Up in the air: Untethered Factors of Auxin Response

Abstract

As a prominent regulator of plant growth and development, the hormone auxin plays an essential role in controlling cell division and expansion. Auxin-responsive gene transcription is mediated through the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX (TIR1/AFB) pathway. Roles for TIR1/AFB pathway components in auxin response are understood best, but additional factors implicated in auxin responses require more study. The function of these factors, including S-Phase Kinase-Associated Protein 2A (SKP2A), SMALL AUXIN UP RNAs (SAURs), INDOLE 3-BUTYRIC ACID RESPONSE5 (IBR5), and AUXIN BINDING PROTEIN1 (ABP1), has remained largely obscure. Recent advances have begun to clarify roles for these factors in auxin response while also raising additional questions to be answered.

Keywords: auxin; cell division; gene.

Figure 1.. Auxin signal transduction pathways.

( A) Model of the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX (TIR1/AFB) signaling pathway. Auxin promotes the formation of the TIR1/AFB Auxin/INDOLE-3-ACETIC ACID INDUCIBLE (Aux/IAA) co-receptor to promote the ubiquitylation and subsequent degradation of the Aux/IAA repressor. Aux/IAA degradation relieves repression of AUXIN RESPONSE FACTOR (ARF) transcription factors, allowing for auxin-responsive gene expression. One of the transcript families upregulated by auxin is the SAUR family. The small SMALL AUXIN UP RNA (SAUR) proteins encoded by these transcripts have been suggested to play roles in multiple processes, one of which is interaction with and inhibition of members of the PP2C.D family of phosphatases, which act to regulate H ⁺-ATPase activity. Further, INDOLE-3-BUTYRIC ACID RESPONSE5 (IBR5) and MITOGEN-ACTIVATED PROTEIN KINASE12 (MPK12) have been implicated in regulating auxin-responsive gene transcription; this regulation is not through destabilization of the Aux/IAA repressors, suggesting a yet-to-be discovered mechanism of regulating auxin-responsive gene expression. For in-depth reviews of the TIR/AFB signaling pathway, please refer to , . For an in-depth review of SAUR proteins, please refer to . ( B) Model of the S-PHASE KINASE ASSOCIATED PROTEIN 2A (SKP2A) signaling pathway. E2FC/DPB repress expression of cell cycle genes. The F-box protein SKP2A binds auxin and promotes degradation of E2FC/DPB in an auxin-dependent manner. Degradation of E2FC/DPB relieves repression of cell cycle genes and allows for binding by activating E2F+/DP+ complexes. For an in-depth review of the SKP2A signaling pathway, please refer to . ( C) Model of the putative AUXIN BINDING PROTEIN1 (ABP1) signaling pathway. Apoplastic auxin is bound by ABP1, which allows for interaction with the TRANSMEMBRANE KINASE RECEPTOR (TMK) family of leucine-rich repeat receptor-like kinases. ABP1 binding of auxin regulates RHO-LIKE GTPASE 2 (ROP2) and ROP6 activation and binding to ROP interactive CRIB motif-containing proteins (RICs) proteins to positively regulate microtubule polymerization and F-actin polymerization and also alter PINFORMED1 protein localization to alter auxin efflux. For an in-depth review of the ROP/RIC system, please refer to .