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The High Light Response in Arabidopsis Involves ABA Signaling between Vascular and Bundle Sheath Cells^[W]

^a Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, United Kingdom
^b School of Biosciences, University of Exeter, Exeter EX4 4QD, United Kingdom
^c Department of Applied Biological Chemistry, University of Tokyo, Bunkyo, Tokyo 113-8657, Japan
^d Department of Biological Sciences, Lancaster Environment Centre, University of Lancaster, Lancaster LA1 4YQ, United Kingdom
^e Departments of Biology and Pharmacology, University of North Carolina, Chapel Hill, North Carolina 27599-3280

² Address correspondence to mullin{at}essex.ac.uk.

Previously, it has been shown that Arabidopsis thaliana leaves exposed to high light accumulate hydrogen peroxide (H₂O₂) in bundle sheath cell (BSC) chloroplasts as part of a retrograde signaling network that induces ASCORBATE PEROXIDASE2 (APX2). Abscisic acid (ABA) signaling has been postulated to be involved in this network. To investigate the proposed role of ABA, a combination of physiological, pharmacological, bioinformatic, and molecular genetic approaches was used. ABA biosynthesis is initiated in vascular parenchyma and activates a signaling network in neighboring BSCs. This signaling network includes the G subunit of the heterotrimeric G protein complex, the OPEN STOMATA1 protein kinase, and extracellular H₂O₂, which together coordinate with a redox-retrograde signal from BSC chloroplasts to activate APX2 expression. High light–responsive genes expressed in other leaf tissues are subject to a coordination of chloroplast retrograde signaling and transcellular signaling activated by ABA synthesized in vascular cells. ABA is necessary for the successful adjustment of the leaf to repeated episodes of high light. This process involves maintenance of photochemical quenching, which is required for dissipation of excess excitation energy.

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