The roles of a tomato(Lycopersicon esculentum) chloroplast-targeted DnaJ protein(LeCDJ1) were investigated using wild-type(WT) and sense transgenic tomatoes. The LeCDJ1 expression was upregulated by 38 °C, 42 °C, 45 °C, NaCl, PEG, methyl viologen(MV) and hydrogen peroxide(H2 O2), but not by 30 °C and 35 °C. Meanwhile, LeCDJ1 was involved in the response of plants to abscisic acid(ABA). Under heat stress, the sense plants showed better growth, higher chlorophyll content, lower malondialdehyde(MDA) accumulation and relative electrical conductivity(REC), and also less PSII photoinhibition than WT. Interestingly, the sense plants treated with streptomycin(SM), an inhibitor of organellar translation, still showed higher maximum photochemistry efficiency of PSII(Fv/Fm) and D1 protein levels than the SM-untreated WT, suggesting that the protective effect of LeCDJ1 on PSII was, at least partially, independent of D1protein synthesis. Furthermore, the relatively lower superoxide radical(O2·-) and H2 O2 levels in the sense plants were considered to be due to the higher ascorbate peroxidase(APX) and superoxide dismutase(SOD) activity, which seemed unlikely dependent on their transcription level. These results indicated that LeCDJ1 overexpression facilitated heat tolerance in transgenic tomatoes.
Ascorbate peroxidases are directly involved in reactive oxygen species (ROS) scavenging by reducing hydrogen peroxide to water. The tomato thylakoid-bound ascorbate peroxidase gene (StAPX) was introduced into tobacco. RNA gel blot analysis confirmed that StAPX in tomato leaves was induced by methylviologen-mediated oxidative stress. The sense transgenic seedlings exhibited higher tAPX activity than that of the wild type (WT) plants under oxidative stress conditions, while the antisense seedlings exhibited lower tAPX activity. Lower APX activities of antisense transgenic seedlings caused higher malondialdehyde contents and relative electrical conductivity. The sense transgenic seedlings with higher tAPX activity maintained higher chlorophyll content and showed the importance of tAPX in maintaining the optimal chloroplast development under methylviologen stress conditions, whereas the antisense lines maintained lower chlorophyll content than WT seedlings. Results indicated that the over-expression of StAPX enhanced tolerance to methylviologen-mediated oxidative stress in sense transgenic to- bacco early seedlings, whereas the suppression of StAPX in antisense transgenic seedlings showed high sensitivity to oxidative stress.
