Dual Role of SlNOR and SlNOR-like1 Revealed in Tomato Pollen Development

Successful fertilization in flowering plants depends on healthy pollen development and structural integrity of the pollen wall. The outer exine layer, rich in fatty acids and phenylpropanoids, shields pollen from environmental stress while enabling its adhesion and germination on the stigma. Disruption of this wall can cause pollen collapse and male sterility, leading to poor fruit set. Although ABC transporters and lipid metabolism enzymes are known to participate in pollen wall formation, the transcriptional regulators controlling these pathways in tomato remain unclear. Due to these challenges, in-depth research is needed to elucidate how key transcription factors coordinate pollen wall biosynthesis and maintain reproductive success.

Researchers from China Agricultural University, Chongqing University, and the University of California, Davis, have uncovered the dual role of SlNOR and SlNOR-like1 in tomato pollen development. The study, published (DOI: 10.1093/hr/uhaf003) on April 1, 2025, in Horticulture Research, reports that these NAC transcription factors act redundantly to ensure pollen fertility by maintaining exine structure and lipid transport. Double-knockout mutants (nor/nor-like1) exhibited collapsed pollen walls, loss of nuclei, and failed fruit formation, linking these genes-previously known for fruit ripening-to male reproductive success.

Using CRISPR/Cas9-generated double mutants, the team demonstrated that simultaneous loss of SlNOR and SlNOR-like1 causes complete pollen abortion in tomato. Histological and ultrastructural analyses revealed that the pollen walls in mutants collapsed during the mature stage, leaking intracellular contents and leading to adhesion between grains. RNA sequencing of stamens at key developmental stages identified over 3,500 differentially expressed genes, including those in ABC transporter, lipid metabolism, and phenylpropanoid biosynthesis pathways. Specifically, SlABCG8, SlABCG9, and SlABCG23-homologs of known Arabidopsis pollen wall transporters-were strongly downregulated.Functional assays including electrophoretic mobility shift, dual-luciferase reporter, and ChIP-qPCR confirmed that SlNOR and SlNOR-like1 directly bind to and activate promoters of SlABCG8/9/23, SlCER1 (wax biosynthesis), and SlGRP92 (glycine-rich structural protein). This regulatory network ensures the proper transport and deposition of sporopollenin precursors essential for exine formation. The findings establish that SlNOR and SlNOR-like1 redundantly orchestrate pollen wall development, revealing an unexpected extension of their roles beyond fruit ripening.

Our research uncovers a previously overlooked role for SlNOR and SlNOR-like1 beyond fruit ripening. These transcription factors serve as dual safeguards of male fertility by regulating lipid metabolism and pollen wall integrity. The discovery of their redundancy not only deepens our understanding of floral development in tomato but also provides valuable clues for manipulating fertility in crop breeding."

Professor Da-Qi Fu, corresponding author, China Agricultural University

The discovery that SlNOR and SlNOR-like1 redundantly control pollen viability opens new avenues for genetic regulation of plant fertility. The double-knockout's complete male sterility suggests a potential biotechnological route for developing hybrid tomato varieties without the need for manual emasculation. Moreover, the conserved functions of ABC transporters and NAC transcription factors across species imply that similar regulatory modules could be targeted in other crops to improve pollination efficiency and hybrid seed production. This work bridges reproductive biology and agricultural application, offering new molecular tools for enhancing yield and sustainability in horticultural breeding.