No Time for Transcription—Rapid Auxin Responses in Plants

Abstract

Auxin regulates the transcription of auxin-responsive genes by the TIR1/AFBs-Aux/IAA-ARF signaling pathway, and in this way facilitates plant growth and development. However, rapid, nontranscriptional responses to auxin that cannot be explained by this pathway have been reported. In this review, we focus on several examples of rapid auxin responses: (1) the triggering of changes in plasma membrane potential in various plant species and tissues, (2) inhibition of root growth, which also correlates with membrane potential changes, cytosolic Ca 2+ spikes, and a rise of apoplastic pH, (3) the influence on endomembrane trafficking of PIN proteins and other membrane cargoes, and (4) activation of ROPs (Rho of plants) and their downstream effectors such as the cytoskeleton or vesicle trafficking. In most cases, the signaling pathway triggering the response is poorly understood. A role for the TIR1/AFBs in rapid root growth regulation is emerging, as well as the involvement of transmembrane kinases (TMKs) in the activation of ROPs. We discuss similarities and differences among these rapid responses and focus on their physiological significance, which remains an enigma in most cases.

Genome-Wide Identification and Characterization of PIN-FORMED (PIN) Gene Family Reveals Role in Developmental and Various Stress Conditions in Triticum aestivum L.

“…PINs are plant-specific transmembrane proteins involved in polar auxin transport [ 4 , 25 , 26 , 34 ]. PIN family proteins regulate multiple developmental processes, including morphogenesis, embryogenesis, organogenesis, and environmental stimuli [ 14 , 22 , 34 , 36 , 53 , 69 , 70 ].…”

“…Several studies have demonstrated the role of PIN proteins in numerous developmental processes and in response to environmental stimuli [ 2 , 4 , 6 , 10 , 22 , 34 , 43 ]. Expression profiling of the TaPIN gene family revealed that TaPIN expression was observed in different tissues such as TaPIN6 , TaPIN10 , TaPIN11 , TaPIN14 , TaPIN19 , TaPIN23 , TaPIN26 , TaPIN29 , TaPIN34 , TaPIN39 , TaPIN42 and TaPIN43 , which were elevated in roots ( Figure 8 ).…”

“…In addition, TaPIN31 , TaPIN32 , TaPIN35 , TaPIN40 , and TaPIN44 expression was raised in the grain, while TaPIN5 , TaPIN9 , TaPIN13 , TaPIN21 and TaPIN28 showed higher expression in the stem. The PIN proteins have been implicated in a variety of developmental processes, including leaf venation and vascular development, gravitropism, apical dominance, phototropism, embryo axis formation, and root development [ 4 , 5 , 6 , 8 , 10 , 18 , 26 , 70 ]. Different PIN proteins differentially catalyze auxin transport in the root hair cell of Arabidopsis [ 85 ].…”

“…The phytohormone auxin plays a key role in plant developmental processes [ 1 , 2 , 3 , 4 ]. Auxin forms gradients and concentration maxima in tissues and organs to stimulate diverse biological processes including gravitropism [ 5 , 6 ], organ initiation [ 7 ], leaf venation [ 8 ], apical dominance [ 9 ], embryo axis formation [ 10 ], root architecture [ 11 ], leaf vascular development [ 12 ], tropisms [ 13 , 14 ], fruit ripening [ 15 ], phototropism [ 16 ], phyllotactic patterning [ 17 ], lateral root emergence [ 18 ], root hair growth [ 19 ], apical hook development and root patterning [ 20 ], and sporophyte and male gametophyte development [ 21 , 22 ].…”

“…Auxin forms gradients and concentration maxima in tissues and organs to stimulate diverse biological processes including gravitropism [ 5 , 6 ], organ initiation [ 7 ], leaf venation [ 8 ], apical dominance [ 9 ], embryo axis formation [ 10 ], root architecture [ 11 ], leaf vascular development [ 12 ], tropisms [ 13 , 14 ], fruit ripening [ 15 ], phototropism [ 16 ], phyllotactic patterning [ 17 ], lateral root emergence [ 18 ], root hair growth [ 19 ], apical hook development and root patterning [ 20 ], and sporophyte and male gametophyte development [ 21 , 22 ]. Several researchers have demonstrated that metabolic changes and transport of auxin play a key role in tissue differentiation, embryogenesis, organogenesis, differential growth, and tropic responses [ 2 , 3 , 4 , 23 , 24 , 25 , 26 ]. Auxin is synthesized in various plant tissues by several different pathways [ 27 , 28 ] and subjected to long- and short-range transport mediated by influx and efflux auxin transporters [ 25 , 29 ].…”