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 polypetides contributes to the inhibition of photosystem II (PSII) function during water stress, changes in transcript and template levels of chloroplast psb A and psb D 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 psb A and psb D 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 psb A and psb D genes actually declined by 6‐ and 9‐fold, respectively. These results suggest that water stress affects the expression of the psb A and psb D 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 psb A or psb D genes, suggesting that the decreased abundance of psb A and psb D 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.