was linked to the increase in chitinase activity, phenolics and flavonoids. These genes were discovered to be related to the phenylpropanoid and flavonoid pathways which are known to promote growth and improve overall nutrition (Fan et al. 2013). Flavonoids are secondary metabolite which is important for plant development. Chalcone isomerase (CHI), a key enzyme in the biosynthesis of flavanone precursors and phenylpropanoid plant defense compounds were also upregulated in response to seaweed extract treatment. The use of A. nodosum extracts improved the nitrogen use efficiency in barley plants by increasing the N content in barley shoot which was linked with the upregulation of root nitrate transporter (NRT1.1, NRT2.1, and NRT 1.5) (Goñi et al. 2021). Jannin et al. (2012) reported that A. nodosum application to Brassica napus increased the uptake of nitrogen and sulfate content in root and shoot. The expression of genes (BnNRT1.1 and BnNRT2.1) which encode nitrate transporter was shown to increase in roots of the treated plant (Jannin et al. 2012).The investigation of molecular responses in plant to A.nodosum extract treatment revealed that the increase in chlorophyll concentration was primarily due to downregulation of stay green protein (SRG) which was able to limit leaf protein degradation and protect chlorophyll degradation during leaf senescence. Senescence-associated cysteine protease was down-regulated whereas genes involved in photosynthesis, cell metabolism and S and N metabolism were up regulated (Rayorath et al. 2008; Jannin et al. 2012).

Cell wall undergo structural changes upon biotic and abiotic stress and they store carbohydrate for plant growth. Previously it was reported that Kelpak® can alter the allocation of carbon between cellulose synthesis and secondary cell wall by regulating sucrose synthase expression (Kocira et al. 2020). Sucrose is required for cell wall synthesis and several mechanisms contribute to cell wall synthesis (Decker and Kleczkowski 2019). In another study it was reported that with application of S. vulgare in tomato and sweet pepper cell-wall biosynthesis-related transcripts were upregulated. The MYB transcription are significantly upregulated and are related to secondary cell wall formation. The MYB are activated by SND1 (Secondary wall-associate NAC domain protein 1) and it activates the biosynthesis of lignin, cellulose and hemicellulose (Miedes et al. 2014; Decker and Kleczkowski 2019).

Phytohormones are necessary for plant growth and development. Thus, the alteration of the innate phytohormone pool by the seaweed-based biostimulants can have a major impact on plant development. Recent research has shown that seaweed and its components can influence the expression of genes involved in the biosynthesis of growth hormones such as NCED3 (Nine-cis-epoxycarotenoid dioxygenase3), ERF2 (Ethylene-responsive transcription factor 2), ICS2 (Isochorismate synthase 2) etc. The genes are upregulated in plants upon treatment of seaweed extracts (Ali et al. 2019).

When extracts are applied at the recommended levels, (do Rosário Rosa et al. 2021) the growth hormones present within the extract is insufficient to invoke physiological changes in plants (Górka and Wieczorek 2017). However, the components within the seaweed extract may induce innate pathways for the biosynthesis of growth hormones in plants. Aremu et al. (2015) isolated eckol and phloroglucinol from Ecklonia maxima and investigated their effects on Eucomis autumnails. It was observed that exogenous application of eckol and phloroglucinol caused increase in rise of auxin level which resulted in a 1.5-fold increase in the root length of the treated plant. The compounds influence auxin’s root-lengthening activity by preventing decarboxylation and acting as a cofactor which promotes the breakdown of the IAA oxidase (Aremu et al. 2015). It has been observed that biostimulants promote overall plant growth, physiological processes, agronomic aspects, and yield quality of crops (Yao et al. 2020).

