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In Journal of Experimental Botany: To branch or not to branch? Shade, ABA and auxin

Should a plant branch or not? It’s a critically important question for survival, and intricate signalling would be expected to underpin the ‘decision’. A recent Journal of Experimental Botany paper from Holalu and Finlayson focusing on the response in Arabidopsis to an increased red to far-red light ratio intriguingly puts the emphasis on abscisic acid (ABA) before stem auxin changes. This groundbreaking research is introduced in an accompanying Insight article in the same issue.

Find a plant and snap off a nice upward-growing shoot. What happens? In most cases we know it will recover, with new, vigorous branching growth from meristems in the leaf axils. Every biologist will have learnt about the apical dominance underlying this process at high school, with Thimann and Skoog’s classic demonstration of the repression of lateral bud outgrowth by auxin. Of course the ‘decision’ to branch needs to be modulated, too, in situations other than recovery from damage, and prominent among them is the plant’s position. Is it in a gap or being shaded? And depending on which it is, should it branch or not?

Shade lowers the red to far-red light ratio, sensed by phytochrome. In Arabidopsis plants in that situation branching is inhibited. Holalu and Finlayson were particularly interested in rapid bud responses to an increased red to far-red light ratio and, based on their known involvement, both ABA and auxin.

Dynamic changes
Their detailed time-course experiments included measurements of bud elongation, ABA and auxin (IAA) abundance, and a whole range of specific gene targets. This led them to conclude that ‘the primary early effects of an increased R:FR on bud growth can be attributed to alterations in bud ABA physiology, whereas later elongation responses may also involve altered systemic auxin physiology.’
The work builds on painstaking research by a number of labs, teasing apart complex physiology. For example, roles are also thought to be played by cytokinin, strigolactone and sugar signalling. The authors are certainly careful to note that other hormonal pathways are also likely to be rapidly modulated by the red to far-red ratio. They also speculate on other aspects, such as clock gating of ABA signalling and the possible location of the primary site of perception and signal transduction.

Read more
Journal of Experimental Botany publishes an exciting mix of research, review and comment on fundamental questions of broad interest in plant science. Regular special issues highlight key areas.

Article source: Jonathan Ingram, Journal of Experimental Botany
Image credit: Pixabay, CC0 Public Domain

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