Introduction

Globally, plant growth, development, and yield are challenged by environmental stresses. However, the plants counteract efficiently by activating a cascade of signals that regulates the expression of various stress-responsive genes like AP2/ERF, MAPKs, MYB, NAC, Stress associated proteins (SAPs), ZIP, etc. Currently, SAPs belonging to the zinc finger protein family have gained recognition for their significant role against various abiotic stresses and plant development (Dixit et al. 2018). They are Zinc finger proteins with characteristic AN1/A20 domains at C and N terminals, respectively (Mukhopadhyay et al. 2004) and act as regulatory proteins with diverse functions in various organisms (Takatsuji 1999). The basic domain organization of SAPs in plants is composed of the A20 domain at N terminal and AN1 domain at C terminal separated by a variable stretch of amino acids (Vij and Tyagi 2008). The A20 domain of SAPs was first identified as a tumor necrosis factor (TNF) induced gene from the human umbilical cord consisting of the Cx2-4Cx11Cx2C consensus sequence, where x can be any amino acid (Opipari et al. 1990). Additionally, the AN1 domain was first discovered in Xenopus laevis, a protein encoded by animal hemisphere 1 (AN1) maternal RNA and comprises of Cx4Cx9-12Cx1-2Cx4Cx2Hx5Hx6 consensus sequence at C terminal (Jin et al. 2007). OsSAP1 was the first SAP that was reported in rice, and its expression was induced in response to several abiotic stresses, like ABA, cold, drought, salt, heavy metals, and wounding. Also, the overexpression of OsSAP1 in transgenic tobacco enhanced the tolerance to salt, dehydration, and cold stress (Mukhopadhyay et al. 2004). Since then, several homologs of SAPs have been identified and characterized in diverse plant species like Arabidopsis thaliana (Vij and Tyagi 2006), Solanum lycopersicum (Solanke et al. 2009), Gossypium hirsutum (Gao et al. 2016), Malus domestica (Dong et al. 2018), Glycine max (Zhang et al. 2019), Brassica napus (He et al. 2019), Populus trichocarpa (Li et al. 2019), and Cucumis sativa (Lai et al. 2020) (Fig. 1).

Fig. 1
figure 1

Molecular phylogenetic analysis of AtSAP5 homologs in different plants along with their domain architecture. The phylogenetic tree was constructed using MEGA 7.0 by Maximum Likelihood method based on the JTT matrix-based model with a bootstrap value of 1000 replicates. The tree with the highest log likelihood (-1551.81) is shown. The percentage of trees in which the associated taxa clustered together is shown next to the branches. The tree is drawn to scale, with branch lengths measured in the number of substitutions per site. The domains on the right were predicted using NCBI CDD and visualized using Domain Graph v 1.0 (http://dog.biocuckoo.org/). Zm-Zea mays; Ph-Panicum hallii; Si-Setaria italica; Os-Oryza sativa; Pv-Panicum virgatum; Gm-Glycine max; Mt-Medicago truncatula; Eg-Eucalyptus grandis; At-Arabidopsis thaliana; Br-Brassica rapa; Cs-Cucumis sativus; Pp-Prunus persica; Md-Malus domestica; Bd-Brachypodium distachyon; Ma-Musa acuminata; St-Solanum tuberosum

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Most SAPs have been characterized to confer stress tolerance in transgenic plants (Giri et al. 2013). For instance, the overexpression of SAP (AtSAP5) from Arabidopsis in cotton conferred tolerance to drought and salt stress (Hozain et al. 2012). Similarly, overexpression of SAP (AlSAP) from Aeluropus littoralis improves the tolerance against cold, drought, and salt stress in transgenic rice (Ben et al. 2012). Hitherto, various homologs of AtSAP5 were identified in different plant species. Apart from abiotic stress tolerance, SAPs are also associated with pathogen defense. A study has revealed that GhSAP17A/D negatively regulates the defense response of cotton to Vertillium dahlia (Gao et al. 2016). Besides, AtSAP5 and its homolog Pha13 from orchid could mediate antiviral immunity in plants (Chang et al. 2018). SAPs also intervene in physiological processes, like growth and development in plants. For example, elevated expression of GhSAPs in the stamen and pistil indicates their possible role in regulating flower development in cotton (Gao et al. 2016). In summary, SAPs are now recognized as potential regulators of plant development and stress responses. Despite the recent progress, understanding the mode of action and precise functions in stress response remains elusive. In this context, the present review enumerates the progress made in understanding the evolution and interplay of SAPs during the stress response and the mechanism underlying plant defense.

