Regulation of ascorbic acid synthesis and accumulation in Arabidopsis thaliana leaves by light and respiration

BARTOLI CARLOS G; GÓMEZ FACUNDO; FERNÁNDEZ LAURA; YU JIANPING; MCINTOSH LEE; FOYER CHRISTINE

Bristol, Inglaterra

Congreso; Oxygen metabolism, ROS and redox signalling in plants; 2005

Society for Free Radical Research

The following study was undertaken to determine the factors affecting the synthesis and accumulation of ascorbate (AA) in Arabidopsis thaliana leaves. We report the effects of growth irradiance on the capacity for respiration, AA accumulation and L-galactone-1, 4-lactone dehydrogenase (L-GalLDH) activity. Moreover, we have used transformed plants with modified amounts of the alternative oxidase (AOX) protein to examine the relationship between AOX activity and AA synthesis. Wild type plants were grown under low (LL; 50 µmol photons.m-2 s-1) intermediate (IL; 100µmol photons.m-2 s-1) or high (HL; 250 µmol photons.m-2 s-1) light. Increasing growth irradiance progressively elevated the steady state leaf AA contents, such that amounts of AA were highest in the rosette leaves in the HL-grown plants and lowest in those grown under LL. The extent of the dark-induced decline in leaf AA was also dictated by the day time light level. A dark-induced decline of AA content was observed in the HL plants but it was absent under LL conditions. Hence, light not only regulates leaf content in the light but it also regulates the extent of AA degradation in the dark either directly or indirectly. Feeding experiments using L-galactone-1, 4-lactone (L-Gal) the precursor of AA, revealed that the HL leaves accumulated twice as much AA and had double the maximal L-GalLDH activities of leaves from LL plants. The leaves of IL plants had over 70% more AA and 60% higher L-GalLDH activities than the LL leaves. Hence, growth irradiance increases the capacity of leaves to accumulate AA and to synthesise AA through higher L-GalLDH activities. Growth under HL also enhanced dehydroascorbate reductase and monodehydroascorbate reductase activities.  Leaf respiration rates were highest in the HL leaves. Growth at HL led to increased levels of leaf cytochrome c and cytochrome c oxidase (CCO) activity as well as enhancing fluxes through both the AOX and CCO electron transport pathways.  Intact mitochondria, isolated from transgenic lines with high AOX activities had higher capacities to synthesise AA than those from the wild type or antisense lines. In agreement with this finding, the leaves of the AOX over-expressing lines had accumulated more AA than leaves of the WT or antisense plants. We conclude that light affects the capacity for AA synthesis and degradation, and that the light-dependent regulation of AA synthesis involves complex interactions with light-dependent controls on the capacity for leaf respiration that involve both AOX and CCO electron transport pathways.