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It has been shown that under high light (HL) conditions, Arabidopsis bundle sheath cell (BSC) , chloroplasts accumulate hydrogen peroxide (H202) as part of a retrograde signalling network associated with the induction of HL-responsive ASCORBATE PEROXIDASE2 (APX2). It was postulated that this network is mediated by both the "core" abscisic acid (ABA) signalling pathway and G protein signalling and involves the heterotrimeric Ga subunit (GP AI), type 2C protein phosphatases (PP2Cs), SNFI-related kinases 2 (SnRK2s) and respiratory burst oxidase homo logs (RBOHs). The main objective of this study was to determine to what extent these two pathways were implicated in the regulation of APX2 and establish whether the same pathway regulates the expression of other HL-responsive genes in Arabidopsis. Finally, the project looked at identifying if GPAI and SnRK2.6 were integrated into the same HL, ABA-directed signalling pathway. Screening of null mutants under HL showed that GP Al was responsible for the negative regulation of APX2 and extracellular H202 together with ABI2. It was predicted that RBOH F acts upstream of RBOH D where it may interact with SnRK2.6 and in turn regulates ROS production from RBOH D. Screening of the F3 population from the cross between ost1-1 and gpa1-4 under HL indicated that both GPAI and OSTI do not feature on the same signalling pathway. A microarray analysis of HL- responsive gene overlaps between PP2C and SnRK2 mutants revealed that only a proportion of genes regulated by ABA and HL were regulated by this pathway. The promoters of these genes contained both heat shock elements (HSEs) and ABA-responsive elements (ABREs). This highlighted the fact that only a few genes conformed to the same signalling that occurs for APX2. In summary, ABA-directed signalling may control three separate pathways that are regulated by HSFs, ABFs/ AREBS and genes that respond indirectly to HL via other regulatory pathways.