Transcription of Two Photosynthesis-Associated Nuclear Gene Families Correlates with the Presence of Chloroplasts in Leaves of the Variegated Tomato ghost Mutant

Abstract

Leaves of the tomato ghost mutant show a variegated green/white phenotype due to a somatically unstable genetic block in carotenoid biosynthesis. Colored carotenoids are not synthesized in white leaves; consequently, chlorophyll is destroyed by photooxidation and the plastids formed show little development of internal membrane structures. Carotenoid biosynthesis proceeds to wild type levels in green tissue, thus chlorophyll accumulates and chloroplasts develop normally. The presence of green sectors allows for the production through tissue culture of variegated green/white plants, in which growth is supported by the photosynthetic green tissue. Thus, ghost is the first plant carotenoid mutant that can be grown to maturity. We determined the steady state mRNA levels for two nuclear gene families that code for chloroplast proteins: rbcS, which codes for the small subunit of ribulose-1-5-bisphosphate carboxylase; and cab, which codes for chlorophyll a/b binding protein. In ghost plants grown in light, the steady state mRNA levels for both gene families were low in white leaves but were similar to wild type in green leaves. Light regulation of the transcripts studied was observed in both ghost green and white leaves. Transcription experiments conducted on nuclei isolated from green and white leaves indicate that the low levels of cytoplasmic mRNAs observed in the absence of colored carotenoids and/or light are due to reduced rates of transcription. We conclude that maximum transcription of rbcS and cab genes in leaves of mature tomato plants requires both light and mature chloroplasts.