Structural features and evolution of SAPs

The identification of SAPs in plants was influenced by their pre-existing occurrence in humans. In humans, the Zinc finger protein A20 serves as an important regulator of innate and adaptive immunity (Mc Dermott and Aksentijevich 2002; Ngo et al. 2014; Peckham et al. 2017). It plays a major role in innate immunity by inhibiting NF-kB signaling, required for immune cell activation in humans (Lu et al. 2013; Ma and Malynn 2012; Wertz et al. 2004). In addition, characterization of other A20 like domain-containing proteins, such as ZNF216, AWP1, and ZNF216 from mice unraveled their involvement in NF-kB signaling. Further, a study revealed that the human zinc finger protein, ZNF216, can regulate immune responses due to the presence of an additional domain, AN1, at the C-terminal (Heyninck and Beyaert 2005). Additionally, many structural studies revealed that four cysteine residues at C-terminal were found to be conserved between OsSAP1, ZNF216, and AWP1 (Mukhopadhyay et al. 2004). However, domain analysis of A20/AN1 protein revealed diversity in domain architecture and variation in domain organization across taxa. Besides, domain comparison of SAPs from plants and animals reported maximum diversity in domain architecture of animal origin SAPs (Vij and Tyagi 2008).

The A20/AN1 zinc finger domains containing SAPs were also identified in some primitive organisms like protists, algae, and fungi. In some protists and fungi, like Plasmodium falciparum, Entamoeba histolytica, Saccharomyces cerevisiae, the AN1 domain exists alone, indicating its primitive nature (Jin et al. 2007; Vij and Tyagi 2008). SAPs from animals showed the presence of additional domains, like OTU, AAA, R3H, UBQ, UIM, and VPS9, associated with specific functions (Hurley et al. 2006). On the contrary, plants show much simpler domain organization (Fig. 2; Table 1) and consist of only one additional domain, i.e., C2H2 linked with AN1. In addition, only a few domains are shared between plants and animals, such as AN1 + A20, AN1, A20, and 2AN1. Among all reported domain architectures in plant SAPs, A20/AN1 is most prevalent, indicating its distinct lineage-specific expansion (LSE) and dynamic evolution in land plants (Table 1). The distinct LSE of a particular domain organization in plants might be due to whole-genome duplication (Lespinet et al. 2002). The presence of distinct domain organization enables SAPs to regulate diverse physiological functions in plants (Vij and Tyagi 2008). Further, the identification and characterization of SAPs in various plant species showed the occurrence of intron-less genes. For example, among 30 SAP genes of Malus domestica, 25 were intron-less (Dong et al. 2018), and in Glycine max, 18 from 27 SAP genes were reported to be intron-less (Zhang et al. 2019). Hence, the occurrence of intron-less SAP encoding genes supports their primitive origin and resemblance with prokaryotic genes. Altogether, these facts indicated that due to reduced post-transcriptional processing of SAPs, they have a primary role in early responses to stress conditions (Grzybowska 2012).

Fig. 2
figure 2

Distribution of distinct domain architectures (except A20 + AN1) across the diverse group of plants. Among 16 different plant species, maximum diversity was seen in Oryza sativa

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Table 1 Domain organization of stress associated proteins and their corresponding genes identified in plants
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In addition, Kang et al. (2013) had reported that AtSAP5 shared close homology with animal proteins like Rabex-5 and A20 protein. These animal orthologs of AtSAP5 could perform E3 ubiquitin ligase activity and similar action (Kang et al. 2013). Further insights in this study were gained by the prediction of three-dimensional structures of AtSAP5 and its homologs in different plants, Oryza sativa (OsSAP7), Orchid (Pha13), Brassica rapa, Glycine max, wheat, and Cucumis sativus (Fig. 3). The three-dimensional and secondary structure distribution used for the modelling were conserved in all plants. However, due to the lack of X-ray crystallographic structural information of SAPs, formulating a conclusion on the mechanism and alteration of their conformation during ubiquitination will be a daunting task. Nevertheless, the predominance of random coil and alpha-helix in AtSAP5 and its homologs, and their resemblance with animal ortholog Rabex-5, suggested the function as E3 ubiquitin ligase in ubiquitination (Penengo et al. 2006).

