# A High-Thousand-Seed-Weight Mutant of Brassica napus

## TL;DR

This study identifies a high-seed-weight mutant in rapeseed and explores its genetic and physiological mechanisms to improve breeding strategies.

## Contribution

The discovery of a high-thousand-seed-weight mutant and its genetic regulation provides new resources for rapeseed breeding.

## Key findings

- GRG177 mutant shows significantly higher thousand-seed weight and seed volume compared to control.
- High TSW in GRG177 is controlled by two dominant epistatic genes and polygenes.
- Endogenous hormones and cell activity changes underpin the high TSW trait.

## Abstract

Thousand-seed weight (TSW) is a critical determinant of yield in rapeseed (Brassica napus L.). Developing germplasm with high TSW is therefore a key strategy in high-yield rapeseed breeding. However, the genetic and molecular mechanisms underlying TSW in rapeseed remain poorly understood. In our earlier work, we identified a mutant, designated GRG177, which exhibits a remarkably high TSW exceeding 7 g. To unravel the mechanisms driving this elevated TSW, we conducted a comprehensive analysis of GRG177, integrating morphological, genetic, developmental, anatomical, and physiological approaches. Compared with the control germplasm GRD328 (TSW ≈ 3.5 g), GRG177 displayed a significant increase in seed weight and seed volume, larger silique surface area, and higher yield per plant. However, it also showed a notable reduction in both silique number per plant and seed number per silique. Genetic analysis of a segregating population revealed that the high-TSW trait in GRG177 is governed by two pairs of dominant epistatic major genes plus polygenes. Endogenous hormone analysis revealed significantly higher zeatin riboside (ZR) content in the early stage of seed development in GRG177, whereas indole-3-acetic acid (IAA) and abscisic acid (ABA) levels were significantly up-regulated in the late stage of seed development. Anatomical observation using paraffin sections further confirmed that enhanced cell division activity in the early stage and improved cell expansion capacity in the later stage underpin the formation of high TSW. Furthermore, BSA-seq was utilized to map four TSW-related Quantitative Trait Loci (QTLs) and screen 13 candidate genes involved in IAA, ZR, and ABA signaling pathways. In conclusion, these findings provide novel insights into the regulatory mechanisms governing high-TSW formation in rapeseed and present valuable genetic resources for high-yield breeding.