Stone BB, Stowe-Evans EL, Harper RM, Celaya RB, Ljung K, Sandberg G, Liscum E
Disruptions in AUX1-dependent auxin influx alter hypocotyl phototropism in Arabidopsis
Molecular Plant: 2008 1:129-144

Abstract
Phototropism represents a differential growth response by whichplant organs can respond adaptively to changes in the directionof incident light to optimize leaf/stem positioning for photosyntheticlight capture and root growth orientation for water/nutrientacquisition. Studies over the past few years have identifieda number of components in the signaling pathway(s) leading todevelopment of phototropic curvatures in hypocotyls. These includethe phototropin photoreceptors (phot1 and phot2) that perceivedirectional blue-light (BL) cues and then stimulate signaling,leading to relocalization of the plant hormone auxin, as wellas the auxin response factor NPH4/ARF7 that responds to changesin local auxin concentrations to directly mediate expressionof genes likely encoding proteins necessary for developmentof phototropic curvatures. While null mutations in NPH4/ARF7condition an aphototropic response to unidirectional BL, seedlingscarrying the same mutations recover BL-dependent phototropicresponsiveness if co-irradiated with red light (RL) or pre-treatedwith either ethylene. In the present study, we identify second-siteenhancer mutations in the nph4 background that abrogate theserecovery responses. One of these mutations—map1 (modifierof arf7 phenotypes 1)—was found to represent a missenseallele of AUX1—a gene encoding a high-affinity auxin influxcarrier previously associated with a number of root responses.Pharmocological studies and analyses of additional aux1 mutantsconfirmed that AUX1 functions as a modulator of hypocotyl phototropism.Moreover, we have found that the strength of dependence of hypocotylphototropism on AUX1-mediated auxin influx is directly relatedto the auxin responsiveness of the seedling in question.


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