Mitigating plant abiotic stress using seaweed-based biostimulants

Abiotic stress such as temperature extremes (heat and cold), drought, salinity, and heavy metals are key issues restricting agricultural growth and sustainability across the world. It negatively impacts growth and yield formation. These abiotic stresses are interrelated and can manifest osmotic stress, ion distribution failure, and plant cell homeostasis. Developing stress-tolerant plants through genetic engineering is a challenging task and has achieved limited success (Agarwal et al. 2012). As an early reaction to abiotic stress plants undergo internal oxidative stress by excess overproduction of reactive oxygen species (ROS). An imbalance is created between ROS production and the ability to detoxify and remove intermediate substances (Bencze et al. 2004). The increased production of ROS causes structural alteration to DNA and proteins, inhibition of antioxidant enzymes, and activation of programmed cell death (PCD) (Huang et al. 2019). To counteract the deleterious effects, plants use a wide range of defense mechanisms (Fig. 2). In order to scavenge the ROS, plants employ both enzymatic and nonenzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR) and phenolic compounds (Gruszka et al. 2018; Yadav et al. 2019). Prominent results prove that seaweed can mitigate abiotic stress in plants (Table 2). For example, pre-treatment with A. nodosum based biostimulant had a favourable effect on Arabidopsis under drought-stressed conditions. It also altered the expression levels of genes implicated in ABA-responsive and antioxidant system pathways resulting in improved photosynthetic performance than untreated plants (Santaniello et al. 2017). The use of polysaccharides extracted from brown seaweed Lessonia nigrescens improved the salt tolerance level in wheat plants (Zou et al. 2019). The treatment minimized the oxidative damage in plants by increasing the antioxidant activity of SOD, CAT, and POD (peroxidase) enzymes by controlling the relative electric leakage and malondialdehyde content which are two parameters that determine membrane permeability and lipid peroxidation. Similarly, the use of U. rigida improved the salt tolerance in wheat plant and protected it from oxidative degradation, and increased the enzymatic activities in plants (Chernane et al. 2015). The application of polysaccharides extracted from Grateloupia filicina mitigated the tolerance of rice seedling to salinity stress and improved rice growth under salinity conditions (Liu et al. 2019). Milkweed seedling treated S. angustifolium extract can tolerate salinity up to 15 dS m−1 and the survival rate was increased by 69%. The treatment increased the growth parameters with a decrease in electrolyte leakage (Bahmani Jafarlou et al. 2022). Similarly, supplementation of S. muticum and Jania rubens provided salt stress amelioration in chickpea and enhanced activity of SOD and APX were related with improved growth parameters. Moreover, four key amino acids including serine, threonine, proline, and aspartic acid were identified from roots that contribute to stress reduction (Abdel Latef et al. 2017). Garcilaria dura extract applied to wheat elevated drought stress tolerance and increased the biomass and crop yield (Sharma et al. 2019). The foliar application of K. alvarezii significantly mitigated the water stress in maize with increased seed yield primarily by increasing all yield attributes, particularly cob length, the number of grains per cob, and length of grain fill. It also resulted in increased antioxidant activity and reduced reactive oxygen species levels, leading to a reduced degree of oxidative stress in treated plants. (Trivedi et al. 2018).

Fig. 2
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The schematic illustration highlights the application of seaweed extract on plants and the response of plants to abiotic stresses. Seaweed extract application to plants is perceived by a complex sensing mechanism in which various secondary messenger’s function (Abscisic acid (ABA), phytohormones, Ca2+, etc.,). The secondary messengers cause a myriad of downstream processes such as the activation of MAPKs (Mitogen-activated protein kinases), CIPKs (Calcineurin B-like proteins), PPs (Protein phosphatases) and CDPKs (Calcium-dependent protein kinases) activating the transcriptional factors and they regulate the stimulation of antioxidant enzymes thus scavenging ROS production. The other stress-inducible genes are involved in the alteration of cell wall biosynthesis, carbohydrate metabolism, secondary metabolites, and regulation of Na+ and K+ transporters thus altering the physiological adaption of plants to abiotic stress and enhancing the tolerance to abiotic stress. Note MAPK mitogen-activated protein kinase, CDPKs calcium-dependent protein kinases, PPs protein phosphatases, CIPKs calcineurin B-like protein (CBL)-CBL-interacting protein kinase, WRKY tryptophan arginine lysine tyrosine, MYB myeloblastosis, DREB dehydration responsive element-binding, AP2-APETALA2, EREB ethylene responsive element binding factor, LEA late embryogenesis abundant, DHN dehydrin, FIB1a fibrillarin1a, GST glutathione S-transferase, SOD superoxide dismutase, CAT catalase, GSH glutathione, APX ascorbate peroxidase, POD peroxidase, PGA-3 phosphoglyceric

