Auxin Information Processing; Partners and Interactions beyond the Usual Suspects

Abstract

Auxin plays a major role in a variety of processes involved in plant developmental patterning and its adaptation to environmental conditions. Therefore, an important question is how specificity in auxin signalling is achieved, that is, how a single signalling molecule can carry so many different types of information. In recent years, many studies on auxin specificity have been published, unravelling increasingly more details on differential auxin sensitivity, expression domains and downstream partners of the auxin receptors (transport inhibitor response 1 (TIR1) and other auxin signaling F-box proteins (AFB)), transcriptional repressors that are degraded in response to auxin (AUX/IAA) and downstream auxin response factors (ARF) that together constitute the plant’s major auxin response pathways. These data are critical to explain how, in the same cells, different auxin levels may trigger different responses, as well as how in different spatial or temporal contexts similar auxin signals converge to different responses. However, these insights do not yet answer more complex questions regarding auxin specificity. As an example, they leave open the question of how similar sized auxin changes at similar locations result in different responses depending on the duration and spatial extent of the fluctuation in auxin levels. Similarly, it leaves unanswered how, in the case of certain tropisms, small differences in signal strength at both sides of a plant organ are converted into an instructive auxin asymmetry that enables a robust tropic response. Finally, it does not explain how, in certain cases, substantially different auxin levels become translated into similar cellular responses, while in other cases similar auxin levels, even when combined with similar auxin response machinery, may trigger different responses. In this review, we illustrate how considering the regulatory networks and contexts in which auxin signalling takes place helps answer these types of fundamental questions.

Specification and regulation of vascular tissue identity in the Arabidopsis embryo

“…Indeed, MP controls a variety of vascular-specific genes and pathways (De Rybel et al, 2013;Donner et al, 2009;Möller et al, 2017;Schlereth et al, 2010;Yoshida et al, 2019). However, all reported perturbations of auxin activity (synthesis, transport and response) that affect vascular development also affect a range of other processes (Bennett et al, 1996;Cheng et al, 2006;Marchant, 1999;van den Berg and ten Tusscher, 2017). It is therefore difficult to separate a role for auxin in the specification of vascular identity from its many other functions, which warrants the use of a simple developmental model system for studying initial vascular tissue specification in the absence of a certain process, such as differentiation.…”

Small Auxin Up RNAs influence the distribution of indole-3-acetic acid and play a potential role in increasing seed size in Euryale ferox Salisb

“…Furthermore, it is increasingly interesting to investigate the role of genes included in IAAsignal transduction pathway in biological process mediated by auxin [16]. Up to present, a large number of IAA-related factors included in IAA-signal transduction pathway have been documented in plants, including the F-box TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX PROTEIN (TIR1/AFB) auxin coreceptors, the AUXIN RESPONSE FACTOR (ARF) transcription factors, and the Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) transcriptional repressors [17,18]. Auxin stimulates the interaction between TIR1/AFB and Aux/IAA proteins and then induces the degradation of Aux/IAA and the release of ARF factors, thereby contributing to the rapid transcription of a set of auxinresponsive genes which implicate in diverse processes in plants, such as embryogenesis and organogenesis [19,20].…”

Small Auxin Up RNAs influence the distribution of indole-3-acetic acid and play a potential role in increasing seed size in Euryale ferox Salisb.

“…Furthermore, it is increasingly interesting to investigate the role of genes included in IAAsignal transduction pathway in biological process mediated by auxin [16]. Up to present, a large number of IAA-related factors included in IAA-signal transduction pathway have been documented in plants, including the F-box TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX PROTEIN (TIR1/AFB) auxin co-receptors, the AUXIN RESPONSE FACTOR (ARF) transcription factors, and the Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) transcriptional repressors [17,18]. Auxin stimulates the interaction between TIR1/AFB and Aux/IAA proteins and then induces the degradation of Aux/IAA and the release of ARF factors, thereby contributing to the rapid transcription of a set of auxin-responsive genes which implicate in diverse processes in plants, such as embryogenesis and organogenesis [19,20].…”