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1 Introduction

Nitrogen, phosphorus, and water are the most limiting factors to increased crop yields in Africa. With the high cost of chemical fertilizers, interest has increased in seeking new approaches to promote N nutrition in crops. In global terms, biological N2 fixation contributes about 65 % of N used in agriculture today, and therefore provides an alternative to chemical fertilizers. N2 fixation in root nodules of symbiotic legumes represent the major source of N for food production in the traditional cropping systems in Africa. During nodule formation, some legumes release flavonoid and isoflavonoid signal molecules, which act as chemo-attractants to invading root-nodule bacteria or “rhizobia” [1]. Additionally, these phenolic molecules fulfill various other functions, including promotion of bacterial growth [2] and induction of nodulation (nod) genes in compatible bacterial strains [3, 4]. Different legumes release different profiles of flavonoids and isoflavonoids from their roots and germinating seeds. Alfalfa grown in the absence of its N2-fixing symbiont, releases five distinct flavonoids [3, 5, 6] and two betaines [7] that induce transcription of nod genes in homologous Sinorhizobium meliloti. The common bean plant also releases nine compounds from seeds and roots that cause expression of nod genes in its microsymbiont [8, 9], while sterile Vicia roots release only two nod gene-inducing compounds [10].

The early steps in nodule formation involves a two-way molecular communication between the legume and rhizobial symbiont. Flavonoid and isoflavonoid signal molecules released by leguminous host induce expression of rhizobial nod genes followed by a coordinated synthesis of rhizobial signal compounds, such as lipo-chito-oligosaccharide nod factors. It is these nod factors that affect morphological changes in legume root hairs, leading to nodule formation. In compatible interactions, rhizobial signaling to host root hairs results in root hair deformation, root hair branching, root hair curling, cortical cell division, infection thread formation [11], and ultimately nodule development. It is in the mature nodules that N2 fixation occurs through the activity of the enzyme nitrogenase.

2 Microsymbiont-Induced Biosynthesis of Flavonoid and Isoflavonoid Compounds in Legumes

Studies with white clover [12] and later Vicia sativa subsp.nigra [13] were the first to demonstrate that inoculating host legume with infective rhizobial strains increased nod gene-inducing activity. Because that finding implied increased production of new and/or existing compounds, Recourt et al. [10] found from analysis of root exudates that, with Rhizobium inoculation, Vicia released 8 new additional nod gene-inducing compounds. However, there was no evidence that the inoculated roots released isoflavonoid phytoalexins [10] even though most of the 5-deoxy flavonoids produced by inoculated seedings could easily be metabolized to medicarpin, a common phytoalexin in Vicia faba [14].

Like Vicia, alfalfa root exudates also showed increased nod gene-induction following seedling inoculation with Rhizobium meliloti [15]. Similarly, root exudates of the common bean exhibited greater nod gene-inducing activity when plant roots were inoculated with Rhizobium leguminosarum bv phaseoli compared to uninoculated controls [16]. HPLC analyses followed by NMR and MS studies revealed that, unlike Vicia, those two symbioses exuded isoflavonoid phytoalexins in the presence of rhizobial symbionts. Rhizobium meliloti elicited the exudation of medicarpin, medicarpin-3-Oglycoside, and formononetin-7-O-(6″-O-malonylglycoside) from the roots of alfalfa plants. In addition to genistein, eriodictyol, and naringenin which are normally present in root exudates of sterile-grown bean plants [9]; the phytoalexin coumestrol and its precursor daidzein (Fig. 1a) were released by roots of bean in response to Rhizobium inoculation [16]. Tests for biological activity showed that the two compounds were active nod gene inducers. Thus, coumestrol can function both as a nod gene inducer in bean rhizobia and as a phytoalexin for host plant defense.

