MdILL6 regulates xylem and vessel development to control internode elongation in spur-type apple

Spur-type varieties play an important role in facilitating high-density plantings. However, the underlying mechanisms of internode elongation in spur-type varieties are poorly understood. In this research, we investigated the morphological phenotype of annual shoots in four spur-type varieties ('Miyazak', 'Jinfu 18', 'Yanfu No. 6', and 'Liquan spur') and four standard-type varieties ('Aomorifu', 'Shou Fuji', 'Yanfu No. 10', and 'Yanfu No. 3'). Compared with standard-type varieties, spur-type varieties had a shorter shoot length, an average internode length and a smaller xylem size. The content of Jasmonic acid (JA) and Jasmonic acid isoleucine (JA-Ile) significantly increased in spur-type varieties, accompanied by an increase in the expression of JA biosynthesis and signal transduction genes. Exogenous methyl jasmonate (MeJA) inhibited plant height, xylem size, and vessel area. Additionally, we identified an IAA-Leucine Resistant1-like Hydrolase family member, MdILL6, which was highly expressed in spur-type varieties and mature stems. MdILL6 was mainly expressed in the shoot tips and stem, and its protein was located on the endoplasmic reticulum. Overexpression of MdILL6 in apple inhibited plant height and average internode length by decreasing xylem size and vessel area. Our results revealed a molecular mechanism of spur-type variety development affected by the JA pathway and suggest strategies for genetic improvement and regulation of spur-type varieties.