The Branching Gene RAMOSUS1 Mediates Interactions among Two Novel Signals and Auxin in Pea

Abstract

In Pisum sativum, the RAMOSUS genes RMS1, RMS2, and RMS5 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 RMS1 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 RMS1 transcript level, a potentially important mechanism for regulation of shoot branching by IAA. In addition, RMS1 transcript levels are dramatically elevated in rms3, rms4, and rms5 plants, which do not contain elevated IAA levels. This degree of upregulation of RMS1 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 RMS1 transcript levels in rms4 plants. Therefore, the long-distance signaling network controlling branching in pea involves IAA, the RMS1 inhibitor, and an IAA-independent feedback signal. Consistent with physiological studies that predict an interaction between RMS2 and RMS1, rms2 mutations appear to disrupt this IAA-independent regulation of RMS1 expression.

Strigolactone Acts Downstream of Auxin to Regulate Bud Outgrowth in Pea and Arabidopsis

“…In addition to causing bud outgrowth, decapitation has been shown to deplete IAA levels in the main stem (Foo et al, 2005;Morris et al, 2005), which in turn may reduce strigolactone levels (Foo et al, 2005;Johnson et al, 2006). If auxin in the stem inhibits bud outgrowth via strigolactones, then strigolactone application to wild-type buds should prevent their outgrowth regardless of decapitation.…”

“…Auxin has been shown to promote the expression of SMS synthesis genes (Sorefan et al, 2003;Bainbridge et al, 2005;Foo et al, 2005;Johnson et al, 2006;Arite et al, 2007). As a result, branching mutants defective in auxin response may have reduced strigolactone biosynthesis, which may be the cause of their increased branching phenotype.…”

“…Decapitation resulted in reduced IAA levels and decreased expression of SMS synthesis genes (Sorefan et al, 2003;Foo et al, 2005;Johnson et al, 2006). Applying IAA to the decapitated plants rescued the drop in gene expression (Sorefan et al, 2003;Foo et al, 2005;Johnson et al, 2006).…”

“…Auxin is known to promote the expression of SMS synthesis genes (Sorefan et al, 2003;Bainbridge et al, 2005;Foo et al, 2005;Johnson et al, 2006;Arite et al, 2007). Decapitation resulted in reduced IAA levels and decreased expression of SMS synthesis genes (Sorefan et al, 2003;Foo et al, 2005;Johnson et al, 2006).…”

Strigolactones fine-tune the root system

“…Auxin application was also shown to promote the expression of strigolactone synthesis genes in plant stems (Foo et al 2005;Hayward et al 2009), suggesting that auxin should reduce adventitious rooting. However, auxin-induced strigolactone signaling is not the key process regulating adventitious rooting, because both auxin and strigolactone mutants respond to strigolactone and auxin treatments, respectively (Rasmussen et al 2012).…”