OsTKPR2 is part of a sporopollenin-producing metabolon required for exine formation in rice

The sporopollenin polymer is a major component of the pollen exine. Fatty acid derivatives synthesized in the tapetum are among the precursors of sporopollenin. Progress has been made to understand sporopollenin metabolism in rice; however, the underlying molecular mechanisms remain elusive. We found that OsTKPR2 and OsTKPR1 share a similar expression pattern, and their coding proteins have a similar subcellular localization and enzyme activities towards reduced tetraketide alpha-pyrone and hydroxylated tetraketide alpha-pyrone. Unexpectedly, OsTKPR1(pro):OsTKPR2-eGFP could not rescue the phenotype of ostkpr1-4. Three independent ostkpr2 mutant lines generated by CRISPR/Cas9 displayed reduced male fertility to various extents which were correlated with the severity of gene disruptions. Notably, the anther cuticle, Ubisch bodies, and pollen development were affected in the ostkpr2-1 mutant, where a thinner pollen exine was noticed. OsTKPR1 and OsTKPR2 were integrated into a metabolon including OsACOS12 and OsPKS2, which resulted in a significant increased enzymatic efficiency when both OsTKPR1 and OsTKPR2 were present, indicating the mutual dependence of OsTKPR2 and OsTKPR1 for their full biochemical activities. Thus, our results demonstrated that OsTKPR2 is required for anther and pollen development where an OsTKPR2-containing metabolon is functional during rice sporopollenin synthesis. Furthermore, the cooperation and possible functional divergence between OsTKPR2 and OsTKPR1 is also discussed. OsTKPR2 plays a role in the formation of the anther cuticle, the Ubisch body and the pollen exine through efficient sporopollenin precursor production within a metabolon containing OsTKPR1, OsPKS2, and OsACOS12.

Automated summary

It has been discovered that OsTKPR2 plays a critical role in sporopollenin synthesis during rice pollen wall formation, and there is a complementary relationship between OsTKPR1 and OsTKPR2. Within a metabolon formed in collaboration with other enzymes, OsTKPR2 facilitates the development of anthers and pollen, thereby affecting the formation of the pollen wall.