Nitrate inhibition of nodule formation in Medicago truncatula is mediated by ACC SYNTHASE 10
Legumes form a mutualistic endosymbiosis with nitrogen-fixing rhizobia. These rhizobia are housed intracellularly in specialised lateral root organs, called nodules. Initiation of these nodules is triggered by bacterial derived signalling molecules, lipochitooligosaccharides (LCO). The process of nitrogen fixation is highly energy-demanding and therefore nodule initiation is tightly regulated. Nitrate is a potent inhibitor of nodulation. However, the precise mechanisms by which nitrate inhibits nodulation is poorly understood. Here, we demonstrate that in Medicago truncatula nitrate interferes with the transcriptional regulation of the ethylene biosynthesis gene ACC SYNTHASE 10. ACSs commit the rate limiting step in ethylene biosynthesis and in M. truncatula ACS10 is highly expressed in the zone of the root where nodulation occurs. Our results show that a reduction in ACS10 expression in response to LCO exposure correlates with the ability to form nodules. In addition, RNAi-mediated knockdown of ACS10 confers nodulation ability under otherwise inhibitory nitrate conditions. This discovery sheds new light on how ethylene is involved in the inhibition of nodulation by nitrate, bringing us one step closer to understanding how plants regulate their susceptibility towards rhizobia.