Fig. 1
figure 1

Flavonoid compounds

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Other studies have shown that exposing soybean plants to infective strains of Bradyrhizobium japonicum led to the exudation of glyceollin, a major phytoalexin of soybean [17]. However, the amount of glyceollin released in the presence of Bradyrhizobium japonicum was less compared to levels exuded in the presence of Phytophthora pathogen [17]. With Sinorhizobium fredii (another soybean symbiont), however, the level of glyceollin exudation was comparable to that caused by pathogenic Phytophthora [17]. Those studies [15–17] clearly indicate that both pathogens and rhizobial symbionts can elicit phytoalexin exudation in N2-fixing legumes. Even symbiotic fungi have been shown to trigger the formation of isoflavonoid phytoalexins in their host plants. In one study, analysis of soybean root extracts revealed increased tissue concentrations of glyceollin, coumestrol, and its precursor daidzein following Glomus infection of roots [18]. In alfalfa, Glomus infection resulted in accumulation of only formononetin in the roots [19], suggesting that phytoalexins are formed in both alfalfa and bean in response to fungal symbionts. These findings demonstrate that isoflavonoid phytoalexin production is caused by both pathogenic and symbiotic microbes, so to view these molecules simply as occurring in diseased tissue is misleading.

In contrast, some studies have found no accumulation of phytoalexins with fungal or rhizobial symbionts. Soybean cv. Maple Arrow, for example, showed no accumulation of glyceollin in response to Glomus mosseae, though with Rhizoctonia solani pathogen, glyceollin accumulation was very high [20]. The observed lack of phytoalexin accumulation in response to mycorrhizal fungus is comparable to Bradyrhizobium japonicum infection of soybean roots without inducing phytoalexin in accumulation [21]. However, with loss of symbiotic ability, rhizobial strains can be perceived as pathogens by host plants. For example, infection of soybean root by fix mutants of Bradyrhizobium japonicum led to rapid development of necrosis and instant phytoalexin accumulation [21], symptoms reminiscent of pathogen invasion.

Apart from these inconsistencies in overall experimental observations, similarities exist between rhizobial and fungal symbioses, especially with respect to host plant response to symbionts. Sinorhizobium inoculation of alfalfa seedlings stimulates the formation and release of conjugated formononetin into root exudates [15]. With mycorrhizal infection, the aglycone form of the same molecule is also found in alfalfa root extracts [19] and possibly exudates, suggesting that host plant recognition of symbiont, be it a fungus or rhizobium, is by the same mechanism. In that regard, mycorrhizal roots have been found to produce proteins which are immunologically related to nodulins in N2-fixing nodules [22]. Another study has also shown that both pathogens and symbionts of soybean identify their hosts by recognizing the same chemical signals [23].

3 Phytoalexin Effects on Rhizobial Symbionts

Phytoalexin accumulation to higher concentrations in host plant tissue can have detrimental effects, especially where micro-symbiont cells are in contact with such metabolites. An early study by Cruickshank [24] showed that pisatin, (a classical phytoalexin from pea and vetch) significantly inhibited growth of rhizobia compared with other rhizosphere microorganisms. Interestingly, Rhizobium leguminosarum biovar viciae, the microsymbiont of pea and vetch, exhibited a much broader range of pisatin tolerance relative to other rhizobial species. A survey on the inhibitory effects of selected isoflavonoid phytoalexins revealed three levels of sensitivity: pisatin, coumestrol, formononetin, vestitol, biochanin A, genistein, and rotenone were inhibitory; phaseollin, and maackiain, moderately inhibitory; while medicarpin and kievitone were strongly inhibitory [25]. In that study, kievitone and medicarpin, the latter a common photoalexin of agriculturally important legumes, such as Vigna unguiculata, Cicer arietinum, Canavalia ensiformis, Vicia faba, Medicago, Melilotus, and Trifolium spp., showed considerably marked inhibition of growth in slow-growing root-nodule bacteria compared to fast-growing strains (Table 1). Phaseollin also exhibited significant inhibition of growth in bradyrhizobia but had little effect on fast-growing species.

