Untitled - 中国植物生理与分子生物学学会

Function and Mechanism of Photorespiration*
Xinxiang Peng
Center for Plant Stress Biology, College of Life Sciences, South China Agricultural University,
Guangzhou 510642
Photorespiration is fundamentally important in plant physiology since it is metabolically closely
coupled with photosynthetic CO 2 assimilation (i.e. the Calvin cycle). As characterized by CO 2
release, photorespiration is apparently counter-productive to the Calvin cycle, may account for
25-50% loss of the carbon fixed by photosynthesis in C 3 plants. However, despite its seemingly
negative impact, this pathway is believed to play various functional roles in plants. First, from an
evolutionary point of view, it may act as an ancillary metabolism to compensate for the futile
withdrawal of ribulose-1,5-bisphosphate from the Calvin cycle under aerobic conditions by serving
as a carbon recovery system reconverting 2-phosphoglycolate to 3-phosphoglycerate. More
intriguingly, a growing body of evidence has shown that photorespiration may function in amino
acid metabolism, resistance to various stresses, and even signal transduction. During the past years,
various efforts have been made in our lab aiming to realize a molecular regulation and improvement
on photorespiration in rice. We used both over-expressing and know-down approaches such as
antisense RNA and RNAi in creating rice photorespiratory transgenic plants. Consequently various
lines have been successfully generated which displayed different levels of enzyme activities
including glycolate oxidase (GLO), serine-glyoxylate aminotransferease/glutamate-glyoxylate
aminotransferease (SGAT/GGAT), glycine decarboxylase (GDC), serine hydroxymethyl transferase
(SHMT), and hydroxypyruvate reductase (HPR). Further functional analyses on these transgenics
revealed a number of interesting results. For instance, it is found that GLO is able to exert a strong
regulation over photosynthesis, possibly through a feed-back inhibition on Rubisco activase (RCA),
and that the glyoxylate cycle may be involved in the anaplerotic reaction to compensate for
photorespiration in rice. Novel functions for the other photorespiratory enzymes are also being
disclosed.
Keywords:Photorespiration; function; mechanism
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