Developmental role of the tomato Mediator complex subunit MED 18 in pollen ontogeny

Summary Pollen development is a crucial step in higher plants, which not only makes possible plant fertilization and seed formation, but also determines fruit quality and yield in crop species. Here, we reported a tomato T‐ DNA mutant, pollen deficient1 ( pod1 ), characterized by an abnormal anther development and the lack of viable pollen formation, which led to the production of parthenocarpic fruits. Genomic analyses and the characterization of silencing lines proved that pod1 mutant phenotype relies on the tomato Sl MED 18 gene encoding the subunit 18 of Mediator multi‐protein complex involved in RNA polymerase II transcription machinery. The loss of Sl MED 18 function delayed tapetum degeneration, which resulted in deficient microspore development and scarce production of viable pollen. A detailed histological characterization of anther development proved that changes during microgametogenesis and a significant delay in tapetum degeneration are associated with a high proportion of degenerated cells and, hence, should be responsible for the low production of functional pollen grains. Expression of pollen marker genes indicated that Sl MED 18 is essential for the proper transcription of a subset of genes specifically required to pollen formation and fruit development, revealing a key role of Sl MED 18 in male gametogenesis of tomato. Additionally, Sl MED 18 is able to rescue developmental abnormalities of the Arabidopsis med18 mutant, indicating that most biological functions have been conserved in both species.