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Gametophyte Development Needs Mitochondrial Coproporphyrinogen III Oxidase Function1

Mutation in the mitochondrial HEMN1 gene of Arabidopsis impairs tetrapyrrole biosynthesis in flower buds, which generates ROS and prevents polar cell fusion in the embryo sac and also affects pollen development. Tetrapyrrole biosynthesis is one of the most essential metabolic pathways in almost all organisms. Coproporphyrinogen III oxidase (CPO) catalyzes the conversion of coproporphyrinogen III into protoporphyrinogen IX in this pathway. Here, we report that mutation in the Arabidopsis (Arabidopsis thaliana) CPO-coding gene At5g63290 (AtHEMN1) adversely affects silique length, ovule number, and seed set. Athemn1 mutant alleles were transmitted via both male and female gametes, but homozygous mutants were never recovered. Plants carrying Athemn1 mutant alleles showed defects in gametophyte development, including nonviable pollen and embryo sacs with unfused polar nuclei. Improper differentiation of the central cell led to defects in endosperm development. Consequently, embryo development was arrested at the globular stage. The mutant phenotype was completely rescued by transgenic expression of AtHEMN1. Promoter and transcript analyses indicated that AtHEMN1 is expressed mainly in floral tissues and developing seeds. AtHEMN1-green fluorescent protein fusion protein was found targeted to mitochondria. Loss of AtHEMN1 function increased coproporphyrinogen III level and reduced protoporphyrinogen IX level, suggesting the impairment of tetrapyrrole biosynthesis. Blockage of tetrapyrrole biosynthesis in the AtHEMN1 mutant led to increased reactive oxygen species (ROS) accumulation in anthers and embryo sacs, as evidenced by nitroblue tetrazolium staining. Our results suggest that the accumulated ROS disrupts mitochondrial function by altering their membrane polarity in floral tissues. This study highlights the role of mitochondrial ROS homeostasis in gametophyte and seed development and sheds new light on tetrapyrrole/heme biosynthesis in plant mitochondria.