Table 1 Legume flavonoids and isoflavonoids involved in nod gene expression and N2 fixation
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While the concentrations used in in-vitro studies [24, 25] probably surpass those encountered in plant tissues, the data obtained nevertheless provide a useful indication of the spectrum of symbiont sensitivity to isoflavonoids. Classical phytoalexins produced and released in purely symbiotic systems (Table 2), include medicarpin from alfalfa-Rhizobium meliloti symbiosis [15], coumestrol from bean-Rhizobium leguminosarium biovar phaseoli interaction [15], coumestrol from bean-Rhizobium leguminosarum biovar phaseoli interaction [16], and glyceollin from soybean-Bradyrhizobium japonicum association [17]. Although the in-vitro sensitivity of Rhizobium meliloti to medicarpin, the major phytoalexin of alfalfa, is low, it is difficult to imagine how the bacterial symbiont copes with increasing metabolite concentration in tissues and in the rhizosphere. Initial experiments revealed that at 150 micromolar concentration, glyceollin inhibited growth of Bradyrhizobium japonicum [26], suggesting that the microsymbiont must have a mechanism for overcoming glyceollin toxicity in soybean tissue and/or rhizosphere.

Table 2 Mechanisms of bioprotection in plants by symbiotic rhizobia
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Various mechanisms have been suggested for phytoalexin tolerance in pathogens and symbionts. Phytoalexin detoxification is one such mechanism by which microbial symbionts and pathogens overcome plant defence response. The phytoalexins medicarpin and maackiain, which are produced by a number of legumes, can be degraded by Nectria haematococca and Ascochyta rabiei, (two common fungal pathogens of chickpea and other legumes) via NADPH-dependent reductase conversion of the molecule to less toxic isoflavan, isoflavanone, or 6a-hydroxyptercocarpan (see Van Etten et al. [27]). Whether Rhizobium meliloti employs such a mechanism to detoxify medicarpin and other phytoalexins, remains to be seen Table 3.

Table 3 Isoflavonoid compounds from legume roots with potential for use as defense molecule against soil borne pests (insect larvae and pathogens) in cropping systems
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The bean pathogen, Fusarium solani f. sp. Phaseoli, can also detoxify four major bean phytoalexins, namely kievitone, phaseollin, phaseollidin, and phaseollinisoflavan; kievitone detoxification occurs through kievitone hydratase-catalyzed hydration of the isopentenyl side chain to yield the less toxic kievitone hydrate [27]. There is evidence that Rhizobium and Bradyrhizobium strains modify various flavonoid molecules within their vicinity [28], suggesting that bacterial and fungal symbionts probably overcome isoflavonoid phytoalexin toxicity in tissues and soil through biotransformation of these compounds.

Besides chemical modification, bacterial tolerance of phytoalexins may be flavonoid-induced. Growing cells of a pathogen or symbiont in low-level concentrations of a phytoalexin can confer resistance to the microbe against the toxic effects of phytoalexins. It has been shown recently that this induction of resistance against phytoalexins may be effected by low concentrations of a second, structurally-related molecule other than the phytoalexin itself. In a classical study by Parniske et al. [26], it was demonstrated that preculturing Bradyrhizobium japonicum with 10 μmM concentration of genistein or daidzein induced resistance in the bacterium against the phytoalexin glyceollin. In that study, cell viability tests involving bacterial growth in geneistein-free medium followed by transfer to 300 μM glyceollin showed a strong bactericidal effect, while rhizobia precultured with genistein were unaffected. Kape et al. [29] have also found that isoliquiritigenin (2′,4′,4-trihydroxychalcone) is both a strong nod gene inducer and glyceollin resistance inducer. This confirms the role of flavonoids in phytoalexin resistance induction. While these findings may be the first for bacterial symbionts, induced phytoalexin resistance is not new in fungal pathogens. Denny van Etten [30, 31] have reported and induced resistance to pisatin in Nectria haematococca.

