Transcriptomics-Assisted Discovery of Hormone Regulation during Nodule Development in Soybean

Lateral roots (LRs) and nodules are two important nutrient acquisition organs formed during root development in legumes such as soybean. These organs are developmentally unique in that they arise from differentiated cells whereas lateral organs in the shoot arise from founder cells in the meristem. LRs are initiated in response to developmental and environmental cues while nodules arise due to coordinated interactions between the plant and nitrogen-fixing rhizobia bacteria. LRs arise from a few initial pericycle cells adjacent to the xylem pole while nodules arise from cortex cells in proximity to the xylem pole. Both organs undergo a defined program of cell division and expansion forming the primordium. While all higher plants produce LRs, nodules are restricted largely to leguminous plants. Parallels observed in the sequence of morphological events in the initiation of these organs led to the speculation that nodules might have evolved from LRs. However, the current body of knowledge indicates that LRs and nodules have distinct hormonal requirements during their development. Unfortunately, components of hormonal and developmental signaling pathways distinct between these root lateral organs are not known. We dissected two distinct stages of LRs and nodules and compared their gene expression profiles. Our results reiterated that auxin and cytokinin play opposite roles during the development of LRs and nodules. We also identify organ-specific biosynthesis, catabolism and signaling components for all major hormones. In addition, we noticed significant differences in the expression patterns of major transcription factor families associated with cell differentiation between these organs.