PARENT SESSION
Posters P6C Photosynthesis, respiration and alternative electron sinks. Abstracts (660-680)

Function of ascorbate oxidase and cytochrome b₅₆₁ in drought/high light-tolerant wild watermelon. Yoshihiko Nanasato^*,1, Kinya Akashi¹, Akiho Yokota¹, ¹ Nara Institute of Science and Technology (NAIST), Ikoma-si, Nara, JAPAN

ABSTRACT- Wild watermelon from the Kalahari desert has an ability to survive severe drought/high light conditions. To understand the molecular mechanisms of the tolerance, genes up-regurated in the stressed leaves was screened by mRNA differential display. We isolated a cDNA that encodes a homologue of mammalian cytochrome b₅₆₁ (Cyt b₅₆₁), named CLb561-A. A cDNA encoding another homologue, CLb561-B was also isolated by degenerate PCR. Protein blot analyses showed that CLb561-A protein was induced in the leaves while the level of CLb561-B protein was unchanged during the stress. Transient expression assays using CLb561-GFP fusions demonstrated that both CLb561-A and B proteins were localized in the plasma membrane. Flux of transplasma membrane electron transport (TPMET) detected in the protoplasts from stressed watermelon leaves was 30% higher than that from unstressed leaves, which correlated well with the abundance of CLb561 proteins. We also found that the activity of apoplastic ascorbate oxidase (AAO) in the leaves grown in high light condition (700 mol photons m^-2 s^-1) was 8 times higher than that grown in low light condition (100 mol photons m^-2 s^-1). These results suggest a novel energy flow from cytosol to apoplast, where apoplastic ascorbate (AsA) regenerated by CLb561 is subsequently oxidized by AAO, thereby dissipating the excess reducing energy safely in the apoplast.

KEY WORDS: cytochrome b₅₆₁, watermelon, ascorbate oxidase, drought