No abstract

Legume plants are able to interact symbiotically with soil bacteria to form nitrogen-fixing root nodules. Although specific recognition between rhizobia and legume species has been extensively characterized, plant molecular determinants that govern the preferential colonization by different strains within a single rhizobium species have received little attention. We found that the C subunit of the heterotrimeric nuclear factor NF-Y from common bean (Phaseolus vulgaris) NF-YC1 plays a key role in the improved nodulation seen by more efficient strains of rhizobia. Reduction of NF-YC1 transcript levels by RNA interference (RNAi) in Agrobacterium rhizogenes-induced hairy roots leads to the arrest of nodule development and defects in the infection process with either high or low efficiency strains. Induction of three G2/M transition cell cycle genes in response to rhizobia was impaired or attenuated in NF-YC1 RNAi roots, suggesting that this transcription factor might promote nodule development by activating cortical cell divisions. Furthermore, overexpression of this gene has a positive impact on nodulation efficiency and selection of Rhizobium etli strains that are naturally less efficient and bad competitors. Our findings suggest that this transcription factor might be part of a mechanism that links nodule organogenesis with an early molecular dialogue that selectively discriminates between high-and low-quality symbiotic partners, which holds important implications for optimizing legume performance.

Plants are exposed to different external conditions that affect growth, development, and productivity. Water deficit is one of these adverse conditions caused by drought, salinity, and extreme temperatures. Plants have developed different responses to prevent, ameliorate or repair the damage inflicted by these stressful environments. One of these responses is the activation of a set of genes encoding a group of hydrophilic proteins that typically accumulate to high levels during seed dehydration, at the last stage of embryogenesis, hence named Late Embryogenesis Abundant (LEA) proteins. LEA proteins also accumulate in response to water limitation in vegetative tissues, and have been classified in seven groups based on their amino acid sequence similarity and on the presence of distinctive conserved motifs. These proteins are widely distributed in the plant kingdom, from ferns to angiosperms, suggesting a relevant role in the plant response to this unfavorable environmental condition. In this review, we analyzed the LEA proteins from those legumes whose complete genomes have been sequenced such as Phaseolus vulgaris, Glycine max, Medicago truncatula, Lotus japonicus, Cajanus cajan, and Cicer arietinum. Considering their distinctive motifs, LEA proteins from the different groups were identified, and their sequence analysis allowed the recognition of novel legume specific motifs. Moreover, we compile their transcript accumulation patterns based on publicly available data. In spite of the limited information on these proteins in legumes, the analysis and data compiled here confirm the high correlation between their accumulation and water deficit, reinforcing their functional relevance under this detrimental conditions.

A C subunit of the heterotrimeric nuclear factor Y (NF-YC1) was shown to play a key role in nodule organogenesis and bacterial infection during the nitrogen fixing symbiosis established between common bean (Phaseolus vulgaris) and Rhizobium etli. To identify other proteins involved in this process, we used the yeast (Saccharomyces cerevisiae) two-hybrid system to screen for NF-YC1-interacting proteins. One of the positive clones encodes a member of the Phytochrome A Signal Transduction1 subfamily of GRAS (for Gibberellic Acid-Insensitive (GAI), Repressor of GAI, and Scarecrow) transcription factors. The protein, named Scarecrow-like13 Involved in Nodulation (SIN1), localizes both to the nucleus and the cytoplasm, but in transgenic Nicotiana benthamiana cells, bimolecular fluorescence complementation suggested that the interaction with NF-YC1 takes place predominantly in the nucleus. SIN1 is expressed in aerial and root tissues, with higher levels in roots and nodules. Posttranscriptional gene silencing of SIN1 using RNA interference (RNAi) showed that the product of this gene is involved in lateral root elongation. However, root cell organization, density of lateral roots, and the length of root hairs were not affected by SIN1 RNAi. In addition, the expression of the RNAi of SIN1 led to a marked reduction in the number and size of nodules formed upon inoculation with R. etli and affected the progression of infection threads toward the nodule primordia. Expression of NF-YA1 and the G2/M transition cell cycle genes CYCLIN B and Cell Division Cycle2 was reduced in SIN1 RNAi roots. These data suggest that SIN1 plays a role in lateral root elongation and the establishment of root symbiosis in common bean.One of the main entry points of molecular nitrogen into ecosystems is the mutualistic symbiosis between legume plants and rhizobia, which results in the formation of a new root organ, the nodule. This interaction involves two separated but coordinated processes, the infection that leads bacteria to the interior of the root and the organogenesis of the nodule, where bacteria reduce atmospheric nitrogen to metabolic intermediates that the plant can incorporate (Oldroyd and Downie, 2008). Entry of rhizobia occurs mainly through an invagination of the root hair plasma membrane that expands toward the interior of the root, forming a cylindrical channel where bacteria allocate and proliferate. This penetration structure formed by the plant is called infection thread (IT). At the same time that infection begins, a yet unknown signal triggers the reactivation of cell divisions in the cortex, leading to the development of the nodule primordia. When the IT reaches the dividing cortex, bacteria are released into the cytoplasm in a process similar to endocytosis, producing a new organelle-like structure, the symbiosome, where the enzymatic nitrogen fixation takes place.Establishment of symbiosis is highly specific, involving several recognition steps between both partners. In response to iso/flavonoids exuded by legume roots, rh...