Arabidopsis Mutants by Activation Tagging in which Photosynthesis Genes are Expressed in Dedifferentiated Calli

In an effort to delineate the precise mechanisms underlying the organ-specific expression of photosynthesis genes, Arabidopsis lines homozygous for each transgene construct made with the gene for hygromycin B phosphotransferase or β-glucuronidase (GUS) placed under control of the promoter of the nuclear gene for the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (RBCS-3B) were constructed. Furthermore, activation tagging with T-DNA possessing quadruply repeated enhancers derived from the cauliflower mosaic virus 35S promoter was applied to a transgenic line of Arabidopsis. Mutants resistant to hygromycin B during the growth of calli generated from non-green roots on callus-inducing medium resulted from the expression of hygromycin B phosphotransferase driven by the RBCS-3B promoter. Three mutant lines, ces101 to ces103 (callus expression of RBCS), were obtained from approximately 4,000 calli resistant to a selectable marker for transformation. The active transcription driven by the RBCS-3B promoter in all the calli of ces mutants was confirmed by expression of both the GUS reporter gene and endogenous RBCS-3B. Chlorophyll and carotenoids, as well as light-dependent O2 evolution, have been detected in the calli of all ces mutants. The loci where T-DNA was integrated in the ces101 line were determined by thermal asymmetric interlaced (TAIL)-PCR. The introduction of a DNA fragment harboring the gene for receptor-like kinase placed under the influence of enhancers into the parental line reproduced the phenotype of ces mutants. We have thus concluded that CES101 is a receptor-like kinase. The strategy presented in this investigation may promise to select a greater number of ces mutants.