Small RNA in Legumes

Legumes (Fabaceae) are major crops for food and fodder production worldwide. They have the ability to develop nitrogen-fixing root nodules in symbiosis with soil bacteria of the Rhizobium genus. Plant small RNA (sRNA) from 20 to 24 nt, either microRNA (miRNA) or short interfering RNA (siRNA), negatively regulate the expression of specific target genes, at transcriptional or posttranscriptional levels, and can control development, growth, and adaptation to environmental constraints. Since 2007, in silico analysis and deep sequencing of sRNA in legume model and crop species allowed to identify 167 novel miRNA families, in addition to the conserved ones. Although many miRNA differentially accumulate in organs, during symbiosis or in response to abiotic stresses, only few physiological roles could be assigned as yet to these miRNA. In Lotus japonicus, the role of trans-­acting siRNA on auxin-dependent regulation of leaf and flower development was shown. In Medicago truncatula and soybean, several miRNA were shown to act in the regulation of symbiotic interactions. The functions of other legume-specific siRNA, such as stress-responsive natural-antisense siRNA or repeat-associated siRNA involved in heterochromatic silencing, or miRNA remain largely unexplored.