# Structural and Genomic Bases of Branching Traits in Spur-Type Apple: Insights from Morphology and Whole-Genome Resequencing

## TL;DR

This study explores the genetic and structural basis of compact growth in spur-type apples using genome sequencing and analysis to support breeding improvements.

## Contribution

The paper identifies candidate genes and genomic variations linked to spur-type branching traits in apples through whole-genome resequencing.

## Key findings

- Spur-type apple lines showed smaller, more compact cell structures compared to non-spur-type lines.
- Genomic analysis identified 29,157 candidate genes with deleterious mutations associated with spur-type traits.
- Candidate genes were enriched in pathways related to molecular function, phosphorylation, and metabolism.

## Abstract

Background: Plant architecture, particularly branching patterns, plays a crucial role in plant growth, photosynthetic performance, and yield. Spur-type apple, characterized by compact growth, early fruiting, high productivity, and manageable canopy structure, represent valuable germplasm for establishing dwarf and high-density apple orchards. While hybrid breeding of spur-type varieties offers significant potential for genetic advancement, severe segregation of traits in hybrid progeny and the difficulty of combining multiple favorable traits still significantly limit breeding efficiency. Moreover, the genetic basis and molecular mechanisms of the spur-type trait remain poorly understood at the genomic level, hindering the development of precise molecular breeding approaches. Methods: To address this, we used the spur-type line ‘0301-13-14’ and the non-spur-type line ‘0301-50-32’ from hybrid progenies of the spur-type cultivars ‘Miyazaki Spur Fuji’ and ‘Starkrimson’ to elucidate the regulatory mechanisms underlying apple branch formation and spur-type trait development by characterizing their branching traits, performing whole-genome resequencing analysis, and identifying candidate genes using bioinformatics analyses. Results: Anatomical observations revealed that the spur-type line ‘0301-13-14’ possessed smaller cells with a more compact spatial arrangement compared to the non-spur-type line ‘0301-50-32’. Whole-genome resequencing generated 5,003,968 high-quality single-nucleotide polymorphisms (SNPs) and 577,886 high-quality insertions/deletions (InDels). We further identified 29,157 candidate genes harboring predicted deleterious mutations (classified as high or moderate impact). Gene Ontology (GO) enrichment analysis indicated that genes associated with the spur-type trait were mainly enriched in molecular function and biological process categories. Specifically, variant genes related to molecular function were enriched in transferase and catalytic activities, while those in biological process were mainly involved in phosphorylation and phosphorus metabolism. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed that candidate genes were significantly enriched in environmental information processing and metabolic pathways. Conclusions: These results will provide a genomic foundation for identifying genes controlling spur-type branching traits and facilitate the genetic improvement of spur-type apple.