# BrPIF4/BrBBX24-BrHB52-mediated hypocotyl modularity unlocks mechanized harvesting potential in Brassica rapa

## TL;DR

This study identifies a genetic module that controls hypocotyl length in Chinese cabbage, which could help improve mechanical harvesting efficiency.

## Contribution

The discovery of a BrPIF4/BrBBX24-BrHB52 regulatory module that controls hypocotyl elongation for mechanized harvesting.

## Key findings

- BrHB52 is a key regulator of hypocotyl elongation in Chinese cabbage.
- A 251-bp insertion in the BrHB52 promoter of a long-hypocotyl variety introduces light-responsive GT-1 motifs.
- PIF4 and BBX24 transcription factors regulate BrHB52 expression through different promoter elements.

## Abstract

Chinese cabbage production faces critical mechanization challenges due to traditional plant architectures that limit mechanical harvesting efficiency. Traditional breeding prioritized short-hypocotyl varieties to prevent damping-off, but long hypocotyls are now critical for mechanical harvesting. We identified BrHB52, an HD-Zip transcription factor, as a key regulator of hypocotyl elongation through quantitative trait locus (QTL) mapping, RNA-seq, and haplotype analysis. BrHB52 expression was significantly higher in the long-hypocotyl variety R031L than in the short-hypocotyl variety R032S. Overexpression of BrHB52 in both Chinese cabbage and Arabidopsis led to elongated hypocotyls. The silencing of BrHB52 in R031L resulted in a reduction of hypocotyl length. Sequence alignment revealed a 251-bp insertion in the BrHB52 promoter of the long-hypocotyl variety R031L, which introduced the light-responsive GT-1 motifs. The upstream transcription factors Phytochrome-interacting factor4 (PIF4) and B-box zinc finger 24 (BBX24) were identified through yeast one-hybrid screening using the BrHB52R031L promoter sequence. PIF4 were found to bind to the both BrHB52R031L and BrHB52R032S promoters and activate their expression through G-box, while light-induced factor BBX24 only bind to the BrHB52R031L promoter and activate its expression by light-responsive element GT-1. Our findings elucidate a BrPIF4/BrBBX24-BrHB52 regulatory module that controls plant architecture through hypocotyl elongation. These findings not only provide critical genetic targets for developing mechanization-compatible Chinese cabbage, but also develop transgenic prototypes with elongated hypocotyls, offering practical resources for mechanized breeding.