Egg cell‐secreted aspartic proteases ECS1/2 promote gamete attachment to prioritize the fertilization of egg cells over central cells in Arabidopsis

Abstract

Double fertilization is an innovative phenomenon in angiosperms, in which one sperm cell first fuses with the egg cell to produce the embryo, and then the other sperm fuses with the central cell to produce the endosperm. However, the molecular mechanism of the preferential fertilization of egg cells is poorly understood. In this study, we report that two egg cell‐secreted aspartic proteases, ECS1 and ECS2, play an important role in promoting preferential fertilization of egg cells in Arabidopsis. We show that simultaneous loss of ECS1 and ECS2 function resulted in an approximately 20% reduction in fertility, which can be complemented by the full‐length ECS1/2 but not by corresponding active site mutants or by secretion‐defective versions of ECS1/2. Detailed phenotypic analysis revealed that the egg cell–sperm cell attachment was compromised in ecs1 ecs2 siliques. Limited pollination assays with cyclin‐dependent kinase a1 (cdka;1) pollen showed that preferential egg cell fertilization was impaired in the ecs1 ecs2 mutant. Taken together, these results demonstrate that egg cells secret two aspartic proteases, ECS1 and ECS2, to facilitate the attachment of sperm cells to egg cells so that preferential fertilization of egg cells is achieved. This study reveals the molecular mechanism of preferential fertilization in Arabidopsis thaliana.

Do egg cell‐secreted aspartic proteases promote gamete attachment?

“…As a consequence, partially semigamous zygotes and haploid offspring were generated (Zhang et al, 2023). More recently, Jiang et al (2022) confirmed the critical role of ECS1/2 in plant fertility and discussed that the endopeptidases are also involved in gamete interaction. These authors concluded, based on their data, that ECS1 and ECS2 were secreted from the egg cell already before fertilization and function to promote sperm cell attachment to the egg cell.…”

“…Notably, Jiang et al (2022) observed that sperm cells remained outside of the unfertilized egg cells at 30 hap, and unfused sperm cells were found detachable at 24 hap from egg cells of the ecs1‐1 ecs2‐1 mutant. Therefore, the authors concluded that ECS1 and ECS2 significantly contribute to egg cell–sperm cell attachment.…”

“…(B) Additional sperm cell pairs in ovules of ecs1 ecs2 mutants after fertilization appear to occur from polytubey rather than gamete attachment. Pairs of sperm cells instead of single sperm cells are visible in ecs1‐1 ecs2‐1 at 24 h after pollination after zooming into the corresponding images in Figure S10 of Jiang et al (2022). sp, sperm cell.…”

“…In ecs1 ecs2 mutant ovules, about 16.4% of seeds showed occurrence of extra pairs of sperm cells as early as 6–8 hap (Yu et al, 2021). Therefore, it is not surprising that Jiang et al (2022) found extra sperm cells outside of the egg cell that are detached from the egg cell at 24 hap. The authors further reported that single fertilization of the central cell occurred and one sperm cell remained outside of the egg cell.…”

“…In this case, it is likely that the second sperm cell pair could not adhere anymore to fertilized egg cells. Along the same lines, Jiang et al (2022) compared wild type (WT) ovules pollinated by hap2 pollen with ecs1‐1 ecs2‐1 ovules pollinated by WT pollen. The authors found that hap2‐ sperm cells were tightly attached in 74.3% of ovules to WT egg cells, and this attachment was stronger compared to WT sperm that were attached to ecs1‐1 ecs2‐1 egg cells (22.4%).…”