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

Abstract

During the last century, two key hypotheses have been proposed to explain apical dominance in plants: auxin promotes the production of a second messenger that moves up into buds to repress their outgrowth, and auxin saturation in the stem inhibits auxin transport from buds, thereby inhibiting bud outgrowth. The recent discovery of strigolactone as the novel shootbranching inhibitor allowed us to test its mode of action in relation to these hypotheses. We found that exogenously applied strigolactone inhibited bud outgrowth in pea (Pisum sativum) even when auxin was depleted after decapitation. We also found that strigolactone application reduced branching in Arabidopsis (Arabidopsis thaliana) auxin response mutants, suggesting that auxin may act through strigolactones to facilitate apical dominance. Moreover, strigolactone application to tiny buds of mutant or decapitated pea plants rapidly stopped outgrowth, in contrast to applying N-1-naphthylphthalamic acid (NPA), an auxin transport inhibitor, which significantly slowed growth only after several days. Whereas strigolactone or NPA applied to growing buds reduced bud length, only NPA blocked auxin transport in the bud. Wild-type and strigolactone biosynthesis mutant pea and Arabidopsis shoots were capable of instantly transporting additional amounts of auxin in excess of endogenous levels, contrary to predictions of auxin transport models. These data suggest that strigolactone does not act primarily by affecting auxin transport from buds. Rather, the primary repressor of bud outgrowth appears to be the auxindependent production of strigolactones.

Strigolactones fine-tune the root system

“…Strigolactones affect auxin fluxes (see above) (Bennett et al 2006;Brewer et al 2009;Crawford et al 2010;Ruyter-Spira et al 2011;Shinohara et al 2013), so it was expected that strigolactones may also have an effect on lateral root formation. Thus far, two reports describe a negative influence of strigolactones on lateral root density in a MAX2-dependent manner (Fig.…”

“…In the shoot there are clear links between strigolactone and auxin (Bennett et al 2006;Brewer et al 2009;Crawford et al 2010;Ruyter-Spira et al 2011;Shinohara et al 2013). In particular, it seems likely that strigolactones regulate local auxin levels.…”

Strigolactones as mediators of plant growth responses to environmental conditions

“…7 This is especially important since SLs application to growing buds were shown to reduced their growth, with no effect on auxin transport from the bud. 8 …”

A petunia ABC protein controls strigolactone-dependent symbiotic signalling and branching

“…This idea is compatible with both current models of strigolactone-dependent branching control 27 . According to the 'second messenger model', strigolactones are transported to the bud as a second messenger of auxin 28 , hence cellular transport of strigolactones in the axillary regions would be indispensable. In the 'auxin transport canalization-based model', strigolactones are thought to dampen polar auxin transport, resulting in the accumulation of auxin to levels that inhibit bud outgrowth 29 .…”