Fig. 3
figure 3

The three-dimensional structures of candidate SAPs reported in different plant species predicted using SOPMA tool (https://npsa-prabi.ibcp.fr/cgi-bin/secpred_sopma.pl)

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Multitude function of stress associated proteins in plants

SAPs are widely characterized in plants and have been recognized to play a crucial role in enhancing tolerance against abiotic stresses in plants. Comprehensive studies on SAPs revealed multiple modes of action in plants, ranging from ubiquitin ligase (Kang et al. 2011, 2017; Liu et al. 2019; Lloret et al. 2017) to redox sensors (Stroher et al. 2009; Giri et al. 2011), and in the regulation of stress-responsive genes (Li et al. 2019; Lloret et al. 2017; Giri et al. 2011; Hozain et al. 2012; Ben et al. 2012). Moreover, SAPs are also involved in other physiological processes of plants, such as regulation of phytohormone signaling (Chang et al. 2018; Liu et al. 2019; Ben et al. 2020), defense responses against pathogens (Chang et al. 2018; Liu et al. 2019; Sreedharan et al. 2012), cell elongation (Liu et al. 2011), glandular trichome development (Wang et al. 2020) and cell expansion (Lloret et al. 2017), which altogether indicate the diverse functions of SAPs in plants.

SAPs and ubiquitination

Ubiquitination is reversible and often controlled by the alternating action of ubiquitin ligase and deubiquitinases, which are key regulators of various stress responses in plants and animals. The cytoplasm localized mammalian A20 protein mediates the dual function of ubiquitination and de-ubiquitination in NF-kB signaling in humans. The OTU domain of A20 protein at N –terminal is responsible for the K-63 linked de-ubiquitination of receptors like RIP1 (Receptor Interacting serine/threonine Kinase Protein 1) and NEMO (Nf- Kappa B essential modulator), which further disrupts the downstream signaling of NF-kB activation (Mauro et al. 2006; Wertz et al. 2004). Also, the Zinc finger 4 and Zinc finger 7 domain at the C-terminal of the A20 protein is crucial in K48 linked ubiquitination of RIP receptors. Thus, A20 protein disrupts the receptor of NF-kB signaling and hinders the shuttling of NF-kB into the nucleus, thus inhibiting the signaling (Verhelst et al. 2012; Tokunaga et al. 2012). Similar functions of SAPs are reported in plants due to the presence of A20 domain at the N-terminus. Some SAPs have E3 ligase activity and often facilitate abiotic and biotic stress tolerance in plants. In contrast, some play an essential role in ubiquitination by interacting with other proteins of ubiquitination pathways. In 2011, Kang et al. had reported the E3 ligase activity of the AtSAP5 in Arabidopsis. The domain mapping with in-vitro ubiquitin assay revealed ligase activity in the AN1 domain of AtSAP5 due to the replacement of cys6/7 by histidine and his4/5 by cysteine (Kang et al. 2011). This conformational alteration in the AN1 domain resembled the structure of RING Finger proteins, which function as ligase enzyme during ubiquitination (Kang et al. 2011). Further, it was revealed that a highly conserved diaromatic patch in animal orthologs of AtSAP5, i.e., A20 protein and Rabex-5, was important for binding of polyubiquitin. However, the dialipathic patch in AtSAP5 mediates linkage-specific polyubiquitin binding and recognizes K-63 linked polyubiquitination to regulate the downstream signaling of target protein (Choi et al. 2012). A study by Kang et al. (2013) revealed the direct interaction of AtSAP5 with AtMBP-1 to regulate developmental processes and stress response. The expression of AtMBP-1 makes plants hypersensitive to ABA and abiotic stress. However, yeast two-hybrid screening showed direct interaction of AtMBP-1 with AtSAP5. The co-expression analysis of these proteins resulted in the ubiquitination of AtMBP-1 in-vivo, restoring the developmental abnormalities caused by the expression of AtMBP-1 (Kang et al. 2013). Similarly, the ligase activity of SAPs was also reported in other plants, like wheat, where in-vitro ubiquitination assay performed with TaSAP5 resulted in the formation of polyubiquitin chain (Zhang et al. 2017). It was observed that the TaSAP5 mediated the ubiquitination of DRIP (DREB2A–Interacting protein), which is responsible for destabilizing DREB2A, a drought tolerance protein. TaSAP5 enhances DRIP degradation by ubiquitination process, resulting in increased accumulation of DREB2A (Dehydration Responsive Element Binding protein 2A) proteins, thereby indicating the function of TaSAP5 in mediating drought tolerance in wheat (Zhang et al. 2017).

