Expression of chloroplast photosynthesis genes during leaf senescence

Abstract

Transcripts for thylakoid proteins encoded in the chloroplast genome that show decreased synthesis in senescing primary leaves of bean (Phaseolus vulgaris) have been examined by Northern and dot blot analyses. In young, 7‐day‐old leaves pulse‐labelled with 35S‐methionine, all of the major chloroplast DNA‐encoded thylakoid proteins become radiolabelled, including the 68 kDa protein of PSI encoded by psaAl and psbA2, the D‐1, D‐2, CP47 and CP43 proteins of PSII encoded by psbA, psbD, psbB, and psbC, respectively, and cytochrome f and cytochrome b6 encoded by petA and petB, respectively. By contrast, in pulse‐labelled, senescent 28‐day‐old leaves, the only chloroplast‐encoded thylakoid protein showing active synthesis is the D‐1 protein of PSII. Total RNA levels decline by ≅ 9 fold between 7 and 28 days on a fresh weight basis. Dot blot analyses using gene‐specific hybridization probes indicate that steady state transcript levels for all of the chloroplast DNA‐encoded proteins examined maintain the same proportion of total RNA over this period. This means that these transcripts also decline by =9 fold between 7 and 28 days. Thus, reduced synthesis of the 68 kDa protein of PSI, the CP47 and CP43 proteins of PSII, cytochrome f and cytochrome b6 can presumably be attributed, at least in part, to a decline in steady state levels of corresponding message. It is apparent from Northern blot hybridizations that there are also qualitative changes in transcript pattern for petA, psbC and psbD between 7 and 28 days that perhaps reflect alterations in post‐transcriptional processing. These alterations may also contribute to the decreased synthesis of the CP43 protein and cytochrome f. The continued active synthesis of the D‐1 protein of PSII in the senescent 28‐day‐old leaves, despite the =9 fold decrease in the steady state level of its transcript, suggests that the psbA transcript is preferentially translated relative to transcripts for other chloroplastencoded thylakoid proteins.