Plant and Cell Physiology Advance Access originally published online on July 14, 2005
Plant and Cell Physiology 2005 46(9):1515-1524; doi:10.1093/pcp/pci164
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Co-expression of Cytochrome b 561 and Ascorbate Oxidase in Leaves of Wild Watermelon under Drought and High Light Conditions
Graduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma, Nara, 630-0192 Japan
* Corresponding author: E-mail, yokota{at}bs.naist.jp; Fax, +81-743-72-5569.
Despite carrying out C3 photosynthesis, wild watermelon (Citrullus lanatus sp.) exhibits exceedingly good tolerance to severe drought at high light intensities. However, the mechanism(s) by which this plant protects itself from photodamage has yet to be elucidated. In this study, we characterized wild watermelon cytochrome b 561 (cyt b 561), which potentially mediates regeneration of apoplastic ascorbate by transferring electrons from cytosolic ascorbate across the plasma membrane. Two cDNA species for wild watermelon cyt b 561, designated CLb561A and CLb561B, were isolated. Levels of both CLb561A mRNA and protein were significantly elevated in the leaves during drought at a light intensity of 700 µmol photons m–2 s–1. The transcript of CLb561B was detected to a much lesser extent, but no CLb561B protein was produced under any condition used in this study. A transient expression assay with the CLb561A::green fluorescent protein fusion construct showed clear fluorescence on the plasma membrane of onion epidermal cells. The CLb561A protein was enriched in the plasma membrane fraction in leaves of transgenic tobacco expressing CLb561A. Moreover, the high activity of apoplastic ascorbate oxidase (AO), which was able to dispose of cyt b 561-transferred reducing equivalents, increased in leaves of wild watermelon grown at high light intensity, but not lower light intensities. Taken together, these observations suggest the occurrence of a novel pathway for excess light energy dissipation in wild watermelon leaves, where excessive energy absorbed by chloroplasts can be transported to and dissipated safely in the apoplasts through the cooperative action of cyt b 561 and AO.
The nucleotide sequences reported in this paper have been submitted to DDBJ/EMBL/GenBank under the accession numbers AB205222
(Received March 15, 2005; Accepted July 7, 2005)
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