Effects of Elevated Temperature on Pisum sativum Nodule Development: I-Detailed Characteristic of Unusual Apical Senescence

Despite global warming, the influence of heat on symbiotic nodules is scarcely studied. In this study, the effects of heat stress on the functioning of nodules formed by Rhizobium leguminosarum bv. viciae strain 3841 on pea (Pisum sativum) line SGE were analyzed. The influence of elevated temperature was analyzed at histological, ultrastructural, and transcriptional levels. As a result, an unusual apical pattern of nodule senescence was revealed. After five days of exposure, a senescence zone with degraded symbiotic structures was formed in place of the distal nitrogen fixation zone. There was downregulation of various genes, including those associated with the assimilation of fixed nitrogen and leghemoglobin. After nine days, the complete destruction of the nodules was demonstrated. It was shown that nodule recovery was possible after exposure to elevated temperature for 3 days but not after 5 days (which coincides with heat wave duration). At the same time, the exposure of plants to optimal temperature during the night leveled the negative effects. Thus, the study of the effects of elevated temperature on symbiotic nodules using a well-studied pea genotype and Rhizobium strain led to the discovery of a novel positional response of the nodule to heat stress.

Effects of Elevated Temperature on Pisum sativum Nodule Development: II-Phytohormonal Responses

High temperature is one of the most important factors limiting legume productivity. We have previously shown the induction of senescence in the apical part of nodules of the pea SGE line, formed by Rhizobium leguminosarum bv. viciae strain 3841, when they were exposed to elevated temperature (28 °C). In this study, we analyzed the potential involvement of abscisic acid (ABA), ethylene, and gibberellins in apical senescence in pea nodules under elevated temperature. Immunolocalization revealed an increase in ABA and 1-aminocyclopropane-1-carboxylic acid (ACC, the precursor of ethylene biosynthesis) levels in cells of the nitrogen fixation zone in heat-stressed nodules in 1 day of exposure compared to heat-unstressed nodules. Both ABA and ethylene appear to be involved in the earliest responses of nodules to heat stress. A decrease in the gibberellic acid (GA3) level in heat-stressed nodules was observed. Exogenous GA3 treatment induced a delay in the degradation of the nitrogen fixation zone in heat-stressed nodules. At the same time, a decrease in the expression level of many genes associated with nodule senescence, heat shock, and defense responses in pea nodules treated with GA3 at an elevated temperature was detected. Therefore, apical senescence in heat-stressed nodules is regulated by phytohormones in a manner similar to natural senescence. Gibberellins can be considered as negative regulators, while ABA and ethylene can be considered positive regulators.

Effect of Triazole Fungicides Titul Duo and Vintage on the Development of Pea (Pisum sativum L.) Symbiotic Nodules

Triazole fungicides are widely used in agricultural production for plant protection, including pea (Pisum sativum L.). The use of fungicides can negatively affect the legume-Rhizobium symbiosis. In this study, the effects of triazole fungicides Vintage and Titul Duo on nodule formation and, in particular, on nodule morphology, were studied. Both fungicides at the highest concentration decreased the nodule number and dry weight of the roots 20 days after inoculation. Transmission electron microscopy revealed the following ultrastructural changes in nodules: modifications in the cell walls (their clearing and thinning), thickening of the infection thread walls with the formation of outgrowths, accumulation of poly-β-hydroxybutyrates in bacteroids, expansion of the peribacteroid space, and fusion of symbiosomes. Fungicides Vintage and Titul Duo negatively affect the composition of cell walls, leading to a decrease in the activity of synthesis of cellulose microfibrils and an increase in the number of matrix polysaccharides of cell walls. The results obtained coincide well with the data of transcriptomic analysis, which revealed an increase in the expression levels of genes that control cell wall modification and defense reactions. The data obtained indicate the need for further research on the effects of pesticides on the legume-Rhizobium symbiosis in order to optimize their use.

A variable gene family encoding nodule-specific cysteine-rich peptides in pea (Pisum sativum L.)

Various legume plants form root nodules in which symbiotic bacteria (rhizobia) fix atmospheric nitrogen after differentiation into a symbiotic form named bacteroids. In some legume species, bacteroid differentiation is promoted by defensin-like nodule-specific cysteine-rich (NCR) peptides. NCR peptides have best been studied in the model legume Medicago truncatula Gaertn., while in many other legumes relevant information is still fragmentary. Here, we characterize the NCR gene family in pea (Pisum sativum L.) using genomic and transcriptomic data. We found 360 genes encoding NCR peptides that are expressed in nodules. The sequences of pea NCR genes and putative peptides are highly variable and differ significantly from NCR sequences of M. truncatula. Indeed, only one pair of orthologs (PsNCR47-MtNCR312) has been identified. The NCR genes in the pea genome are located in clusters, and the expression patterns of NCR genes from one cluster tend to be similar. These data support the idea of independent evolution of NCR genes by duplication and diversification in related legume species. We also described spatiotemporal expression profiles of NCRs and identified specific transcription factor (TF) binding sites in promoters of "early" and "late" NCR genes. Further, we studied the expression of NCR genes in nodules of Fix^- mutants and predicted potential regulators of NCR gene expression, one among them being the TF ERN1 involved in the early steps of nodule organogenesis. In general, this study contributes to understanding the functions of NCRs in legume nodules and contributes to understanding the diversity and potential antibiotic properties of pea nodule-specific antimicrobial molecules.