Many studies have reported the interaction of SAPs with other ubiquitination pathway proteins. For instance, Kang et al. (2017) have reported the interaction of AtSAP9 with AtRAD23d (Radiation sensitive 23) protein, which is involved in the transfer of ubiquitinated protein to ubiquitin proteasomes (Kang et al. 2017). Similarly in tomato, Liu et al. (2019) observed the interaction of SlSAP4 with SlRAD23d, mediated by the A20 domain of SlSAP4 and the UBA domain of SlRAD23d. The study also revealed the interaction of SISAP4 with ubiquitin and ubiquitin ribosomal fusion protein (Liu et al. 2019). Interestingly, PpSAP1 regulates leaf morphological alteration by interacting with polyubiquitin proteins (Lloret et al. 2017). Chang et al. (2018) reported E3 ligase and ubiquitin-binding activity in the A20 domain of Pha13, a homolog of AtSAP5. The detailed analysis confirmed the interaction of Pha13 with multiple ubiquitinated proteins (Chang et al. 2018). The deletion and mutant studies of AN1 and A20 domains of Pha13 demonstrated the importance of A20 domain for its function (Chang et al. 2018). However, further structural analysis of A20/AN1 domains is needed to understand their coordination in ubiquitination. Functional analysis of domains across different plant species will assist in understanding the functional deviation in domains of SAPs in different plant species.

SAP-mediated redox regulation

The redox-dependent function of SAPs was reported in AtSAP12 (Ströher et al. 2009). The structural analysis of AtSAP12 showed the presence of 16 cysteine residues in two AN1 domains, of which 12 residues are involved in coordinating zinc ions, indicating their role in redox homeostasis (Ströher et al. 2009). Further, the 2D SDS-PAGE analysis showed the formation of monomers, dimers, and oligomers under oxidizing and reducing conditions, indicating redox-dependent conformational changes by the formation of intra- or inter-molecular disulfide bonds in AtSAP12. Similarly, OsSAP1/11 from rice showed redox-dependent conformational changes to promote its interaction with OsRLCK253, leading to activation of stress response in rice (Giri et al. 2011). However, this hypothesis needs further experimental proof. Although in animals, it has been reported that the NF-kB is dependent on the A20 domain activity for redox control (Bubici et al. 2006). Besides, the function of cysteine residues in mediating the formation of disulphide linkages in SAPs for conformational changes during stress-induced redox changes remains elusive.

Regulation of growth and development by SAPs

Interests have been developed in examining the role of SAPs in various other physiological processes, including growth and development. It has been reported that expression of MtSAP1 in Medicago truncatula was enhanced by six to eight folds during seed maturation, thereby increasing tolerance to the desiccation phase; however, the expression was reduced after few hours of seed imbibition (Gilles et al. 2011). MtSAP1 expression is necessary to accumulate storage proteins, such as globulin, vecilin, and legumin, for efficient germination of seeds (Gilles et al. 2011). Further, overexpression of MtSAP1 in tobacco confers tolerance to abiotic stress by enhancing chlorophyll content and proline accumulation (Charrier et al. 2013). Nevertheless, in rice, OsDOG, dwarf rice with overexpression of gibberellin-induced gene, an A20/AN1 ZFP family protein, acts as a negative regulator of gibberellic acid mediated cell-elongation (Liu et al. 2011). The study reported a reduction in cell elongation of leaf sheath and internodes in the OsDOG overexpressing lines due to a reduced level of GA1. The study reported the novel function of OsDOG in regulating GA homeostasis by modulating GA metabolism genes. Although, the detailed molecular mechanism underlying this signaling needs further work (Liu et al. 2011).