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Table 2 Effect of different seaweed extracts in mitigating abiotic stress in plants
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Seaweed extract could be another approach for dealing with temperatures that impede the growth and development of plants. Application of A. nodosum extract (ANE) to tomato seedling enhanced heat stress tolerance and showed significant improvement in flower development and fruit production. The constituents in ANE formulation were associated with an accumulation of soluble sugars, and increase in heat shock proteins (HSPs) gene transcription in heat exposed tomato flowers before fertilization (Carmody et al. 2020). Algafect a commercial seaweed extract (A. nodosum, Fucus spp., and Laminaria spp.) was applied to maize and it enhanced the low temperature tolerance of root zone during early growth. On application of seaweed extract, the SOD activity was increased in root and leaf tissue as it plays an important role in antioxidative stress defense (Bradáčová et al. 2016).

Seed priming has emerged as an essential approach for producing plants that are resistant to various stress and it is a simple and alternative practice compared to conventional methodologies (breeding or transgenics). Seaweed extract is successfully utilized as a priming agent to enhance stress tolerance in plants (Bhanuprakash and Yogeesha 2016). Priming wheat seeds with Hormophysa cuneiformis and Actinotrichia fragilis reduced salt stress and it also enhanced the growth and increased the level of antioxidant activity in plants (Latef et al. 2021).

Maize and black-eyed pea seeds primed with seaweed liquid extract (SLE) prepared from Ulva fasciata, Cystoseira compressa and Laurencia obtusa exhibited remarkable tolerance to salinity stress and increased the seedling morphological parameter (Hussein et al. 2021). The negative effects of salinity in tomato seedlings were attenuated by priming the seeds with Ulva lactuca methanol extract. The hydrogen peroxide concentration and antioxidant activity were decreased in plants treated with methanol extract and a high concentration of total phenol and soluble sugars were recorded in methanol extract suggesting these compounds might play a key role in mitigating salinity stress in plants (El et al. 2021). The use of natural plant biostimulants has been recommended to boost plant resilience to abiotic environmental stresses (Nephali et al. 2020; Deolu-Ajayi et al. 2022).

Mitigating plant biotic stress using seaweed-based biostimulants

The emergence of infectious plant pathogens has increased due to changes in climatic conditions and intensive agriculture (Anderson et al. 2004). Even though new technology, research, and products help agriculture in maintaining integrated management and farming practices, pathogens (primarily bacteria, viruses, and fungi) have decreased agricultural productivity inflicting economic damage on at least 10% world's food supply. To counteract the biotic stresses induced by pathogenic infection, plants activate different defense mechanisms. Mitigation of plant biotic stress has been successfully achieved by priming the plant molecular defense by treatment with different chemicals, natural substances, and phytohormones (Aranega-Bou et al. 2014). SEs and their bioactive compounds are successfully used as defense priming agents in plants to trigger an immune response in plants (Craigie 2010; Islam et al. 2020).

