Abstract
Transcription factors of the Nuclear Factor Y (NF-Y) family play essential functions in plant development and plasticity, including the formation of lateral root organs such as lateral root and symbiotic nodules. NF-Ys mediate transcriptional responses by acting as heterotrimers composed of three subunits, NF-YA, NF-YB, and NF-YC, which in plants are encoded by relatively large gene families. We have previously shown that, in the Phaseolus vulgaris × Rhizobium etli interaction, the PvNF-YC1 subunit is involved not only in the formation of symbiotic nodules, but also in the preference exhibited by the plant for rhizobial strains that are more efficient and competitive in nodule formation. PvNF-YC1 forms a heterotrimer with the PvNF-YA1 and PvNF-YB7 subunits. Here, we used promoter:reporter fusions to show that both PvNF-YA1 and PvNF-YB7 are expressed in symbiotic nodules. In addition, we report that knock-down of PvNF-YA1 and its close paralog PvNF-YA9 abolished nodule formation by either high or low efficient strains and arrested rhizobial infection. On the other hand, knock-down of PvNF-YB7 only affected the symbiotic outcome of the high efficient interaction, suggesting that other symbiotic NF-YB subunits might be involved in the more general mechanisms of nodule formation. More important, we present functional evidence supporting that both PvNF-YA1 and PvNF-YB7 are part of the mechanisms that allow P. vulgaris plants to discriminate and select those bacterial strains that perform better in nodule formation, most likely by acting in the same heterotrimeric complex that PvNF-YC1.
Highlights
Nitrogen (N) is an essential macronutrient for plant growth and development since it is part of many biological molecules such as nucleic acids, proteins, vitamins, and chlorophyll (Wang et al, 2012); its availability is frequently limited in soils of both natural and agronomical ecosystems
Since the individual subunits of the complex are encoded by relatively large gene families in plant genomes, it is crucial to elucidate the composition of the heterotrimers that would be acting in specific tissues or during the activation of morphogenetic programs, such as rhizobial infection and nodule organogenesis, which will largely rely on the tissue-specific expression pattern of individual members of these gene families
The use of promoter:GFPGUS constructs revealed that the promoters of both PvNFYA1 and Pv-Nod factors (NFs)-YB7 are active in infected root hairs, as well as in the central tissue of N-fixing nodules developed by the strain of R. etli carrying the nodC-α allele
Summary
Nitrogen (N) is an essential macronutrient for plant growth and development since it is part of many biological molecules such as nucleic acids, proteins, vitamins, and chlorophyll (Wang et al, 2012); its availability is frequently limited in soils of both natural and agronomical ecosystems. Most legume species overcome N limitation by establishing a symbiosis with N-fixing bacteria from different genera known as rhizobia. This interaction results in the formation of a new postembryonic root organ, the nodule, where bacteria allocate and convert atmospheric N to reduced forms that will be incorporated into the plant metabolism. The second stage is the infection, which consists in the penetration of bacteria into host tissues through a tubular structure referred to as the infection thread (IT) With this infection process, cell divisions are initiated in the root cortex of the host to form the nodular primordium. The third and last stage consists in the development of the nodule and the release of bacteria from the ITs into the host cells to form organelle-like structures called symbiosomes (Popp and Ott, 2011), where biological N fixation will take place
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