Although experimental results show that isoflavonoids can induce resistance in soybean microsymbionts against glyceollin phytoalexins [26], the mechanism by which this resistance is induced is still not properly understood. It is, however, clear that, unlike rhizobial nod gene induction which requires flavonoid interaction with nodD protein, induction of glyceollin resistance in Bradyrhizobium does not involve the common nod genes. This was evidenced by the successful induction of glyceollin resistance in a nodD1D2YABC deletion mutant [26]. Equally unknown is the mechanism by which soybean rhizobia and pathogens overcome the inhibitory effects of glyceollin. Even the mode of action of phytoalexins on pathogens and/or symbionts remains undefined. In the case of the glyceollins, however, the lethal effect of these phytoalexins on pathogens and symbionts is exerted via inhibition of plasma membrane and tonoplast H+-transporting ATPases [32], and/or NADH-ubiquinone-oxidoreductase [33].

4 Nodulation Significance of Phytoalexins From Legumes

The release of phytoalexin compounds by legume roots, whether in response to pathogen invasion, symbiont infection, or both, is likely to lead to higher concentrations of phytoalexins in the rhizosphere. Root exudation of phytoalexins, such as medicarpin, coumestrol, and glyceollin is known to occur from challenge by symbionts [15–17] and pathogens [26]. This suggests that the rhizosphere of nodulating legumes must be the site of continuous accumulation of phytoalexins triggered by invading rhizobia, root-borne pathogens, and various environmental stimuli. Also, because phytoalexin exudation is a localised response in roots, rhizosphere soil associated with symbiotic field legumes is likely to consist of spatial, and temporal phytoalexin concentration gradients. The accumulation of nod-gene inducing isoflavonoid phytoalexins in the rhizosphere is likely to increase the chance of enhancing nodulation while warding off pathogens. In fact, differences in legume nodulation have been shown to relate to limitation in the synthesis and release of nod gene-inducing flavonoids [34, 35].

A number of studies [36] have shown that soybean plants release the isoflavones daidzein and genistein into root exudates and these induce nod genes in Bradyrhizobium japonicum. Coumestrol is another phytoalexin from soybean which moderately induces nod genes in the host’s microsymbiont [37]. Thus, the nodulation potential of soybean depends largely on adequate availability of daidzein, genistein, and coumestrol in the rhizosphere to transcribe nod genes in Bradyrhizobium japonicum. Besides nod gene induction, daidzein, and coumestrol released into the rhizosphere also promote growth of the soybean bacterial symbiont [38]. The growth-promoting effect of these isoflavonoids on bradyrhizobial populations in the rhizosphere increases the chance for enhanced nodulation and N2 fixation in soybean from increased rhizobial numbers. Furthermore, in soybean, daidzein, genistein, and their glycosyl conjugates act in concert against pathogen invasion of this legume [39], resulting in healthy plant growth for better nodulation.

Phytoalexins released by legume roots play other roles essential for nodule formation. As indicated before, the isoflavonoids daidzein and genistein can each independently induce glyceollin resistance in Bradyrhizobium japonicum [26], thus permitting selective development of bacterial strain population needed for host plant nodulation. This means that with soybean release of daidzein and genistein into the rhizosphere, bradyrhizobia are induced to develop resistance to glyceollin exuded by legume to ward-off pathogens. Nutritionally, this offers a competitive advantage to microsymbiont, and enhances its ability to survive in the rhizosphere of phytoalexin-producing host roots. Biochemically, daidzein and genistein are important precursors for glyceollin formation [40], indicating that the synthesis and release of glyceollin would depend on the pool size of isoflavones in tissues as well as their rates of exudation from roots. The ecological significance of induced glyceollin resistance is not only to increase the survival of the microsymbiont in the rhizosphere which is constantly challenged by microsymbionts, pathogens, and environmental stimuli, but also to provide bacteria with physiological plasticity for adaptation to changing phytoalexin concentrations within the rhizosphere.

