Double fertilization in angiosperms is a highly specialized reproductive process in which two sperm cells fuse with the egg cell and central cell, respectively, giving rise to the embryo and endosperm—the two fundamental components of the seed. At the core of this process is the precise recognition, adhesion, and fusion of male and female gametes. For decades, deciphering the molecular mechanisms that govern gamete recognition and fusion has been a central question in plant sexual reproduction. Understanding these mechanisms has profound implications for both basic reproductive biology and agricultural innovation.
As the global population continues to grow and food security pressures intensify, the development of efficient breeding technologies has become increasingly urgent. Unlike conventional breeding, which requires multiple generations of selfing to obtain homozygous lines, doubled haploid (DH) technology combines haploid induction with chromosome doubling to generate completely homozygous DH lines within only two generations, thereby dramatically shortening the breeding cycle. Identifying highly efficient and broadly applicable haploid induction genes is therefore of great importance for accelerating crop improvement.
In two back-to-back studies published online in Plant Communications, the teams of Prof. LI Hongju and Prof. YANG Weicai at the Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, uncovered key molecular mechanisms underlying gamete adhesion and fusion in flowering plants and reported a major advance in haploid induction technology.
In the first study, the researchers identified GEX3, a sperm surface protein containing an extracellular β-propeller domain, as a key factor controlling gamete adhesion and fusion. Loss-of-function mutations in GEX3 caused severe silique abortion. Although pollen viability, pollen tube growth, and pollen tube targeting to the embryo sac were unaffected in the gex3 mutant, sperm cells became arrested within the embryo sac because double fertilization failed. Further analyses showed that GEX3 depletion impaired both gamete adhesion and fusion, with a stronger effect on adhesion.
Mechanistically, genetic and biochemical assays demonstrated that GEX3 interacts with another adhesion protein, GEX2, through their extracellular domains to form a complex that mediates male–female gamete adhesion. In addition, GEX3 cooperates with the sperm membrane proteins DMP8/9 to promote the translocation of the fusogen HAP2 from the cytoplasm to the sperm plasma membrane, thereby enabling gamete membrane fusion. Based on these findings, the authors proposed a three-step model for the molecular regulation of double fertilization.
Model of GEX3-mediated gamete adhesion and fusion. (Image credit: LI Hongju’s group)
In the second study, the researchers showed that disruption of GEX3 in Arabidopsis produced haploids at a haploid induction rate (HIR) of 0.57%. Strikingly, combining the gex3 mutation with mutations in DMP8/9 significantly increased the HIR to 3.13%. The researchers further identified GEX3 orthologs in rice, tomato, and soybean and found that mutations in these orthologs also induced haploid production, with the HIR reaching up to 8.99% in soybean. Notably, loss of OsGEX3 in rice also generated 9.82% triploid offspring, suggesting that GEX3-mediated fertilization is both evolutionarily conserved and species-specifically diversified.
Together, these two studies reveal GEX3 as a central sperm surface protein that coordinates gamete adhesion, membrane fusion, and haploid induction. They not only deepen our understanding of the molecular basis of double fertilization in flowering plants but also provide valuable genetic resources for advancing doubled haploid technology and synthetic apomixis in crops.
These studies were supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, and the CAS Project for Young Scientists in Basic Research.
Contact:
Prof. LI Hongju
Institute of Genetics and Developmental Biology, Chinese Academy of Sciences
Email: hjli@genetics.ac.cn
CAS
中文
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Model of GEX3-mediated gamete adhesion and fusion. (Image credit: LI Hongju’s group)In the second study, the researchers showed that disruption of GEX3 in Arabidopsis produced haploids at a haploid induction rate (HIR) of 0.57%. Strikingly, combining the gex3 mutation with mutations in DMP8/9 significantly increased the HIR to 3.13%. The researchers further identified GEX3 orthologs in rice, tomato, and soybean and found that mutations in these orthologs also induced haploid production, with the HIR reaching up to 8.99% in soybean. Notably, loss of OsGEX3 in rice also generated 9.82% triploid offspring, suggesting that GEX3-mediated fertilization is both evolutionarily conserved and species-specifically diversified.Together, these two studies reveal GEX3 as a central sperm surface protein that coordinates gamete adhesion, membrane fusion, and haploid induction. They not only deepen our understanding of the molecular basis of double fertilization in flowering plants but also provide valuable genetic resources for advancing doubled haploid technology and synthetic apomixis in crops.These studies were supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, and the CAS Project for Young Scientists in Basic Research.Contact:Prof. LI HongjuInstitute of Genetics and Developmental Biology, Chinese Academy of SciencesEmail: hjli@genetics.ac.cn