Yactayo-Chang, J. P. | Search bioRxiv

bioRxiv2024

Arabidopsis Gluconolactonase, the First Enzyme Involved in Ascorbate Biosynthesis Localized in the Chloroplast Protects Plants from Light Stress.

Vitamin C (L-ascorbic acid, AsA) is the most abundant water-soluble antioxidant in plants. Ascorbate scavenges free radicals, is an enzyme cofactor, and a donor and acceptor of electrons in the chloroplast. Ascorbate protects tissues against damage caused by reactive oxygen species (ROS) produced through normal metabolism or generated from stress. The inositol route to AsA involves four enzymes: myo-inositol oxygenase, glucuronate reductase, gluconolactonase (GNL), and L-gulono-1,4-lactone oxidase. The third enzyme, GNL, has been characterized in rat and bacteria but not in plants. Eighteen putative GNLs were identified in Arabidopsis, one of which, AtGNL, is interesting because it possesses a chloroplastic signal peptide. Plastids can accumulate up to 50 m M As A but until now no chloroplastic AsA biosynthetic genes have been described. This study includes the characterization of the first plant GNL enzyme in vitro and in planta. A knockout on this gene had lower foliar As A and stunted growth compared to controls. The functional gene restored the phenotype of the knockout, and those restored plants had higher AsA content, enhanced photosynthetic capacity, and higher seed yield. These results highlight the importance of AtGNL in As A formation and in maintaining a healthy redox balance in the leaves particularly under low light stress.

biochemistry↗

bioRxiv2019

Mechanisms Underlying the Enhanced Biomass and Abiotic Stress Tolerance Phenotypes of an Arabidopsis MIOX Over expresser

Myo-inositol oxygenase (MIOX) is the first enzyme in the inositol route to ascorbate (L-ascorbic acid, AsA, vitamin C). We have previously shown that Arabidopsis plants constitutively expressing MIOX have elevated foliar AsA content and displayed enhanced growth rate, biomass accumulation, and increased tolerance to multiple abiotic stresses. In this work, we used a combination of transcriptomics, chromatography, microscopy, and physiological measurements to gain a deeper understanding of the underlying mechanisms mediating the phenotype of the AtMIOX4 line. Transcritpomic analysis revealed increased expression of genes involved in auxin synthesis, hydrolysis, transport, and metabolism, which are supported by elevated auxin levels both in vitro and in vivo, and confirmed by assays demonstrating their effect on epidermal cell elongation in the AtMIOX4 over-expresser plants. Additionally, we detected up-regulation of transcripts involved in photosynthesis that was validated by increased efficiency of the photosystem II and proton motive force. We also found increased expression of amylase leading to higher intracellular glucose levels. Multiple gene families conferring plants tolerance to cold, water limitation, and heat stresses were found to be elevated in the AtMIOX4 line. Interestingly, the high AsA plants also displayed up-regulation of transcripts and hormones involved in defense including jasmonates, defensin, glucosinolates, and transcription factors that are known to be important for biotic stress tolerance. These results overall indicate that elevated levels of auxin and glucose, and enhanced photosynthetic efficiency in combination with up-regulation of abiotic stresses response genes underly the higher growth rate and abiotic stresses tolerance phenotype of the AtMIOX4 over-expressers.

plant biology↗