Tubulin Cytoskeleton Organization in Cells of Determinate Nodules

Plant cell differentiation is based on rearrangements of the tubulin cytoskeleton; this is also true for symbiotic nodules. Nevertheless, although for indeterminate nodules (with a long-lasting meristem) the organization of microtubules during nodule development has been studied for various species, for determinate ones (with limited meristem activity) such studies are rare. Here, we investigated bacteroid morphology and dynamics of the tubulin cytoskeleton in determinate nodules of four legume species: Glycine max, Glycine soja, Phaseolus vulgaris, and Lotus japonicus. The most pronounced differentiation of bacteroids was observed in G. soja nodules. In meristematic cells in incipient nodules of all analyzed species, the organization of both cortical and endoplasmic microtubules was similar to that described for meristematic cells of indeterminate nodules. In young infected cells in developing nodules of all four species, cortical microtubules formed irregular patterns (microtubules were criss-crossed) and endoplasmic ones were associated with infection threads and infection droplets. Surprisingly, in uninfected cells the patterns of cortical microtubules differed in nodules of G. max and G. soja on the one hand, and P. vulgaris and L. japonicus on the other. The first two species exhibited irregular patterns, while the remaining two exhibited regular ones (microtubules were oriented transversely to the longitudinal axis of cell) that are typical for uninfected cells of indeterminate nodules. In contrast to indeterminate nodules, in mature determinate nodules of all four studied species, cortical microtubules formed a regular pattern in infected cells. Thus, our analysis revealed common patterns of tubulin cytoskeleton in the determinate nodules of four legume species, and species-specific differences were associated with the organization of cortical microtubules in uninfected cells. When compared with indeterminate nodules, the most pronounced differences were associated with the organization of cortical microtubules in nitrogen-fixing infected cells. The revealed differences indicated a possible transition during evolution of infected cells from anisotropic growth in determinate nodules to isodiametric growth in indeterminate nodules. It can be assumed that this transition provided an evolutionary advantage to those legume species with indeterminate nodules, enabling them to host symbiosomes in their infected cells more efficiently.

The Regulation of Pea (Pisum sativum L.) Symbiotic Nodule Infection and Defense Responses by Glutathione, Homoglutathione, and Their Ratio

In this study, the roles of glutathione (GSH), homoglutathione (hGSH), and their ratio in symbiotic nodule development and functioning, as well as in defense responses accompanying ineffective nodulation in pea (Pisum sativum) were investigated. The expression of genes involved in (h)GSH biosynthesis, thiol content, and localization of the reduced form of GSH were analyzed in nodules of wild-type pea plants and mutants sym33-3 (weak allele, "locked" infection threads, occasional bacterial release, and defense reactions) and sym33-2 (strong allele, "locked" infection threads, defense reactions), and sym40-1 (abnormal bacteroids, oxidative stress, early senescence, and defense reactions). The effects of (h)GSH depletion and GSH treatment on nodule number and development were also examined. The GSH:hGSH ratio was found to be higher in nodules than in uninoculated roots in all genotypes analyzed, with the highest value being detected in wild-type nodules. Moreover, it was demonstrated, that a hGSHS-to-GSHS switch in gene expression in nodule tissue occurs only after bacterial release and leads to an increase in the GSH:hGSH ratio. Ineffective nodules showed variable GSH:hGSH ratios that correlated with the stage of nodule development. Changes in the levels of both thiols led to the activation of defense responses in nodules. The application of a (h)GSH biosynthesis inhibitor disrupted the nitrogen fixation zone in wild-type nodules, affected symbiosome formation in sym40-1 mutant nodules, and meristem functioning and infection thread growth in sym33-3 mutant nodules. An increase in the levels of both thiols following GSH treatment promoted both infection and extension of defense responses in sym33-3 nodules, whereas a similar increase in sym40-1 nodules led to the formation of infected cells resembling wild-type nitrogen-fixing cells and the disappearance of an early senescence zone in the base of the nodule. Meanwhile, an increase in hGSH levels in sym40-1 nodules resulting from GSH treatment manifested as a restriction of infection similar to that seen in untreated sym33-3 nodules. These findings indicated that a certain level of thiols is required for proper symbiotic nitrogen fixation and that changes in thiol content or the GSH:hGSH ratio are associated with different abnormalities and defense responses.

