Belin C, Megies C, Hauserova E. Lopez-Molina L
Abscisic acid represses growth of the Arabidopsis embryonic axis after germination by enhancing Auxin signaling
The Plant Cell: 2009 21:2253-2268

Abstract
Under unfavorable environmental conditions, the stress phytohormoneABA inhibits the developmental transition from an embryo ina dry seed into a young seedling. We developed a genetic screento isolate Arabidopsis thaliana mutants whose early seedlingdevelopment is resistant to ABA. Here, we report the identificationof a recessive mutation in AUXIN RESISTANT1 (AUX1), encodinga cellular auxin influx carrier. Although auxin is a major morphogenesishormone in plants, little is known about ABA–auxin interactionsduring early seedling growth. We show that aux1 and pin2 mutantsare insensitive to ABA-dependent repression of embryonic axis(hypocotyl and radicle) elongation. Genetic and physiologicalexperiments show that this involves auxin transport to the embryonicaxis elongation zone, where ABA enhances the activity of anauxin-responsive promoter. We propose that ABA represses embryonicaxis elongation by potentiating auxin signaling in its elongationzone. This involves repression of the AUXIN INDUCIBLE (Aux/IAA)gene AXR2/IAA7, encoding a key component of ABA- and auxin-dependentresponses during postgerminative growth.

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