A recent study has shown that alfalfa growing in iron-deficient medium releases the phytoalexin 2-(3′5′-dihydroxyphenyl)-5,6-dihydroxybenzofuran, which solubilises iron from insoluble ferric compounds for the plant, while controlling Fusarium oxysporum f. sp. phaseoli, a root pathogen of alfalfa [41]. It is, therefore, likely that many legumes growing in poor soils, use compounds such as isoflavonoids to enhance nutrient uptake and control pathogens.

Coumestrol phytoalexin inhibits growth of pathogens, and acts as nod-gene inducer in strains of Rhizobium leguminosarium bv phaseoli, the causal organism of nodulation in the common bean. The molecule also moderately transcribes nod-genes in Bradyrhizobium japonicum, the microsymbiont of soybean. In soybean, isoliquiritigenin is both a nod gene inducer and glyceollin resistance inducer [29]. Breeding for increased levels of such compounds in legumes for enhanced nodulation and improved disease control could be useful for agriculture. Alternatively, increased nodulation and pathogen control can be achieved directly and together by applying isoflavonoid phytoalexin inducer compounds to field legumes.

5 Symbiotic Rhizobia as Biopesticides Against Plant Pathogens

Novel findings from field and glasshouse studies have shown that inoculating legumes and non-legume plants with N2-fixing rhizobia can provide protection against pathogens. In common bean the severity of Fusarium root rot was reduced by inoculating it with rhizobia. Similarly, living and heat-killed bacterial cells of Rhizobium leguminosarum provided total protection against pathogen infection of lentil plants [42]. That study further revealed that the culture filtrate and the killed bacterial cells contained signals able to induce plant resistance. However, those signals were suppressed once Rhizobium was in contact with the plant. Mishra et al. [43] showed that in Rhizobium-inoculated rice plants, synthesis of phenolic compounds was consistently more enhanced than in the control, and maximum accumulation of phenolic compounds was observed in plants co-inoculated with Rhizobium leguminosarum bv. phaseoli and Rhizoctonia solani.

Phenolic acids mediate induced systemic resistance and provide bioprotection to plants during pathogenic stresses. In a related study, Khaosaad et al. [44] showed that arbuscular mycorrhizal fungi (AMF) root colonization provides a bioprotectional effect against a broad range of soil-borne fungal pathogens, including take all disease caused by Gaeumannomyces gramminis var. tritici. Infection by AMF also enhances shoot and root growth in wheat plants infected with Gaeumannomyces gramminis var. tritici compared to non AMF colonized diseased plants. Similarly co-inoculation of Rhizobium leguminosarum and arbuscular aycorrhizal fungi conferred resistance to Bothytis fabae in Vicia faba as a result of elevated Na-uptake and phenolics concentration in the bean plant [45].

Rhizobia are major biocontrol agents in natural and agricultural ecosystems. There is evidence that a strain of Bradyrhizobium japonicum can cause up to 75 % decrease in sporulation of Phytophthora megasperma, 65 % in Pythium ultimum, 47 % in Fusarium oxysporum, and 35 % in Ascochyta imperfecta [46]. Antoun et al. [47] identified 49 strains of Sinorhizobium meliloti that inhibited growth of F. oxysporum by up to 50 %. Rhizobia isolated from root nodules of Acacia pulchella similarly decreased the survival of the zoospores of Phytophthora cinnamoni in vitro [48], thus potentially providing bioprotection for the host plant.

