The branching gene RAMOSUS1 mediates interactions among two novel signals and auxin in pea

In <em>Pisum sativum</em>, the <em>RAMOSUS</em> genes <em>RMS1</em>, <em>RMS2</em>, and <em>RMS5</em> regulate shoot branching via physiologically defined mobile signals. RMS1 is most likely a carotenoid cleavage enzyme and acts with RMS5 to control levels of an as yet unidentified mobile branching inhibitor required for auxin inhibition of branching. Our work provides molecular, genetic, and physiological evidence that <em>RMS1</em> plays a central role in a shoot-to-root-to-shoot feedback system that regulates shoot branching in pea. Indole-3-acetic acid (IAA) positively regulates <em>RMS1</em> transcript level, a potentially important mechanism for regulation of shoot branching by IAA. In addition, <em>RMS1</em> transcript levels are dramatically elevated in <em>rms3</em>, <em>rms4</em>, and <em>rms5</em> plants, which do not contain elevated IAA levels. This degree of upregulation of <em>RMS1</em> expression cannot be achieved in wild-type plants by exogenous IAA application. Grafting studies indicate that an IAA-independent mobile feedback signal contributes to the elevated <em>RMS1</em> transcript levels in <em>rms4</em> plants. Therefore, the long-distance signaling network controlling branching in pea involves IAA, the <em>RMS1</em> inhibitor, and an IAA-independent feedback signal. Consistent with physiological studies that predict an interaction between <em>RMS2</em> and <em>RMS1</em>, <em>rms2</em> mutations appear to disrupt this IAA-independent regulation of <em>RMS1</em> expression.