Characterization of metabolic disturbances closely linked to the delayed senescence of Arabidopsis leaves after γ irradiation

Abstract

Ionizing radiation is a potent mutagen and has been used to induce random mutagenesis in plants for plant breeding. For the first time, it was revealed that γ irradiation of plants at the early reproductive stage could induce specific alterations in gene expressions involving in pigment metabolism and thereby delay the developmental leaf senescence at the later senescing stage. One-month-old Arabidopsis plants exposed to γ rays at a dose of 50 Gy h −1 for 4 h maintained much higher contents of chlorophylls, carotenoids, proteins, and anthocyanins in leaves at the late reproductive (senescing) stage compared with the control. Interestingly, expression levels of AtPAO and AtSAG12, which are inducible by developmental leaf senescence, were significantly lower in the irradiated leaves than in the control ones, while transcription of AtHEMA1, AtPORB, AtPORC, AtCHLG, and AtANS involving in chlorophyll and anthocyanin anabolism was noticeably increased in the former. Ultrastructure analysis of chloroplasts and thylakoid membranes indicated much less disintegration in the irradiated leaves, whilst most thylakoid membrane proteins for photosynthesis subjected to blue-native gel and immunoblot analysis, seemed to be relatively less affected after γ irradiation. Moreover, the maximal photosynthetic electron transport rate, ETRmax, was steeply decreased in the control leaves during the developmental leaf senescence, but maintained relatively constant in the irradiated ones. The delay in leaf senescence of the irradiated leaves was also confirmed by the additional dark-incubation for 5 d. These results strongly suggest specific radiation-induced disturbances in pigment metabolism and photosynthesis to correlate with delay in leaf senescence of plants at the reproductive stage.