The hypocotyl chloroplast plays a role in phototropic bending of Arabidopsis seedlings: developmental and genetic evidence

Chloroplasts of guard cells and coleoptiles have been implicated in the sen sory transduction of blue light. The present study was aimed at establishin g whether the chloroplast of the hypocotyl from Arabidopsis, another blue l ight-responding organ, has similar characteristics to that of sensory-trans ducing guard cell and coleoptile chloroplasts, Results showed that the phot otropic curvature and arch length induced by blue light in Arabidopsis seed lings matched the distribution of mature chloroplasts in the bending hypoco tyl, The bending arch consistently included the region of the hypocotyl con taining mature chloroplasts, and never extended beyond that region. Manipul ation of the extent of greening of dark-grown hypocotyls by varying red lig ht pretreatments elicited blue light-stimulated curvatures and arch lengths that depended on the duration of the red light pretreatment and on the dis tribution of mature chloroplasts in the hypocotyl, Albino psd2 mutants of A rabidopsis, which lack mature chloroplasts, are devoid of phototropic sensi tivity under conditions in which wild-type seedlings show large curvatures, The star mutant of Arabidopsis has a delayed greening and a delayed photot ropic response as compared with wild type. Measurements of photosynthetic o xygen evolution and carbon fixation, dark respiration, and light-dependent zeaxanthin formation in the hypocotyl showed features similar to those of g uard cells and coleoptiles, and distinctly different from those of mesophyl l tissue. These results indicate that the hypocotyl chloroplast has charact eristics similar to those associated with guard cell and coleoptile chlorop lasts, and that phototropic bending of Arabidopsis hypocotyls appears to re quire mature chloroplasts.