The AT-hook-containing proteins SOB3/AHL29 and ESC/AHL27 are negative modulators of hypocotyl growth in Arabidopsis.

SOB3, which encodes a plant-specific AT-hook motif containing protein, was identified from an activation-tagging screen for suppressors of the long-hypocotyl phenotype of a weak phyB allele, phyB-4. sob3-D (suppressor of phyB-4#3 dominant) overexpressing seedlings have shorter hypocotyls, and as adults develop larger flowers and leaves, and are delayed in senescence compared with wild-type plants. At the nucleotide level, SOB3 is closely related to ESCAROLA (ESC), which was identified in an independent activation-tagging screen. ESC overexpression also suppresses the phyB-4 long-hypocotyl phenotype, and confers an adult morphology similar to sob3-D, suggesting similar functions. Analysis of transgenic plants harboring SOB3:SOB3-GUS or ESC:ESC-GUS translational fusions, driven by their endogenous promoter regions, showed GUS activity in the hypocotyl and vasculature tissue in light- and dark-grown seedlings. A loss-of-function SOB3 allele (sob3-4) was generated through an ethyl methanesulfonate intragenic suppressor screen of sob3-D phyB-4 plants, and this allele was combined with a predicted null allele, disrupting ESC (esc-8), to examine potential genetic interactions. The sob3-4 esc-8 double mutant had a long hypocotyl in multiple fluence rates of continuous white, far-red, red and blue light. sob3-4 esc-8 phyB-9 and sob3-4 esc-8 cry-103 triple mutants also had longer hypocotyls than photoreceptor single mutants. In contrast, the sob3-4 esc-8 phyA-211 triple mutant was the same length as phyA-211 single mutants. Taken together, these data indicate that SOB3 and ESC act redundantly to modulate hypocotyl growth inhibition in response to light.

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