Enhanced tolerance to photoinhibition in transgenic plants through targeting of glycinebetaine biosynthesis into the chloroplasts

Transgenic lines of Brassica juncea with codA gene from Arthrobacter globiformis were generated. The codA gene was targeted into the chloroplasts to enable the transformed plants to synthesize glycinebetaine (GB) in their chloroplasts. Immunoblot analysis confirmed the localization of the choline oxidase in the chloroplasts of transgenic lines. Upto 1 mumol g(-1) fresh weight of glycinebetaine levels were recorded in the transgenic lines. Accumulation of glycinebetaine in the chloroplasts led to increased tolerance of the PS II in transgenic lines to high light intensity under unstressed and stressed (salt as well as low temperature stress) conditions. Enhanced tolerance is due to increased protection as well as accelerated recovery of the PS II from a photo-inactivated state by glycinebetaine. Increased protection of PS II by glycinebetaine appeared to be associated with reduction in generation of toxic oxygen species under high light stress. Increased levels of H2O2 was recorded in both wild type and transformed plants with an increase in the duration of exposure to high light under both stressed and unstressed conditions. Despite having higher levels of H2O2 the transgenic lines were phenotypically normal and recorded higher activities of H2O2 scavenging enzymes under high light conditions. H2O2 produced due to choline oxidase activity in the transgenic lines could have stimulated the expression of antioxidant enzymes to maintain safe levels of H2O2. Protection by glycinebetaine against H2O2 induced photo-oxidative stress and enhanced level of antioxidant system, protects the PS II complex against high light-induced damage in transgenic lines. This we believe, is the first report in B. juncea showing the targeting of chloline oxidase into chloroplasts leading to accumulation of glycinebetaine, is one of the effective strategies of achieving tolerance to photoinhibtion. (C) 2004 Elsevier Ireland Ltd. All rights reserved.