Protoporphyrinogen Oxidase–Overexpressing Transgenic Rice is Resistant to Drought Stress

ABSTRACT Transgenic rice (Oryza sativa L. ‘Dongjin’) lines, AP37 and MX, overexpressing Arabidopsis thaliana and Myxococcus xanthus protoporphyrinogen oxidase (PPO), respectively, were examined for their physiological responses and tolerance mechanism to drought stress. AP37 and MX lines were previously confirmed to be resistant to PPO inhibitors. Transgenic AP37 and MX plants showed significantly improved drought tolerance, as indicated by higher water content, lower leaf injury, less oxidative damage, and more favorable redox balance compared to wild‐type (WT) plants. The magnesium‐protoporphyrin IX (Mg‐Proto IX) content of WT and the transgenic lines did not vary significantly with the drought treatments, but MX lines had higher Mg‐Proto IX methyl ester (ME) content after 90 h of drought treatment compared to WT and AP37. In addition, the expression of most genes involved in porphyrin biosynthesis, particularly in the chlorophyll branch, such as glutamyl‐tRNA reductase, glutamate‐1‐semialdehyde aminotransferase, protoporphyrinogen oxidase 1, Fe‐chelatase2, H‐subunit of Mg‐chelatase, and I‐subunit of Mg‐chelatase, was up‐regulated in the transgenic plants, and this up‐regulation was higher in AP37 than in MX. Overall, the AP37 transgenic line was found to show better drought tolerance than the MX transgenic line.