Regulation of correlative inhibition of axillary bud outgrowth by basal branches varies with growth stage in Trifolium repens.

Roderick G Thomas,Michael J M Hay

doi:10.1093/jxb/erv184

Abstract

In Trifolium repens the decline in bud outgrowth that occurs with distance from basal root systems is due to correlative inhibition by the first-formed basal branches. The apical and axillary buds on these basal branches are the source of the inhibitory effect, but their mode of action is unclear. Inhibition might occur via basal branches being a sink for xylem-transported branching stimulants or alternatively by providing a source of inhibitory signals, or by both mechanisms. To distinguish between these mechanisms, four experiments were conducted on plants varying in initial growth stage from 10 to 19 nodes along their main stems to determine any variation in the relative importance of the operative mechanisms of correlative inhibition. Inhibitory signal exported into the main stem, detected as a branching response to girdling of basal branches, was relatively more significant in smaller (initially with 10-15 nodes on the main stem) than in larger (>19 nodes on main stem) plants. This signal was shown not to involve auxin fluxes, and is unidentified. However, across all stages of growth, the predominant mechanism driving correlative inhibition was the action of axillary and apical buds of basal branches as sinks for the stimulatory signal. This study indicates that the relative importance of the mechanisms regulating bud outgrowth in T. repens varies with growth stage and that, during intermediate stages, regulation has some similarity to that in Pisum.

Highlights

The regulation of axillary bud outgrowth is a major determinant of the development of plant forms adapted to the environmental conditions in which they are growing (Leyser, 2009)
Cytokinin activity is internally modulated as its biosynthesis and degradation are down- and upregulated, respectively, by auxin (Nordström et al, 2004; Tanaka et al, 2006; Werner et al, 2006; Shimizu-Sato et al, 2009) and there is a direct antagonistic effect of strigolactone on cytokinin acting through BRC1 (BRANCHED 1/ TEOSINTE BRANCHED 1-LIKE) at the bud (Dun et al, 2012)
The lack of evidence for the export of inhibitory signals from the basal branches of large highly branched plants of T. repens has led to the suggestion that these branches act largely as sinks for net root-derived stimulus (NRS) (Thomas and Hay, 2011)

Summary

Introduction

The regulation of axillary bud outgrowth is a major determinant of the development of plant forms adapted to the environmental conditions in which they are growing (Leyser, 2009). The mode of action of strigolactone is debated and thought to have its main effect either by directly repressing bud outgrowth (Gomez-Roldan et al, 2008; Brewer et al, 2009; Dun et al, 2012, 2013) or by moderating auxin flux by reducing PIN1 accumulation on the plasma membrane of cells in the polar transport pathway (Crawford et al, 2010; Liang et al, 2010; Shinohara et al, 2013) These inhibitory processes are counterbalanced by the hormone cytokinin, a branching stimulant, which is transported acropetally in the xylem and synthesized in both roots and stem tissues (Sachs and Thimann, 1967; Chen et al, 1985; Nordström et al, 2004; Tanaka et al, 2006). There is a network of interacting shoot- and root-derived signals together with feedback mechanisms that operate as regulators of branching in the erect-stemmed model plant systems in which the inhibitory processes play an important role

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Conclusion