Many seaweed extracts have been reported to exhibit antagonistic activity both in-vitro and in the fields (Table 3). For example, Ulva lactuca reduced the fusarium wilt severity when sprayed onto leaves of common beans (de Borba et al. 2019). The use of Ochtodes secundiramea and Laurencia dendroidea extracts in papaya and banana efficiently inhibited Colletotrichum gloeosporioides, a fungal species that severely hampers fruits post-harvest (Machado et al. 2014). The cucumber plants sprayed with A. nodosum exhibited enhanced resistance against Phytophthora melonis by induction of defense enzymes and genes. Moreover, the plants showed a higher level of phenolic accumulation compared to the control (Abkhoo and Sabbagh 2015). Similarly, the use of A. nodosum in carrot and broccoli successfully controlled the foliar diseases caused by different fungal pathogens (Jayaraj et al. 2008; Mattner et al. 2014). The extracts of brown algae Cystoseria myriophylloides, Laminaria digitata and Fucus spiralis inhibited the growth of tomato pathogens Verticillium dahlia and Agrobacterium tumefaciens both in-vitro and in the greenhouse. It was also reported that defense enzyme polyphenol oxidase and peroxidase activity were substantially increased in comparison to control (Esserti et al. 2016). The extracts of Ulva lactuca, S. filipendula and Gelidium serrulatum suppressed the Alternaria solani and Xanthomonas campestris pv vesicatoria in tomato plants. The seaweed-extract treated plant showed reduced severity of diseases and induced salicylic acid and jasmonate signaling pathways (Ramkissoon et al. 2017). The root-knot diseases in eggplant and watermelon caused by fungi Fusarium solani were suppressed using extracts of Spatoglossum variabile, Stokeyia indica, and Melanothamnus afaqhusainii. The use of Padina pavonica benzene extract caused more mortality of Dsydercus cingulatus an economically important cotton pest (Sahayaraj and Kalidas 2011). In banana and tomato plants the root-knot infection caused by a nematode, (Meloidogyne spp.) was controlled by soil application of SEs such as Ulva lactuca, Ulva fasciata, and Stokeyia indica (El-Ansary and Hamouda 2014; Ghareeb et al. 2019).

Table 3 Effect of different seaweed extracts on disease resistance in different plant species
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Plants activate their defense mechanism against pathogens by recognizing pathogen-associated molecular patterns (PAMP) (Göhre and Robatzek 2008). Seaweed-based elicitors act as PAMPs and the signaling pathways are activated by the phytohormones jasmonic acid (JA), ethylene (ET), and salicylic acid (SA) (Dumas et al. 2010). Elicitors or plant defense activators generally induce partial, broad-range systemic resistance, as indicated by a considerable reduction in disease symptoms caused by various pathogens. The bio-elicitors stimulate the plant’s defense mechanisms, influencing genetic reprogramming (Pršić and Ongena 2020). Seaweeds contain bioactive compounds such as carrageenans, fucans, laminarans and ulvans that have been used as elicitors to protect the plant against various diseases (Shukla et al. 2016). Carrageenans possess antiviral properties and elicit defensive responses in plants. For example, tomato plants treated with carrageenan significantly suppressed tomato chlorotic dwarf viroid replication and expression. Additionally, jasmonic acid-related genes AOS (allene oxide synthase) and LOX (lipoxygenase) were up-regulated during viroid infection in plants treated with carrageenan (Sangha et al. 2011, 2015). The к-carrageenan extracted from Hypnea musciformis enhanced the immunity in tobacco plants infected with Tobacco Mosaic Virus (TMV) and activated SA and JA/ET dependent pathways defense mechanisms (Ghannam et al. 2013). Priming plants with seaweed-based elicitors enhances the plant immunity against phytopathogens by inducing defense-related genes and enzymes. For example, the Arabidopsis thaliana seedling treated with A.nodosum seaweed extract boosted the plant's immunity against various bacterial pathogens and also activated the expression of defense-related genes such as WRKY30 (WRKY DNA binding protein), (cytochrome P450, family 71, subfamily A, polypeptide 12), and PR-1 (Pathogenesis-related protein-1) (Cook et al. 2018). Seaweed-based elicitors are environmentally friendly crop protection strategies available and are a developing paradigm in the prevention of plant diseases that activates the plant immune system prior to pathogen overcoming. It also activates the innate immunity of plants by upregulating the defense-related genes and hormones thus rescuing plants from biotic stress.

