Tocopherol deficiency in transgenic tobacco (Nicotiana tabacum L.) plants leads to accelerated senescence

alpha-Tocopherol constitutes the major lipophilic antioxidant in thylakoid membranes, which cooperates with the soluble antioxidant system to alleviate oxidative stress caused by reactive oxygen species (ROS) during oxygenic photosynthesis. Tocopherol accumulates during leaf senescence, indicating the necessity for increased redox buffer capacity in senescent leaves, and tocopherol deficiency has been shown to restrict sugar export from source leaves by inducing callose plugging in the vasculature. We have generated tocopherol-deficient tobacco plants that contain as few as 1% of wild-type (WT) tocopherol in leaves by silencing homogentisate phytyltransferase (HPT). Employing HPT : RNAi plants, we have assessed the importance of tocopherol during leaf senescence and for sugar export. Irrespective of whorl position, the content of free sugars and starch was lower in HPT : RNAi leaves than in WT during the vegetative phase, and no accumulation of callose or a reduction in sugar exudation compared to WT was evident. Based on our observations, we discuss lipid peroxidation as a potential modulator of tocopherol-mediated signalling. Furthermore, senescence was accelerated in lower leaves of HPT transgenics, as indicated by elevated GS1 and reduced rbcS transcript amounts. Oxidative stress was increased in virescent lower source leaves, suggesting that the lack of tocopherol triggers premature senescence.