The chloroplast and leaf developmental mutant, pale cress, exhibits light-conditional severity and symptoms characteristic of its ABA deficiency

The PALE CRESS gene (PAC) is essential for proper chloroplast and leaf deve lopment in Arabidopsis thaliana. The ability of pac mutants to accumulate s ignificantly more chlorophyll when grown in low light conditions than in hi gh light conditions suggests that carotenoid deficiency is at least partly responsible for premature cessation of chloroplast development. In addition to accumulation of low levels of chlorophyll and carotenoid pigments, pac mutants are abscisic acid (ABA) deficient and have characteristics which ma y be explained by this deficiency. These include reduced seed viability and , in enclosed growth conditions, increased leaf growth. Plants transformed with an antisense PAC construct often bear viviparous embryos which may be symptomatic of a deficiency in ABA. Since carotenoids are precursors of ABA , a role for PAC in carotenoid biosynthesis is further supported. The nucle ar-encoded. chloroplast-localized PAC protein has been implicated in the ma turation of plastid-encoded mRNAs. Thus, PAC may affect the abundance of on e or more chloroplast proteins which function in the synthesis or stability of carotenoids. Using the PROLIFERA gene as a marker for cell division, it is shown that cell division profiles in the pac shoot apex are disrupted. pac leaves are relatively normal in size and shape despite the light intens ity-induced variability of leaf cell defects, (C) 2000 Annals of Botany Com pany.