The transcriptional analysis provides more profound knowledge on the molecular functions of SAPs in various biological processes. For instance, the transcriptome data revealed the significant expression of cotton SAPs in pistil and stamen, suggesting their possible role in flower development (Gao et al. 2016). Likewise, overexpression of AtSAP9 delayed the flowering in transgenic lines by reducing the expression of genes involved in flowering time, such as FT (Flowering locus T), SOC1 (Suppressor of overexpression of CO1), and CO1 (Constants). Hence, AtSAP9 is directly or indirectly involved in determining flowering time by CO-dependent pathway (Kang et al. 2017). The orchid SAP, Pha13, a homolog of AtSAP5, is associated with the expression of NPR and NPR-1 independent genes, which are important in salicylic acid-mediated immune pathways (Chang et al. 2018). Besides, the expression of the JA/ET signaling responsive genes were downregulated in SISAP4 silenced Solanum lycopersicum plants after infection with Botrytis cinerea, indicating the role of SlSAP4 in JA/ET signaling (Liu et al. 2018). The SlSAP10 silenced plants witnessed an accelerated yellowing and low chlorophyll content. These results suggest that SlSAP10 plays a crucial role in maintaining the integrity of chlorophyll in plants (Liu et al. 2018).

Apart from growth and development, SAPs also regulate the phytohormone responses in plants. Recently, the A20 domain of LmSAP in Lobularia maritima was identified in regulating GA homeostasis in plants. Overexpression of LmSAP and LmSAPΔAN1 led to the upregulation of GA biosynthetic genes, thus increasing the endogenous GA content, which further affects the GA responsive genes (Ben et al. 2020). Consequently, it is evident that SAP controls diverse functions in plants, and more comprehensive studies are required to unravel the molecular mechanisms underlying these novel functions of SAPs in plants.

Stress associated proteins and molecular stress response

SAPs regulate defense responses in plants and confer stress tolerance. Therefore, SAPs could be important targets for developing stress-tolerant plants, leading to increased crop productivity under prevailing environmental disturbances. Till now, many abiotic stress tolerant transgenic plants harboring different SAPs have been developed (Table 2). SAPs are not only confined to controlling abiotic stress tolerance but are also associated with pathogen defense.

Table 2 Studies involving overexpression of stress associated protein encoding genes in different plant species to impart tolerance to stresses
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SAPs and multiple abiotic stress tolerance

SAPs mediates stress response by regulating the expression of the stress-responsive genes in plants. Numerous studies showed that the overexpression of SAPs resulted in differential expression of various stress-responsive genes, thereby affecting stress tolerance. The change in expression of stress-related genes was observed in Oryza sativa, Arabidopsis, and Aeluropus littoralis when their respective SAPs, namely OsSAP11, AtSAP5, and AlSAP, were overexpressed in individual host plants (Ben et al. 2012; Giri et al. 2011; Hozain et al. 2012). Further, the transcriptome data also revealed the function of SAPs in regulating the expression of stress-responsive genes (Fig. S1). However, the actual mechanism of the regulation remains elusive and needs further investigation. Similarly, overexpression of PpSAP1 in plum leads to modification of leaf morphology to circumvent drought stress (Lloret et al. 2017). The change in morphology is caused by the downregulation of genes involved in cell growth, like GRF-1 (Growth Regulating Factor), TIP (Tonoplast Intrinsic Protein), and TOR (Target of Rapamycin), in response to overexpressed PpSAP1. Overexpression of PtSAP13 gene in Populus trichocarpa resulted in upregulation of stress-related genes, like BZIP860 (basic leucine zipper transcription factor), DI19-4 (drought-induced gene family), GPX-8 (Glutathione peroxidase 8), NADP-ME2 (NADP malic enzyme 2), RAS1 (Response to ABA and Salt 1), WRKYs (transcription factor associated with drought stress), GSTUs (Glutathione S Transferase), MYBs (MYB domain transcription factor). These genes play a vital role in drought response in plants, and their upregulation conferred tolerance to drought in transgenic Populus plants (Li et al. 2019). Similar results were observed in Glycine max, where overexpression of GmSAP16 affected the expression of stress-responsive genes, such as GmDREB1;1 (Transcription factor involved in drought stress), GmNCED3, and GmRD22, thereby regulating drought stress (Zhang et al. 2019). The elucidation of the function of the first reported SAP, i.e., OsSAP1, in response to various stresses, such as cold, salt, desiccation, and heavy metals, brought new insights for further research in understanding the mechanism of these proteins in stress response (Mukhopadhyay et al. 2004). Consequently, it was identified that SAPs of Sorghum bicolor (SbSAP14), Lobularia maritima (LmSAP), Leymus chinesis (LcSAP), Brassica napus (BnSAP), Sacharum officinarum (ShSAP1), Malus domestica (MdSAP15), Gossypium hirsutum (GhSAP17 A/D), Prunus (PpSAP1), Aeluropus littoralis (AlSAP), Medicago tranculata, and Arabidopsis thaliana (AtSAP5) were induced by multiple abiotic stresses like salt, drought, cold, heat and phytohormone ABA (Ben et al. 2012, 2020; Dong et al. 2018; Gilles et al. 2011; Gao et al. 2016; He et al. 2019; Kang et al. 2011; Liu et al. 2016; Lloret et al. 2017; Wang et al. 2013). Additionally, expression analysis of wheat SAP, TaSAP17D, indicated its involvement in various stresses, such as salt, PEG (Polyethylene glycol), cold, and ABA treatment. Interestingly, transcript levels of TaSAP17D were initially enhanced by NaCl treatment but reduced during later time points of stress (Xu et al. 2018). Similar results were reported in Glycine max, where GmSAP16 showed increased transcript level under drought, salt, and ABA treatment (Zhang et al. 2019), and PtSAP13 was upregulated in Populus trichocarpa, under salt stress (Li et al. 2019). Recently, the transcriptome analysis of cucumber SAPs revealed an upregulation of all CsSAPs under drought treatments (Lai et al. 2020). The result showed that CsSAP5, CsSAP6, CsSAP9, and CsSAP10 were responsive to salt and cold treatment (Lai et al. 2020). Similarly, five SAP genes of the barley, i.e., HvSAP5, HvSAP6, HvSAP11, HvSAP12, and HvSAP15, were found to be upregulated in response to salt stress (Baidyussen et al. 2020).