General Patterns and Species-Specific Differences in the Organization of the Tubulin Cytoskeleton in Indeterminate Nodules of Three Legumes

The tubulin cytoskeleton plays an important role in establishing legume–rhizobial symbiosis at all stages of its development. Previously, tubulin cytoskeleton organization was studied in detail in the indeterminate nodules of two legume species, Pisum sativum and Medicago truncatula. General as well as species-specific patterns were revealed. To further the understanding of the formation of general and species-specific microtubule patterns in indeterminate nodules, the tubulin cytoskeleton organization was studied in three legume species (Vicia sativa, Galega orientalis, and Cicer arietinum). It is shown that these species differ in the shape and size of rhizobial cells (bacteroids). Immunolocalization of microtubules revealed the universality of cortical and endoplasmic microtubule organization in the meristematic cells, infected cells of the infection zone, and uninfected cells in nodules of the three species. However, there are differences in the endoplasmic microtubule organization in nitrogen-fixing cells among the species, as confirmed by quantitative analysis. It appears that the differences are linked to bacteroid morphology (both shape and size).

Mutational analysis indicates that abnormalities in rhizobial infection and subsequent plant cell and bacteroid differentiation in pea (Pisum sativum) nodules coincide with abnormal cytokinin responses and localization

BACKGROUND AND AIMS

Recent findings indicate that Nod factor signalling is tightly interconnected with phytohormonal regulation that affects the development of nodules. Since the mechanisms of this interaction are still far from understood, here the distribution of cytokinin and auxin in pea (Pisum sativum) nodules was investigated. In addition, the effect of certain mutations blocking rhizobial infection and subsequent plant cell and bacteroid differentiation on cytokinin distribution in nodules was analysed.

METHODS

Patterns of cytokinin and auxin in pea nodules were profiled using both responsive genetic constructs and antibodies.

KEY RESULTS

In wild-type nodules, cytokinins were found in the meristem, infection zone and apical part of the nitrogen fixation zone, whereas auxin localization was restricted to the meristem and peripheral tissues. We found significantly altered cytokinin distribution in sym33 and sym40 pea mutants defective in IPD3/CYCLOPS and EFD transcription factors, respectively. In the sym33 mutants impaired in bacterial accommodation and subsequent nodule differentiation, cytokinin localization was mostly limited to the meristem. In addition, we found significantly decreased expression of LOG1 and A-type RR11 as well as KNOX3 and NIN genes in the sym33 mutants, which correlated with low cellular cytokinin levels. In the sym40 mutant, cytokinins were detected in the nodule infection zone but, in contrast to the wild type, they were absent in infection droplets.

CONCLUSIONS

In conclusion, our findings suggest that enhanced cytokinin accumulation during the late stages of symbiosis development may be associated with bacterial penetration into the plant cells and subsequent plant cell and bacteroid differentiation.

Efficacy of a Plant-Microbe System: Pisum sativum (L.) Cadmium-Tolerant Mutant and Rhizobium leguminosarum Strains, Expressing Pea Metallothionein Genes PsMT1 and PsMT2, for Cadmium Phytoremediation

Two transgenic strains of Rhizobium leguminosarum bv. viciae, 3841-PsMT1 and 3841-PsMT2, were obtained. These strains contain the genetic constructions nifH-PsMT1 and nifH-PsMT2 coding for two pea (Pisum sativum L.) metallothionein genes, PsMT1 and PsMT2, fused with the promoter region of the nifH gene. The ability of both transgenic strains to form nodules on roots of the pea wild-type SGE and the mutant SGECdt, which is characterized by increased tolerance to and accumulation of cadmium (Cd) in plants, was analyzed. Without Cd treatment, the wild type and mutant SGECdt inoculated with R. leguminosarum strains 3841, 3841-PsMT1, or 3841-PsMT2 were similar histologically and in their ultrastructural organization of nodules. Nodules of wild-type SGE inoculated with strain 3841 and exposed to 0.5 μM CdCl2 were characterized by an enlarged senescence zone. It was in stark contrast to Cd-treated nodules of the mutant SGECdt that maintained their proper organization. Cadmium treatment of either wild-type SGE or mutant SGECdt did not cause significant alterations in histological organization of nodules formed by strains 3841-PsMT1 and 3841-PsMT2. Although some abnormalities were observed at the ultrastructural level, they were less pronounced in the nodules of strain 3841-PsMT1 than in those formed by 3841-PsMT2. Both transgenic strains also differed in their effects on pea plant growth and the Cd and nutrient contents in shoots. In our opinion, combination of Cd-tolerant mutant SGECdt and the strains 3841-PsMT1 or 3841-PsMT2 may be used as an original model for study of Cd tolerance mechanisms in legume-rhizobial symbiosis and possibilities for its application in phytoremediation or phytostabilization technologies.