Paternal miRNAs Play Unexpected Role in Seed Viability and Embryogenesis

By using targeted gene silencing in different pollen cell types, the study reveals that disrupting DCL1 function either in the vegetative cell or in sperm cells leads to defective seed formation. The findings highlight that male-derived miRNAs do more than just support pollen development-they also contribute critically to early embryo patterning after fertilization.

miRNAs are small non-coding RNAs that regulate gene expression and influence key developmental processes. In plants, these molecules are produced through a multistep biogenesis pathway involving the enzyme DCL1, among others. While loss of maternal miRNA biogenesis has long been associated with embryo failure, the role of miRNAs from pollen-especially those generated during or after pollen maturation-remains less understood. Mature pollen contains two sperm cells and a vegetative cell, and increasing evidence suggests these cell types communicate and exchange RNAs. However, how miRNA production in each of these pollen cells contributes to seed formation has remained unclear. Due to these gaps, further investigation into male gametophyte miRNA biogenesis was needed.

A study (DOI: 10.48130/seedbio-0024-0017) published in Seed Biology on 30 October 2024 by Binglian Zheng's team, Fudan University, provides fresh insights into plant fertility regulation and opens new avenues in crop reproductive biology.

To determine the functional importance of DCL1, a key enzyme in miRNA biogenesis, during male gametophyte development and seed formation, researchers employed both genetic mutant analysis and cell-specific gene knockdown strategies in Arabidopsis thaliana. They used the weak allele dcl1-7 to perform hand-pollination assays, allowing viable homozygous mutant plants to produce pollen with a subset of viable grains. These were used to fertilize wild-type pistils, and seed development was subsequently assessed. In parallel, the team generated transgenic lines with artificial miRNA constructs specifically targeting DCL1 in either the vegetative cell (VC) or the generative/sperm cells (GC/SC), driven by the cell-type-specific promoters VCK1 and HTR10, respectively. Results revealed that pollination with dcl1-7 mutant pollen caused severe seed developmental defects, with siliques being significantly shortened and only 20–30% viable seeds forming. Conditional knockdown of DCL1 in the VC led to reduced pollen germination and a high rate of unfertilized ovules, suggesting a critical role in pollen function and fertilization. Conversely, sperm-cell-specific knockdown of DCL1 led to embryo arrest at the globular stage in approximately 30% of seeds, indicating that paternal DCL1 is essential for early embryogenesis. Further analysis showed that transcription of MIR genes occurred predominantly before the second pollen mitosis, while DCL1 protein persisted into maturity, especially in the VC. This spatial and temporal regulation supports a model in which vegetative cell-derived miRNAs may be transferred to sperm cells. The findings conclusively demonstrate that DCL1-mediated miRNA biogenesis in mature pollen is indispensable for successful fertilization and seed development, with distinct and complementary roles played by both the vegetative and generative cell lineages.

The discovery that mature pollen actively contributes to miRNA production critical for fertilization and seed development opens promising avenues for agricultural biotechnology. Manipulating miRNA pathways in pollen may help improve hybrid seed production, overcome fertilization barriers, or increase seed viability in crops under environmental stress.