Impairment of the Uroporphyrinogen Decarboxylase 2 Gene Induces Lesion Mimicry and Downregulates Tetrapyrrole Biosynthesis Pathway Genes in Rice

Lesion-mimic mutants (LMMs) spontaneously develop necrotic lesions in the absence of pathogen infection or environmental stress, serving as ideal models for studying cell death, immune signaling, and the growth-defense balance in plants. In this study, we identified a rice LMM and mapped the causal mutation to the Uroporphyrinogen Decarboxylase 2 gene, which had not been previously functionally characterized within the tetrapyrrole biosynthesis (TBS) pathway. Knockdown of OsUROD2 resulted in lesion mimicry and down-regulated key TBS pathway genes, including OsCPOX, OsPPOX, OsChlH, OsChlD, OsChlI, and OsChlM. Further functional analyses revealed that OsCPOX knockdown or OsPPOX knockout both caused lesion formation and reduced chlorophyll content, whereas triple knockout of OsChlH/OsChlD/OsChlI or knockout of OsChlM resulted in chlorophyll deficiency. Subcellular localization assays showed that OsUROD2, along with OsCPOX, OsPPOX, OsChlH, OsChlD, OsChlI, and OsChlM, localizes to the chloroplast. Our results demonstrate that the chloroplast-localized TBS pathway is integral to viability, lesion mimicry and chlorophyll synthesis in rice, providing crucial insights into the molecular mechanisms linking tetrapyrrole metabolism to regulated cell death.