Disruptions in AUX1-Dependent Auxin Influx Alter Hypocotyl Phototropism in Arabidopsis

Abstract

Phototropism represents a differential growth response by which plant organs can respond adaptively to changes in the direction of incident light to optimize leaf/stem positioning for photosynthetic light capture and root growth orientation for water/nutrient acquisition. Studies over the past few years have identified a number of components in the signaling pathway(s) leading to development of phototropic curvatures in hypocotyls. These include the phototropin photoreceptors (phot1 and phot2) that perceive directional blue-light (BL) cues and then stimulate signaling, leading to relocalization of the plant hormone auxin, as well as the auxin response factor NPH4/ARF7 that responds to changes in local auxin concentrations to directly mediate expression of genes likely encoding proteins necessary for development of phototropic curvatures. While null mutations in NPH4/ARF7 condition an aphototropic response to unidirectional BL, seedlings carrying the same mutations recover BL-dependent phototropic responsiveness if co-irradiated with red light (RL) or pre-treated with either ethylene. In the present study, we identify second-site enhancer mutations in the nph4 background that abrogate these recovery responses. One of these mutations--map1 (modifier of arf7 phenotypes 1)--was found to represent a missense allele of AUX1--a gene encoding a high-affinity auxin influx carrier previously associated with a number of root responses. Pharmacological studies and analyses of additional aux1 mutants confirmed that AUX1 functions as a modulator of hypocotyl phototropism. Moreover, we have found that the strength of dependence of hypocotyl phototropism on AUX1-mediated auxin influx is directly related to the auxin responsiveness of the seedling in question.

Auxin transport routes in plant development

“…The mechanisms that generate auxin asymmetry in response to light remain unclear, but studies with mutants or inhibitors show that phototropism also requires the activity of all auxin transport components, such as PIN3 (Friml et al, 2002b), AUX1 (Stone et al, 2008), ABCB1 (Lin and Wang, 2005) and ABCB19 (Lin and Wang, 2005;Nagashima et al, 2008a;Nagashima et al, 2008b;Noh et al, 2003).…”

Nuclear Phytochrome A Signaling Promotes Phototropism in Arabidopsis

“…In Oryza sativa (rice), it has been shown that CP1, the ortholog of NPH3, acts upstream of auxin redistribution in the coleoptile (Haga et al, 2005). The importance of auxin transport and signaling for phototropism has been confirmed genetically (Tatematsu et al, 2004;Stone et al, 2008;Mö ller et al, 2010;Christie et al, 2011;Ding et al, 2011).…”

Shedding light on auxin movement: Light-regulation of polar auxin transport in the photocontrol of plant development

“…28 Moreover, a requirement of PIN2 ubiquitylation for its vacuolar targeting and degradation in the dark has also been demonstrated. 32 This would suggest that PIN2 might be a direct target of the COP1 E3 ubiquitin ligase, although this remains to be demonstrated. Other auxin transporters, such as PIN1, PIN3 and PIN7 are also targeted to the vacuole in dark-grown RAM, but genetic analyses suggest that only PIN1 and PIN2 are essential for the regulation of root growth in response to light.…”

“…31 Genetic evidence indicated that other auxin transporters, such as PIN7, AUX1 and ABCB19, may also participate in phototropic responses although their precise role is not yet clearly defined. [31][32][33] Light does not regulate auxin transport only in the aboveground tissues. Indeed, auxin transport in the root apex is also influenced by the light environment, despite the fact that roots are not directly exposed to light.…”