Extensive studies on SAPs suggested their involvement in early stress response in plants. For example, OsSAP1 transcript abundance was enhanced after 15 min of salt stress and ABA treatment (Mukhopadhayay et al. 2004). Nearly all SAPs in cotton were upregulated within one hour of salt treatment (Gao et al. 2016). However, the time period varies across different plant species. For instance, LcSAP was upregulated to a maximum level within six hours of exposure to NaCl, and the abundance of transcript remains stable for 24 h (Liu et al. 2016). TaSAP17D also showed an early response, and its expression increased within a half-hour of salt stress (Xu et al. 2018). However, LmSAP expression increased after 12 h of treatment with heavy metals (Ben et al. 2018). Seven PtSAP genes were induced in Populus trichocarpa within an hour of salt treatment and reached its maximum expression level in 6 h (Li et al. 2019). Altogether, these studies support the fact that SAPs have a crucial function during early stress response but can vary in different plant species.

The function of SAPs in regulating environmental stress has been successfully deployed in some plants to develop tolerance. Overexpression of SAPs in different plants enhances the growth under stress conditions. For example, the overexpression of OsSAP8 in rice conferred tolerance to salt and drought stress at the anthesis stage without any growth penalty (Kannegati and Gupta 2008). Further, overexpression of AtSAP10 elicited enhanced tolerance to heavy metal and heat stress in transgenic Arabidopsis (Dixit and Dhankher 2011). Similarly, overexpression of MusaSAP1 imparted stress tolerance in transgenic plants due to reduced membrane damage, inhibition of malondialdehyde formation, and strong up-regulation of polyphenol oxidase in response to salt and drought stress (Sreedharan et al. 2012). Likewise, overexpression of SbSAP14 in rice seedlings showed a high germination and survival rate with better tolerance to salt stress than wild type (Wang et al. 2013).

