Plastid disruption in a thiamine-requiring mutant of Nicotiana sylvestris blocks accumulation of specific nuclear and plastid mRNAs

Abstract

In the absence of supplemental thiamine, cotyledon leaves of the thi mutant of Nicotiana sylvestris are green, but expanding true leaves (TL) become chlorotic upon emergence and the chloroplasts fail to maintain thylakoid membrane structure. Albino TL1 is deficient in transcripts from the plastid genes psbA, psbE, and rbcL. As in carotenoid-deficient mutants of maize, albino TL1 is deficient in accumulation of the chlorophyll a/b binding protein (LHCP) and its mRNA (cab). The small subunit of ribulose bisphosphate carboxylase/oxygenase (SSu) and its mRNA (rbcS) are also deficient, but not to the same extent as cab. Application of supplemental thiamine and its subsequent removal was employed to examine patterns of gene expression during the onset of plastid disruption in older leaves. Accumulation of plastid transcripts (psbA, psbE, rbcL) is blocked shortly after the onset of chlorosis in TL6. In contrast, transcripts for cab and rbcS follow their normal course of accumulation in spite of plastid degeneration, suggesting that nuclear genes are transcribed without regard to chloroplast integrity in older leaves. The conditional nature of the thi mutation provides a valuable tool for investigating temporal patterns of intracellular communication during development of primary leaves.