Mechanisms of action of seaweed extracts under biotic and abiotic stress conditions

The seaweed-based biostimulants obtained from complex extraction contain a wide range of bioactive compounds that in theory can generate multiple positive effects throughout plant development. This is because they trigger signaling pathways, resulting in physiological changes in plants. For example, overexpression of NtLTP4 (Nicotiana tabacum Lipid transfer protein 4) in Nicotiana tabacum increased tolerance to salt and drought stress. The NtLTP4 modulates transcription levels of salt-responsive genes NHX1 (Na+/H+antiporter) and HKT1 (high-affinity K+ transporter1) to reduce Na+ toxicity. (Xu et al. 2018). The tomato and pepper crops treated with seaweed extract were shown to increase the upregulation of gene transcripts Ga2Ox (Gibberellin 2-oxidase), IAA (Indole-3-acetic acid), and IPT (Isopentenyl transferase) which are involved in the production of phytohormones in plants (Ali et al. 2019).

The usage of seaweed extract modulates the level of ABA, commonly known as the plant’s stress hormone. Treatment of Arabidopsis with ethyl acetate extract of A.nodosum under salt stress increased the transcript accumulation of the SnRK2 (SNF1-related protein kinase 2) gene which activates the ABA signaling network (Jithesh et al. 2018). The bioactive components present in the ethyl acetate fraction of the A. nodosum extract enhance salt tolerance and regulate stress-related signal transduction. Shukla et al.(2018) reported that GmCYP707A1a and GmCYP707A3b genes which regulate ABA biosynthesis during dehydration and hydration cycles were overexpressed in soybean attributing to the application of seaweed extract and also induced drought resistance (Shukla et al. 2018). Similarly, the A. nodosum extract regulated GmFIB1a (Glycine max Fibrillin 1a) expression and protected photosystem ll. Proline induces osmotic stress-related genes in frost tolerance (Wei et al. 2022). The use of seaweed extract on A. thaliana accumulated proline by activation of proline biosynthesis and also mediated freezing tolerance (Nair et al. 2012). Similarly in another study, the use of S. angustifolium improved drought tolerance by enhancing the expression of P5CS (Pyrroline-5-carboxylate synthase) a crucial gene in the biosynthesis pathway of proline, and is overexpressed under stressful conditions (Shahriari et al. 2021). According to Ghamdi et al. (2018) the use of seaweed extract had a synergistic effect in reported Asparagus aethiopicus plants subjected to saline stress. There was a significant enhancement of morphological and physiological characteristics of the treated plant compared to the control. The experiment also revealed the salinity stress was mitigated by upregulation of genes ANN1 (Annexins D1), ANN2 (Annexins D2), PIP1 (plasma membrane intrinsic protein 1) and CHS (Chalcone Synthase) that are related to osmotic adjustment and flavonoid biosynthesis (Al-Ghamdi and Elansary 2018). Pre-treatment with seaweed extract induces partial stomatal closure which is linked to alteration in expression of genes associated with ABA-responsive and antioxidant system pathways. NCED3 (Nine-cis-epoxycarotenoid dioxygenase) genes implicated in the ABA-biosynthesis pathway was significantly expressed during dehydration and ABA-responsive gene RAB18 (responsive to ABA) and RD29A (Responsive to Desiccation) transcript were expressed in treated plants during drought stress (Santaniello et al. 2017). The bean (Phaseolus vulgaris) treated with A. nodosum was shown to increase drought tolerance by reducing ROS-induced MDA (Malondialdehyde) generation and increasing CAT (Catalase) activity (Eugenia Amaral Carvalho et al. 2018). The WRKY transcription factors are involved in the development and physiological processes of plants. They interact with MAPK (mitogen-activated protein kinase) and regulate plant function by acting downstream of multiple MAPKs. The application of K. alvarezii (K-sap) increased the expression of the TaMPK10 gene encoding MAP Kinase in wheat and increased the tolerance of plants under drought stress (Patel et al. 2018). Cold stress in Arabidopsis was mitigated by the use of seaweed extract which increased the chlorophyll content and lowered the expression of AtCLH1 and AtCLH2 (chlorophyll degradation genes) and also induced the expression of cold-responsive genes such as COR15A, RD29A, and CBF3 (Rayirath et al. 2009). In pepper plants, the salinity stress and oxidative damage were reduced by the application of seaweed extract which upregulated the antioxidant activity (SOD, CAT, and APX) (Elansary et al. 2017).