SAPs also impart better adaptive abilities to transgenic plants by regulating distinct morpho-physiological, biochemical, and molecular responses during stress. For example, the overexpression of PtSAP13 in Arabidopsis enhanced flavonoid biosynthesis, leading to improved tolerance to salt stress (Li et al. 2019). Similarly, overexpression of LmSAP in tobacco seedlings enhances their tolerance to heavy metals, like copper, cadmium, and manganese, by increasing the activities of enzymes, such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POD). In addition, transcript levels of specific genes like metallothioneins (Met1, Met2, Met3, Met4, and Met5), CCH (Copper transport proteins), cysteine, and histidine domain-containing protein RAR1 (Rar1), and Ubiquitin like protein 5 (PUB1) were also increased in tobacco seedlings. Further, the expression of AlSAP from Aeluropus littoralis in rice enhances the yield under drought conditions by increasing the tillering and panicle fertility (Thaura et al. 2017). Also, overexpression of PpSAP1 in transgenic plum plants led to leaf shape alterations, hence increasing water retention under drought stress (Lloret et al. 2017). MdSAP15 overexpression in Arabidopsis regulates plant physiological traits during osmotic stress by maintaining chlorophyll content and proline concentration (Dong et al. 2018). Recently, a study indicated that overexpression of GmSAP16 in Arabidopsis improved drought tolerance by regulating stomatal closure and reducing the rate of water loss (Zhang et al. 2019). Altogether, these studies elucidated the function of SAPs in enhancing stress tolerance as well as yield.

SAPs in pathogen defense

SAPs are well characterized in response to abiotic stress. However, SAPs are not confined to abiotic stress response but also regulate biotic stress response in plants. The plant innate immunity, like mammals, depends upon the intracellular nucleotide-binding/ leucine-rich repeat proteins or extracellular transmembrane anchored receptor-like kinases (RLK) or receptor-like proteins (RLP), which further activates downstream signaling (Feehan et al. 2020). The interaction of OsSAP11 with OsRLCK253 via A20 domain directs their function in plant innate immunity (Giri et al. 2011). Defense signaling hormones often induce certain SAPs. MusaSAP1 was induced by wounding and methyl jasmonate treatment, indicating its role in biotic stress response in plants (Sreedharan et al. 2012). In Rice, OsSAP1, is also responsive to various biotic stress and its overexpression in tobacco increases resistance against virulent pathogen Pseudomonas syringae pv. tabaci (Tyagi et al. 2014). Similarly, GhSAP17A/D was induced in response to salicylic acid and methyl jasmonates (Gao et al. 2016). Further, overexpression of AtSAP9 enhances sensitivity for P. syringae pv. Phaseolicola, indicating a negative function of AtSAP9 in immunity (Kang et al. 2017). In the case of Phalaenopsis aphrodite (orchid), Pha13 was induced by exogenous salicylic acid treatment. The Cymibidium mosaic virus-induced gene silencing system (CymMV-VIGS) showed that Pha13 silencing reduces the SA-related genes, thereby affecting the immune response. However, overexpression of Pha13 in orchid and Arabidopsis enhances the resistance to different viruses (Chang et al. 2018). Overexpression of Pha13 in Arabidopsis plants also enhanced their resistance to P. syringae pv. Tomato DC30000. Moreover, overexpression of A20 and AN1 domain double mutants of Pha13 elucidated the involvement of AN1 domain with PhaNPR1 expression, and both the domains are required for imparting resistance to different viruses (Chang et al. 2018). SlSAPs were induced on treatment with salicylic acid, methyl jasmonate, and 1-aminocyclopropane-1-carboxylic acid (ACC). SlSAPs expression was induced within 24 h of Botrytis cinerea infection, leading to enhanced immunity against the necrotrophic fungus through interaction with SIRAD23 and activation of JA/ET signaling. Virus-induced gene silencing (VIGS) and disease phenotyping assays identified that silencing of SlSAP4 and SlSAP10 decreases resistance to B. cinerea, depicting the crucial role of SISAP4 in providing resistance to B. cinerea (Liu et al. 2018). Hence, SAPs play an important role in regulating stress response against pathogen attacks; however, more work is required to understand the hidden molecular mechanisms of their functions.

Furthering the research on stress associated proteins

Currently, only limited knowledge is available about the function of SAPs in regulating various biological processes in plants, including stress responses. The current review summarizes the available knowledge on SAPs in different plant species. In particular, SAPs involved in regulating multiple stresses, should be of great interest to develop stress-tolerant cultivars. To our knowledge, the functional aspects of SAPs in plants against stress responses remain poorly understood. Hence, the advancement in omics approaches can provide in-depth knowledge for future research on the molecular functions of SAP proteins. For example, recently, a transcriptome study by Muthuramalingam et al. (2020) provides annotation of OsSAPs and unveils the role of these proteins in regulating hormonal pathways. Similarly, Lai et al. (2020) revealed transcriptional changes in CsSAP genes in cucumber in response to cold, drought, and salt stresses.

