PHYTOCHROME CONTROLS THE NUMBER OF ENDOREDUPLICATION CYCLES IN THE ARABIDOPSIS-THALIANA HYPOCOTYL

A majority of the cells in the Arabidopsis hypocotyl undergo endoredup lication. The number of endocycles in this organ is partially controll ed by light. Up to two cycles occur in light-grown hypocotyls, whereas in the dark about 30% of the cells go through a third cycle. Is the i nhibition of the third endocycle in the light an indirect result of th e reduced cell size in the light-grown hypocotyl, or is it under indep endent light control? To address this question, the authors examined t he temporal and spacial patterns of endoreduplication in light-or dark -grown plants and report here on the following observations: (i) durin g germination two endocycles take place prior to any significant cell expansion; (ii) in the dark the third cycle is completed very early du ring cell growth; and (iii) a mutation that dramatically reduces cell size does not interfere with the third endocycle. The authors then use d mutants to study the way light controls the third endocycle and foun d that the third endocycle is completely suppressed in far red light t hrough the action of phytochrome A and, to a lesser extent, in red lig ht by phytochrome B. Furthermore, no 16C nuclei were observed in dark- grown constitutive photomorphogenic 1 seedlings. And, finally the hypo cotyl of the cryptochrome mutant, hy4, grown in blue light was about t hree times longer than that of the wild-type without a significant dif ference in ploidy levels. Together the results support the view that t he inhibition of the third endocycle in light-grown hypocotyls is not the consequence of a simple feed-back mechanism coupling the number of cycles to the cell volume, but an integral part of the phytochrome-co ntrolled photomorphogenic program.