The application of seaweed extracts and their bioactive components activate the up-regulation of numerous defense-related genes, plant hormones, and defense enzymes and these factors interact and cross talk to build a complex network that aids in disease tolerance in plants. The seaweed polysaccharides induce oxidative burst and activate various signaling pathways such as SA, JA, and ET at a systemic level and this leads to activation of different defense-related players such as pathogenesis-related protein (PR) and enzymes like phenylalanine ammonia-lyase (PAL) and lipoxygenase (LOX) (Vera et al. 2011) (Fig. 3). For instance, the A. nodosum extract application enhanced resistance against Pseudomonas syringae pv. tomato DC3000. The extract activated the JA-dependent pathway by inducing the expression of PDF1.2 JA-related genes and the decrease in diseases symptoms on the leaves is linked to the enhanced expression of JA -related gene transcripts (Subramanian et al. 2011). The application of K-sap in tomato seedlings enhanced the concentration of hormones (ABA, IAA, SA, and zeatin) as well as pathogenesis-related genes PR-1b1, PR-3, and PR-5 during Macrophomina phaseolina infection (Agarwal et al. 2015). In pepper and tomato plants the foliar application of S. vulgare and Acanthophora spicifera reduced the severity of the pathogens Xanthomonas campestris pv. vesicatoria and Alternarai solani by activating genes (PR-1a, Pinll, and ETR-1) which are involved in defense signaling pathways (Ali et al. 2020). Cluzet et al.(2004) reported that the use of 500 µg mL−1 of Ulva spp extract in Medicago truncatula induced expression of genes related to defense signaling pathways, pathogenesis-related protein and cell wall protein (Cluzet et al. 2004). To reduce stress in plants seaweed extracts can be promising strategy and moreover the experiments on both biotic and abiotic stress have proven that seaweed based biostimulants are beneficial.

Fig. 3
figure 3

The schematic illustration highlights the plausible mechanism of action of seaweed extracts against fungal and bacterial disease control. Seaweed extract may directly affect pathogens or indirectly by eliciting plant defense machinery. Such elicitation occurs by activation of several enzymes (PAL, LOX and ACS) which leads to the activation of several metabolic pathways such as (SA, JA and ET) thus upregulating several defense genes which induce the biosynthesis of several secondary metabolites which are involved in diseases suppression. Note: ROS reactive oxygen species, SA salicylic acid, PAL phenylalanine ammonia lyase, LOX 5-lipoxygenase, AOS active oxygen species, JA jasmonic acid, ACC 1-aminocyclopropane-1-carboxylic acid, ACS-ACC synthase, ACO-ACC oxidase, ET ethylene, PR pathogenesis related protein, PPO polyphenol oxidase, PO prophenoloxidase, LTP lipid-transfer proteins, GLU glutamate, PDF1.2 Plant defensin1.2, PINII proteinase inhibitor II, CYP71A12 cytochrome P450, family 71, subfamily A, polypeptide 12, ETR1 Ethylene response 1

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Conclusion

The utilization of seaweeds extracts and their bioactive compounds can influence the performance of crops. The advantages of seaweed products in agriculture are undeniable as they increase the uptake of nutrients, enhance both the morphological and physiological characteristics, and also mitigate stress in plants. It has been already reported that they can act as both biostimulants and bio-elicitors in plants. Moreover, the SEs are environment friendly and are also effective in inducing innate immunity in plants. SEs can be used in combination with other biofertilizers to gain maximum yield and reduce the use of chemical fertilizers. The chemical constituents of SEs must be studied and analysed to determine the mode of application, dosage, and duration of administration to different crops to obtain long-term benefits. To supply consistent and dependable products to end users, efficient production procedure based on an in-depth understanding of the molecular mechanisms by which these extracts elicit beneficial impacts in treated plants is essential. The composition of seaweed is complex and it is difficult to identify the bioactive molecule that induces specific effect in plants. The use of new approaches (high throughput sequencing, metabolomics and genomics) can unravel the mechanisms of action. The seaweed extracts have been utilized in research; they must be marketed in order to reach farmer’s level.