Auxin as compère in plant hormone crosstalk

Abstract

The architecture of many hormone perceptions and signalling pathways has been recently well established, together with an awareness that plant hormone responses are the product of networks of interactions involving multiple hormones. As growth is quantitative, so are hormone responses, which underlie a systems approach to development and response. Auxin is arguably one of the best characterised hormones in plant development, and despite many excellent reviews on auxin perception, polar transport, and signal transduction, too little attention has been given to auxin crosstalk. This review, therefore, gives a précis of recent developments in hormone crosstalk involving auxin. For decades, the literature has described the involvement of multiple hormones in particular processes, although the mechanistic bases underlying points of crosstalk have been harder to pinpoint. Crosstalk falls into different categories, such as direct, indirect, or co-regulation. One conclusion for auxin crosstalk is that crosstalk operates extensively via the metabolism of other hormones, however, microarray approaches are increasingly identifying co-regulated genes and nodes of crosstalk at shared signalling components. Auxin crosstalk is often local, and is spatially and temporally regulated to provide adaptive value to environmental conditions and fine-tuning of responses.

Auxin-Induced Modulation of ETTIN Activity Orchestrates Gene Expression in Arabidopsis

“…Notably, while ETT appears to repress most of its auxin signaling-related targets (targets upregulated in ett-3), ETT emerged as a positive regulator of most of its ethyleneand JA-related targets (reduced in ett-3). These data agree with the proposed role of auxin in promoting both ethylene and JA synthesis (Muday et al, 2012;Chandler, 2009).…”

Dissecting the role of Arabidopsis thaliana AMIDASE 1 in auxin biosynthesis

“…Plant hormone crosstalk can be classified as: direct, indirect and co--regulation types (Chandler 2009a;Hoffmann, Hentrich, and Pollmann 2011). Direct crosstalk is produced when different hormones are affected by the same stimulus, triggering a common response, in a combinatory way over the same promoters or target proteins.…”

“…the effect of cytokinins vs auxins; gibberellic acid vs. abscisic acid). Eventually, synergism is the enhanced response caused via two or more hormones, regulating common target genes, as for instance, the synergy of auxins and BRs (Chandler 2009a). …”

“…In this case, both hormones have usually an opposite action. For instance, CK and IAA have a role on "lateral bud outgrowth" but, where CK promote this developmental process, auxin has an inhibitory effect (Chandler 2009a;Schaller, Bishopp, and Kieber 2015).…”

“…It seems that auxin stimulates the activity of 1--AMINOCYCLOPROPANE--1--CARBOXYLIC ACID (ACC) SYNTHASES (ACSs), in particular by inducing the expression of three specific ACS genes: ACS6, ACS8, and ACS11. The corresponding gene products catalyse the rate--limiting step of ethylene biosynthesis, converting S--ADENOSYL METHIONINE (S--AdoMet) to ACC (Chandler 2009a;Paponov et al 2008;Wang, Li, and Ecker 2002) (Figure 1.1). Thus, alterations in ethylene production can be correlated with changes in endogenous auxin levels, although the fact that auxin stimulates ethylene biosynthesis does not make the presence of auxin a requirement for the production of ET.…”