A virescent gene V₁ determines the expression timing of plastid genes for transcription/translation apparatus during early leaf development in rice

Differentiation of proplastids into functionally active chloroplasts is one of the most significant changes in cellular organization associated with leaf development in higher plants. This process involves activation of a large number of nuclear and chloroplast genes. A central question, therefore, concerns the nature and origin of the signals that initiate and control this process. The rice nuclear mutant, virescent-1 (v₁), is temperature-conditional and develops chlorotic leaves when grown at restrictive temperatures. We report here the effects of v₁ mutation on the expressions of plastid and nuclear genes during leaf development. In the wild-type rice seedlings, the transcripts of the plastid RNA polymerase gene (rpoB) and ribosomal protein genes (rps7, rps15) accumulated during s strictly limited period of early leaf development, prior to the accumulation of the transcripts of photosynthetic genes (rbcL, RbcS, psbA, Lhc). This period coincides very closely with the leaf developmental stage (late P4) at which the v₁ gene gives the signal that determines the virescent phenotype. On the contrary, in the v₁ seedlings grown at a restrictive temperature (20°C), this stage-specific accumulation of the rpo and rps transcripts was missing. Instead, the accumulation of these transcripts occurred during a later stage of leaf maturation. In such mutant seedlings, the expression of other plastid genes (psbA, rbcL, 16S rDNA) was strongly suppressed and the normal chloroplast development was disturbed. These data indicate that the v₁ gene controls the timing of expression of the key plastid genes for the transcription/translation apparatus that are essential for the subsequent activation of other plastid genes.