Ascorbic acid (Asc) plays a multifunctional role in plants. L-galactono-1,4-lactone dehydrogenase (GLDH, EC 1.3.2.3) catalyzes the last step in the main pathway of Asc biosynthesis in higher plants. In this paper, we first examined how a change in Asc content leads to a changed plant growth and seed set using GLDH transgenic rice (Oryza sativa L.) which has different expression level of GLDH. The results showed that suppression of GLDH expression resulted in a loss of chlorophyll, a lower Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) protein content, and a lower rate of CO₂ assimilation. As a consequence, a slower rate of plant growth and lower seed set were observed. Reduced seed set and growth rate as measured by plant height, root length, leaf weight, and root weight were consistent with the GLDH-mediated reduction of photosynthetic function. Increasing GLDH expression maintained high levels of chlorophyll, Rubisco protein, and a higher rate of net photosynthesis, resulting in higher seed set. The observation that increasing the level of GLDH expression correlated with reduced lipid peroxidation whereas reducing GLDH expression correlated with increased lipid peroxidation was consistent with the foliar level of Asc, indicating that GLDH functions to protect against ROSmediated damage. When taken together, this work suggests that level of Asc in transgenic rice for GLDH is associated with plant growth and seed set.
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