Branching Genes Are Conserved across Species. Genes Controlling a Novel Signal in Pea Are Coregulated by Other Long-Distance Signals

Abstract

Physiological and genetic studies with the ramosus (rms) mutants in garden pea (Pisum sativum) and more axillary shoots (max) mutants in Arabidopsis (Arabidopsis thaliana) have shown that shoot branching is regulated by a network of long-distance signals. Orthologous genes RMS1 and MAX4 control the synthesis of a novel graft-transmissible branching signal that may be a carotenoid derivative and acts as a branching inhibitor. In this study, we demonstrate further conservation of the branching control system by showing that MAX2 and MAX3 are orthologous to RMS4 and RMS5, respectively. This is consistent with the longstanding hypothesis that branching in pea is regulated by a novel long-distance signal produced by RMS1 and RMS5 and that RMS4 is implicated in the response to this signal. We examine RMS5 expression and show that it is more highly expressed relative to RMS1, but under similar transcriptional regulation as RMS1. Further expression studies support the hypothesis that RMS4 functions in shoot and rootstock and participates in the feedback regulation of RMS1 and RMS5 expression. This feedback involves a second novel long-distance signal that is lacking in rms2 mutants. RMS1 and RMS5 are also independently regulated by indole-3-acetic acid. RMS1, rather than RMS5, appears to be a key regulator of the branching inhibitor. This study presents new interactions between RMS genes and provides further evidence toward the ongoing elucidation of a model of axillary bud outgrowth in pea.

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).…”

“…Likewise, RMS5, MAX3, and D17/HTD1 were found to encode CCD7 (Booker et al, 2004;Johnson et al, 2006;Zou et al, 2006). In contrast, the SMS response mutants, max2, rms4, and d3, were found to be mutated in an orthologous gene encoding an F-box protein (Stirnberg et al, 2002;Ishikawa 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).…”