Auxin and strigolactone signaling are required for modulation of Arabidopsis shoot branching by nitrogen supply.

Maaike De Jong,Hayley Mcculloch,Barbara Willetts,Veronica Ongaro,Karin Ljung,Ottoline Leyser,Lisa Williamson,Gilu George

doi:10.1104/pp.114.242388

Abstract

The degree of shoot branching is strongly affected by environmental conditions, such as nutrient availability. Here we demonstrate that nitrate limitation reduces shoot branching in Arabidopsis (Arabidopsis thaliana) both by delaying axillary bud activation and by attenuating the basipetal sequence of bud activation that is triggered following floral transition. Ammonium supply has similar effects, suggesting that they are caused by plant nitrogen (N) status, rather than direct nitrate signaling. We identify increased auxin export from active shoot apices, resulting in increased auxin in the polar auxin transport stream of the main stem, as a likely cause for the suppression of basal branches. Consistent with this idea, in the auxin response mutant axr1 and the strigolactone biosynthesis mutant more axillary growth1, increased retention of basal branches on low N is associated with a failure to increase auxin in the main stem. The complex interactions between the hormones that regulate branching make it difficult to rule out other mechanisms of N action, such as up-regulation of strigolactone synthesis. However, the proposed increase in auxin export from active buds can also explain how reduced shoot branching is achieved without compromising root growth, leading to the characteristic shift in relative biomass allocation to the root when N is limiting.

Highlights

Plants continuously adjust their development to suit the environmental conditions in which they are growing
There has been considerable progress in understanding how local NO32-regulated changes in the root system are effected, but much less is known about how NO32 availability regulates shoot branching and how this is coordinated with the root
Among six experiments examining the effects of NO32 supply or resupply after NO32 deprivation (Wang et al, 2003; Scheible et al, 2004; Gifford et al, 2008, 2013; Patterson et al, 2010; Krapp et al, 2011), we found no evidence of a consistent effect of N supply on the transcription of any of these genes (Supplemental Table S1)

Summary

Introduction

Plants continuously adjust their development to suit the environmental conditions in which they are growing. A patch of high NO32 can stimulate local proliferation of lateral roots into the patch and can suppress root growth outside the patch (Drew, 1975; Scheible et al, 1997; Zhang and Forde, 1998; Zhang et al, 1999; Linkohr et al, 2002) Some of these responses, such as the proliferation of roots into a high-NO32 patch, are the result of direct local NO32 signaling, but there is clear evidence for systemic nitrogen (N) status effects. There is ample evidence that hormones regulate bud activity, and these same hormones are excellent candidates to act as systemic coordinators of nutrient signaling (Leyser, 2009) This suggests the hypothesis that NO32-triggered changes in shoot system architecture are mediated by changes in hormone activity. Two additional systemically mobile hormones have been strongly implicated in shoot branching control, namely auxin and strigolactones (SLs), both of which are generally considered to inhibit bud growth (for review, see Dun et al, 2009; Domagalska and Leyser, 2011)

Methods

Results

Discussion

Conclusion