Changes in transcript levels of chloroplast psbA and psbD genes during water stress in wheat leaves

In order to examine whether the decrease in gene expression of chloroplast DNA-encoded polypeptides contributes to the inhibition of photosystem II (PSII) function during water stress, changes in transcript and template levels of chloroplast psbA and psbD genes (encoding the D1 and D2 reaction center proteins of PSII, respectively) were investigated in spring wheat leaves (Triticum aestivum L. cv. Longchun No. 10) using northern, Southern and dot blot analyses. The results of northern hybridization indicated that stressing wheat seedlings in polyethylene glycol (PEG) solutions with an osmotic potential of -0.5 MPa for 0, 24, 48 and 72 h, caused marked declines in the steady state levels of the psbA and psbD transcripts but did not alter their transcript processing patterns. RNA dot blot analysis further demonstrated that over the whole range of water stress investigated, the transcript levels of the two genes declined by 2- and 3-fold, respectively, relative to the same amount of total RNA. As total RNA decreased 3-fold during the process of stress, the transcript levels of psbA and psbD genes actually declined by 6- and 9-fold, respectively. These results suggest that water stress affects the expression of the psbA and psbD genes, possibly at the transcriptional level. Southern and DNA dot blot analyses consistently showed that water stress did not affect the template levels of either psbA or psbD genes, suggesting that the decreased abundance of psbA and psbD transcripts under water stress is not due to limited gene templates but likely a result of lowered gene transcriptional activity and/or changed mRNA stability.