Field and glasshouse studies show that inoculating plants with rhizobia can be a cheap and effective method of controlling soil-borne pathogens in cropping systems. For example, inoculating soybean and common bean plants with their respective microsymbionts significantly decreased the severity of Phytophthora and Fusarium root rot in these species [46, 49]. As with most parasitic interactions, the level of root rot decreased with increasing rhizobial numbers in soil [46]. Whether applied as seed dressing or soil drench, different rhizobial strains successfully protected field-grown soybean, mungbean, sunflower, and okra plants from infection by the root-borne pathogens Macrophomina phaseolina, Rhizoctonia solani, and Fusarium species [34]. Although Tu [46] has suggested that rhizobia achieve this bioprotection by parasitizing the hyphal tips of the fungal pathogens and decreasing contact with the host plant cells, other mechanisms may exist. For example, the elicitation of isoflavonoid phytoalexins by rhizobial cells [15, 16] and/or by their nod factors [50] can indirectly control pathogens in legumes. However, it is still unclear whether the same protection can be achieved in non-legume hosts such as the cereals and vegetables in mixed cropping systems.

Similar observations of biocontrol of plant pathogens have been reported for mycorrhizae, the second most important mutualism after the rhizobial symbiosis. Following infection, the AM fungus Glomus mosseae is claimed to confer bioprotection against Phytophthora parasitica in roots of tomato plants [51]. This was shown by the presence of pathogenesis-related proteins in both mycorrhizal and non-mycorrhizal roots, suggesting that the tomato plant probably acquired both localized and systemic resistance against the pathogen. This observation parallels the finding that inoculating legumes with infective rhizobial cells [15, 16], or with their nod factors [50], induces the synthesis, and release of isoflavonoid phytoalexins that confer bioprotection to the plant. Interestingly, some rhizobia are themselves protected from the antimicrobial effects of induced phytoalexins by their nod gene inducers [26, 29]. Thus, pathogens are kept under control as rhizobia infect their host-plant roots. Although no specific studies have been done, it is possible that root hair infection by rhizobial bacteria also induces resistance against various pathogens of the host plant, as observed with G. mosseae [51]. Hopefully, future studies will provide direct evidence for this claim. Biological control of fungal pathogens can therefore be achieved through a range of mechanisms, including mycoparasitism, production of phytoalexins, the induction of host plant defense proteins and peptides, as well as competition for nutrients [52]. Tu [46] suggested parasitisation hyphal tips by rhizobia. Purified nod factors from rhizobia induced the biosythesis and increased exudation of phytoalexins (daidzein, genistein, and coumestrol) from soybean roots [53]. Cyclic β-glucans produced by bradyrhizobia elicit glyceollin, a phytoalexin that controls pathogens in soybean [53]. Inoculating alfalfa with Sinorhizobium induced the release of medicarpin, a classical phytoalexin that control fungal pathogens of the host plant [15]. Similarly inoculating Phaseolus vulgaris bean with Rhizobium leguminosarum bv phaseoli elicited the exudation of coumestrol, daidzein, and genistein, isoflavonoid phytoalexins that control many fungal and bacterial pathogens of beans and other legumes. Bradyrhizobium japonicum secretes rhizobitoxine which inhibits charcoal rot fungus, Macrophomina phaseolina [54].

The pre-treatment of chickpea seedlings with Rhizobium isolates before challenging them with Fusarium spp. significantly increased levels of total phenolics and the levels of constitutive isoflavonoids such as formononetin and biochanin A [55]. It has been suggested that pathogen invasion there is increased the mRNA of the defense gene encoding phenylalanine ammonia lyase (PAL) and leading to the biosythesis of high levels of secondary phenolic metabolites.

Limited data are currently available on flavonoids as signaling compounds for fungal pathogens of legumes. Information about possible effects of flavonoids in root exudates on root infecting/colonizing fungi of non-legume plants remains scanty. Ruan et al. [56] have, however, shown that certain flavonoids, including isoflavonoid phytoalexins, stimulate spore germination of Fusarium solani formae specia-les pathogenic on pea and bean. Thus flavonoids in legume root exudates may be perceived as signals in a number of plant–microbe interactions [56]. For example, Steinkellner et al. [57] have reported that the flavonoid compounds, myricetin and luteolin, exhibited a low stimulating activity on microconidia germination of Fusarium oxysporum f. sp. lycopersici.

