The Function and Regulation of B6 Biosynthetic Genes by Oxidative Stress Conditions

Abstract

Vitamin B6 has consistently been associated in diverse organisms with conditions of cellular oxidative stress. Plants contain an alternate, and previously uncharacterized pathway for vitamin B6 biosynthesis, distinct from the well characterized pathway in Escherichia coli. The specific objectives of this work were to further characterize vitamin B6 biosynthetic genes in tobacco and Arabidopsis, assess the regulation of B6 biosynthetic genes during biotic and abiotic stress responses, and determine the antioxidant abilities of B6 vitamers.

Two genes unique to the alternative pathway, PDX1 and PDX2, have been described. PDX2 has previously been shown to encode a glutaminase. Complementation studies with E. coli pdxA and pdxJ mutants are consistent with the hypothesis that PDX1 is responsible for synthesis of the pyridoxine ring.

PDX1 and PDX2 genes were isolated and characterized from tobacco and Arabidopsis. Quantitative RT-PCR demonstrated that, in Arabidopsis, PDX1 (3 copies) and PDX2 are differentially regulated in response to high light, chilling, drought, and ozone. In tobacco, PDX1 and PDX2 transcript levels decreased following inoculation with the avirulent pathogen Pseudomonas syringae pv. phaseolicola and transiently increased in response to salicylic acid and methyl jasmonate. Excess vitamin B6 in tobacco leaves interfered with the development of a hypersensitive defense response and increased disease severity caused by P. syringae pv. tabaci.

B6 vitamers quenched superoxide and prevented lipid peroxidation in in vitro assays.

Our findings indicate that B6 vitamers have antioxidant capabilities in chemical assays and in planta, and that synthesis of vitamin B6 is regulated during plant defense responses in a manner consistent with this vitamin's activity as an antioxidant and modulator of active oxygen species in vivo.