Auxin signaling - PubMed (original) (raw)

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Auxin signaling

Marcel Quint et al. Curr Opin Plant Biol. 2006 Oct.

Abstract

Auxin regulates a host of plant developmental and physiological processes, including embryogenesis, vascular differentiation, organogenesis, tropic growth, and root and shoot architecture. Genetic and biochemical studies carried out over the past decade have revealed that much of this regulation involves the SCF(TIR1/AFB)-mediated proteolysis of the Aux/IAA family of transcriptional regulators. With the recent finding that the TRANSPORT INHIBITOR RESPONSE1 (TIR1)/AUXIN SIGNALING F-BOX (AFB) proteins also function as auxin receptors, a potentially complete, and surprisingly simple, signaling pathway from perception to transcriptional response is now before us. However, understanding how this seemingly simple pathway controls the myriad of specific auxin responses remains a daunting challenge, and compelling evidence exists for SCF(TIR1/AFB)-independent auxin signaling pathways.

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Auxin regulation of gene expression. (a) Under sub-threshold auxin concentrations the Aux/IAA proteins heterodimerize with the ARF transcription factors, thereby repressing auxin-inducible gene expression. (b) The active SCFTIR1 complex, containing RUB-modified CUL1, is shown at the bottom. The CSN complex can cleave the RUB modifier from CUL1, thus facilitating CAND1 binding to CUL1 and SCF disassembly. Conjugation of RUB to CUL1 by the AXR1-ECR1 and RCE1 enzymes might free CUL1 from CAND1, promoting re-assembly of the active complex. Genetic studies have shown that all of the components depicted in this figure are required for optimal SCFTIR1 activity in vivo. (c) Auxin binding to the TIR1/AFB receptors promotes the recruitment of Aux/IAA proteins to the SCF complex. Subsequent Aux/IAA ubiquitinylation and proteasome-mediated degradation results in a decline in Aux/IAA protein levels, thus de-repressing auxin-inducible gene expression. In addition to genes responsible for specific auxin responses, the Aux/IAA genes themselves are auxin-inducible. This might represent a negative feedback loop that ensures a transient response, with the nascent Aux/IAA proteins attenuating the signaling pathway as auxin levels fall by restoring repression of the ARF transcription factors. DBD, DNA-binding domain; E1, ubiquitin-activating enzyme; E2, ubiquitin-conjugating enzyme; U, ubiquitin; R, RUB. *,AFB1, AFB2, or AFB3.

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References

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