6 Mechanisms for Rhizobium Control of Plant Diseases

Many studies have provided both glasshouse and field evidence for biocontrol of plant pathogens by symbiotic rhizobia. These include Phytophthora root rot control by B. japonicum [46, 58], Fusarium root rot by Sinorhizobium melitoti [59], Fusarium root rot, bean bacterial wilt, and Fusarium wilt control by Rhizobium leguminosarum ([42, 49, 60, 61]; see Table 1). However, except for studies by Tu [46, 58], there are no data from microscopy to support rhizobial entry and intercellular/extracellular localization within the host plant cells. Some evidence for rhizobial invasion of host plant cells, especially where the test plants were non-legumes, has been provided by Matiru and Dakora [62]. However, a huge gap still exists in our understanding of possible mechanisms inducing bioprotection with rhizobial inoculation of crop plants.

While it is understood that rhizobial invasion of homologous legume hosts can trigger de novo synthesis of isoflavonoids and/or enhance the production and release of constitutively-present phytoanticipins [63], the same cannot be said of non-legume plants such as sunflower or okra [54]. Even with symbiotic legumes there is evidence of non-rhizobial bacteria present in root nodules. In earlier studies, [64] showed the presence of many non-nodulating agrobacteria-like strains in root nodules of cowpea. [65] also found many non-nodule forming bacteria that cooccupied root nodules of peanut with legitimate peanut rhizobia. Many studies have since shown that cohabitation of legume root nodules by authentic N2-fixing rhizobia and agrobacteria is a common phenomenon in the nodulation world. In fact, we have recently isolated over 700 bacteria from cowpea nodules and many of them have been found to be unable to incite nodulation in both cowpeas and siratro (F. Pule-Meulenburg and F. D. Dakora, unpublished data). The question that then arises is; could rhizobial protection of host plants at least in the case of homologous legumes be caused by induction of phenylpropanoid pathway by illegal bacterial coinhabitant of legume root nodules? The recent report of large numbers of non-rhizobial bacteria isolated from nodules of symbiotic legumes suggest that the so-called bioprotection by rhizobia actually comes from opportunistic rather than the direct effect of nodule forming rhizobia. New studies are needed to answer these questions.

Plant natural products derived from phenylalanine and the phenylpropanoid pathways are impressive in their chemical diversity and are the result of plant evolution, which has selected for the acquisition of large repertoires of pigments, structural, and defensive compounds, all derived from a phenylpropanoid backbone via plant-specific phenylpropanoid pathway [66]. The phenylpropanoid pathway is known to be easily induced by both biotic and abiotic factors [67]. Petunia flavonoids such as kaemferol and its glycosides accumulate with pollen tube development on stigmas [68]. While this could suggest induction of the phenlpropanoid pathway by penentration of the infection thread down legume root hair, no data currently exist on flavonoid formation in response to rhizobial entry into host plant cells. Assuming this was to happen, it would mean that even with “crack entry” by rhizobia [69] internal invasion of host plant cells could trigger the phenylpropanoid pathway leading to the synthesis and/or release of isoflavones that unintetendedly serve in host plant defense. But for the absence of microscopy data on rhizobial localization in root cells of test plants a similar mechanism could be advanced for possible accumulation of flavonoids with rhizobial application as biopesticide to field crops. There is evidence that root tips incur injury during growth in difficult highly compact soils [67]. Such a wounding can also induce the phenylpropanoid pathway leading to tissue accumulation of newly formed isoflavonoids and increase in phytoanticipins for plant defense [67]. A combination of these factors with rhizobial application could provide significant defense against pathogens because of the high levels of host plant phenylpropanoid compounds.

7 Conclusion

Research has shown that in addition to fixing atmospheric nitrogen, rhizobia can have not only a positive influence on plant growth but can also help in its protection. Research on the use of rhizobia as a protectant against plant diseases is scanty and mechanisms for bioprotection are not yet well understood. Future research should reveal these mechanisms for bioprotection by symbiotic rhizobia.