Auxin phytohormone has a role in most aspects of the life of a land plant and is found even in ancient plants such as single-cell green algae. Auxin's ubiquitous but specific effects have been mainly explained by the extraordinary ability of plants to interpret spatiotemporal patterns of auxin concentrations via the regulation of gene transcription. This is thought to be achieved through the combinatorial effects of two families of nuclear coreceptor proteins, that is the TRANSPORT INHIBITOR RESPONSE1 and AUXIN-SIGNALING F-BOX (TIR1/AFB) and AUXIN/INDOLE ACETIC ACID. Recent evidence has suggested transcription-independent roles of TIR1/AFBs localized outside the nucleus and TRANSMEMBRANE KINASE (TMK)-based auxin signaling occurring in the plasma membrane. Furthermore, emerging evidence supports a coordinated action of the intra- and extranuclear auxin signaling pathways to regulate specific auxin responses. Here, we highlight how auxin signaling acts inside and outside the nucleus for the regulation of growth and morphogenesis and propose that the future direction of auxin biology lies in the elucidation of a new collaborative paradigm of intra- and extranuclear auxin signaling.
Keywords: TMK1; TMK4; acid growth theory; cell-surface auxin signaling; nuclear auxin signaling; transcriptional auxin responses.
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