The studies on SAPs hypothesize that these proteins are functionally involved in biological processes such as metabolism, hormonal signaling, translation, developmental regulation, and stress responses. Genome-wide studies have unraveled the tissue level expression profiles of these genes in various plants species. However, it would be worth understanding the interaction of SAPs with various downstream genes involved in biological processes. The role of SAPs as E3 ubiquitin ligase to mediate signal transduction is well discussed in many plant species; however, the detailed mechanism remains elusive. Thus, discovering crystal structures of SAPs (NMR based or X-ray crystallography) having E3 ubiquitin ligase activity will delineate their mechanism of action and underlying pathways associated with ubiquitination. Most of the studies have reported the functions of A20 domain; however, very little information is available for the AN1 domain. Only in AtSAP5, Kang et al. (2011) observed the E3 ligase activity in the AN1 domain whereas most of the studies have suggested the involvement of A20 domain in E3 ligase activity. It would be interesting to unveil such mechanisms in detail to understand their role in plant immunity. Similarly, overexpression of SAPs enhances disease resistance in different plants, indicating their role in pathogen defense, and provides another crucial area of research to explore the detailed signaling underlying these disease resistance mechanisms. In summary, plant SAPs need to be studied comprehensively by employing combinatorial omics approaches and system biology to unravel the molecular mechanisms regulating stress-responsive metabolic and physiological processes in plants.

Conclusions

The zinc finger SAP family could either act as positive or negative regulators in providing plant immunity against biotic and abiotic stresses by the expression of several stress-responsive genes. SAPs can also be utilized to generate disease-resistant plants owing to their role in biotic stress response, hence fulfilling another major objective of the agronomic sector. However, the full potential of SAPs can only be utilized once their mode of action is understood efficiently. We proposed a presumed working model to understand the mechanism of SAPs in regulating the stress responses in plants (Fig. 4.). According to the model, biotic and abiotic stresses induce redox instability, thereby triggering ROS release, leading to the activation of receptors (RLCK), which interacts with SAPs via its A20 domain resulting in the formation of oligomers. SAPs also act as an E3 ubiquitin ligase and can participate in K63 and K48 linked polyubiquitination. In the case of K48 linked polyubiquitination, SAPs might interact with shuttling factors like Rad23b (in the nucleus), leading to degradation of proteins via 26S Proteasome. SAPs also play an important role in regulating stress-responsive genes, either by directly interacting with DNA or interacting with transcription factors, leading to the induction of signalling molecules, such as GA20ox, GA3ox, and GA2ox. Pattern-triggered immunity is generated when bacterial flagellin (Flg22) triggers SA production, followed by activation of NPR1. The activated NPR1 is transported to the nucleus and causes activation of salicylic acid-responsive genes. Salicylic acid might regulate SAPs at the transcriptional and post-transcriptional level, activating other SA-responsive genes leading to pathogen defense. Conclusively, the research exploring the precise involvement of SAPs in stress response requires further attention to gain functional insights that would enable genetic manipulation of these proteins for developing climate-resilient crop species.

Fig. 4
figure 4

A presumed model to depict the mechanism of SAPs in stress responses. The signal of any kind of stress (Biotic or abiotic stress) induces redox instability, triggering ROS release (Reactive Oxygen Species). This will trigger the activation of receptors (RLCK), which will interact with SAPs via its A20 domain and resulting in the formation of oligomers. SAPs also act as E3 ubiquitin ligase facilitated by its A20 domain and can participate in K63 linked polyubiquitination and K48 linked polyubiquitination. In case of K48 linked polyubiquitination, SAPs might interact with shuttling factors like Rad23b (in the nucleus), leading to degradation of proteins via 26S Proteasome. SAPs also play an important role in the regulation of stress-responsive genes, either by directly interacting with DNA or through binding with transcription factors which leads to induction of signalling molecules and causes their up-regulation (GA20ox, GA3ox and GA2ox). On the contrary, pattern-triggered immunity is generated when bacterial flagellin (Flg22) triggers the production of salicylic acid. This leads to monomeric forms of NPR1, which transports to the nucleus and causes salicylic acid-responsive genes. Salicylic acid might regulate SAPs at transcriptional regulation or post-transcriptional level where SAPs assist in the activation of other SA responsive genes, causing pathogen defense. Image created with BioRender.com

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