The Pseudoenzyme PDX1.2 Sustains Vitamin B6 Biosynthesis as a Function of Heat Stress

Plants sense temperature changes and respond by altering growth and metabolic activity to acclimate to the altered environmental conditions. The B vitamins give rise to vital coenzymes that are indispensable for growth and development but their inherent reactive nature renders them prone to destruction especially under stress conditions. Therefore, plant survival strategies would be expected to include mechanisms to sustain B vitamin supply under demanding circumstances. Here, using the example of vitamin B-6, we investigate the regulation of biosynthesis across eudicot and monocot species under heat stress. Most eudicots carry a pseudoenzyme PDX1.2 that is a noncatalytic homolog of the PDX1 subunit of the vitamin B6 biosynthesis protein machinery, PYRIDOXINE BIOSYNTHESIS PROTEIN1. Using Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum) as models, we show that PDX1.2 is transcriptionally regulated by the HSFA1 transcription factor family. Monocots only carry catalytic PDX1 homologs that do not respond to heat stress as demonstrated for rice (Oryza sativa) and maize (Zea mays), suggesting fundamental differences in the regulation of vitamin B6 biosynthesis across the two lineages. Investigation of the molecular mechanism of PDX1.2 transcription reveals two alternative transcriptional start sites, one of which is exclusive to heat stress. Further data suggest that PDX1.2 leads to stabilization of the catalytic PDX1s under heat stress conditions, which would serve to maintain vitamin B6 homeostasis in times of need in eudicots that carry this gene. Our analyses indicate an important abiotic stress tolerance strategy in several eudicots, which has not been evolutionarily adapted (or is not required) by monocots such as grasses.