The MYB-CC transcription factor MdγMYB2 modulates shoot elongation in spur-type apples.

Abstract

Spur-type apple growth, characterized by shortened internodes and compact tree architecture, is an important trait for high-density apple production; however, its underlying molecular basis remains insufficiently understood. Previous work identified the MYB-CC transcription factor MdγMYB2 as a candidate regulator of shoot elongation in spur-type apple. Here, we investigated the phylogenetic relationships, molecular characteristics, and biological functions of MdγMYB2 to elucidate its role in shoot elongation and internode development. During shoot development, MdγMYB2 expression was significantly higher in the standard-type cultivar 'Shansha' ('SS') than in the spur-type cultivar 'Hanfu' ('HF'). MdγMYB2 was localized to the nucleus and exhibited transcriptional activation activity. Phenotypic analysis of transgenic apple plants showed that MdγMYB2 overexpression significantly increased plant height, internode length, leaf area, and longitudinal stem cell length, whereas RNA interference resulted in the opposite phenotypes. Transcriptome profiling revealed that altered MdγMYB2 expression was accompanied by coordinated changes in genes associated with hormone responses, cell wall polysaccharide biosynthesis, pectin and xyloglucan remodeling, secondary cell wall formation, and lignin biosynthesis. Notably, genes involved in cell wall remodeling and lignin deposition exhibited opposite expression patterns between MdγMYB2 overexpression lines (MdγMYB2-OE) and RNA interference lines (MdγMYB2-RNAi) plants, consistent with their contrasting cellular and internode phenotypes. Collectively, these findings establish MdγMYB2 as a positive regulator of apple shoot and internode elongation. MdγMYB2 appears to affect vegetative growth by inducing coordinated transcriptional changes in genes associated with hormone responses and cell wall metabolism. This study identifies MdγMYB2 as a regulator of shoot and internode elongation in apple, providing new insights into the molecular basis of compact growth habits and a valuable genetic resource for the breeding of spur-type apple cultivars.