Enhancement of vitamin B6 levels in rice expressing Arabidopsis vitamin B6 biosynthesis de novo genes

Vitamin B-6 (pyridoxine) is vital for key metabolic reactions and reported to have antioxidant properties in planta. Therefore, enhancement of vitamin B-6 content has been hypothesized to be a route to improve resistance to biotic and abiotic stresses. Most of the current studies on vitamin B-6 in plants are on eudicot species, with monocots remaining largely unexplored. In this study, we investigated vitamin B-6 biosynthesis in rice, with a view to examining the feasibility and impact of enhancing vitamin B-6 levels. Constitutive expression in rice of two Arabidopsis thaliana genes from the vitamin B-6 biosynthesis de novo pathway, AtPDX1.1 and AtPDX2, resulted in a considerable increase in vitamin B-6 in leaves (up to 28.3-fold) and roots (up to 12-fold), with minimal impact on general growth. Rice lines accumulating high levels of vitamin B-6 did not display enhanced tolerance to abiotic stress (salt) or biotic stress (resistance to Xanthomonas oryzae infection). While a significant increase in vitamin B-6 content could also be achieved in rice seeds (up to 3.1-fold), the increase was largely due to its accumulation in seed coat and embryo tissues, with little enhancement observed in the endosperm. However, seed yield was affected in some vitamin B-6-enhanced lines. Notably, expression of the transgenes did not affect the expression of the endogenous rice PDX genes. Intriguingly, despite transgene expression in leaves and seeds, the corresponding proteins were only detectable in leaves and could not be observed in seeds, possibly pointing to a mode of regulation in this organ.