EPAD1 Orthologs Play a Conserved Role in Pollen Exine Patterning

TL;DR

This study shows that EPAD1 genes help form pollen walls in grasses, even though their number varies between species.

Contribution

The study confirms the conserved role of EPAD1 orthologs in pollen exine patterning across Poaceae sub-clades.

Findings

CRISPR-Cas9 mutants showed defects in pollen wall formation in barley, sorghum, and millet.

Barley HvEPAD2 mutants exhibited apical spikelet abortion linked to gene expression patterns.

EPAD1 orthologs maintain primexine integrity for Poaceae-specific pollen exine patterns.

Abstract

The pollen wall protects pollen during dispersal and is critical for pollination recognition. In the Poaceae family, the pollen exine stereostructure exhibits a high degree of conservation with similar patterns across species. However, there remains controversy regarding the conservation of key factors involved in its formation among various Poaceae species. EPAD1, as a gene specific to the Poaceae family, and its orthologous genes play a conserved role in pollen wall formation in wheat and rice. However, they do not appear to have significant functions in maize. To further confirm the conserved function of EPAD1 in Poaceae, we performed an analysis on four EPAD1 orthologs from two distinct sub-clades within the Poaceae family. The two functional redundant barley EPAD1 genes (HvEPAD1 and HvEPAD2) from the BOP clade, along with the single copy of sorghum (SbEPAD1) and millet (SiEPAD1) from the PACMAD clade were examined. The CRISPR-Cas9-generated mutants all exhibited defects in pollen wall formation, consistent with previous findings on EPAD1 in rice and wheat. Interestingly, in barley, hvepad2 single mutant also showed apical spikelets abortion, aligning with a decreased expression level of HvEPAD1 and HvEPAD2 from the apical to the bottom of the spike. Our finding provides evidence that EPAD1 orthologs contribute to Poaceae specific pollen exine pattern formation via maintaining primexine integrity despite potential variations in copy numbers across different species.

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