2018 ASHS Annual Conference

Downward branches characterize the weeping growth habit. In Malus, the weeping tree form is common in crabapples, such as ‘Cheal’s Weeping’ and ‘Red Jade’, which are highly desired for ornamental use. Although most apple cultivars grow their branches upward with certain angles, a branch bending down practice is recommended in several modern orchard training systems, such as Solaxe and Tall Spindle. Term ‘weeping’ here distinguishes from the same term used for ‘ideotype IV’ that described the growth habit and fruit bearing type represented by apple cultivars ‘Granny Smith’ and ‘Rome Beauty’, where the weeping-like trait is caused by the bending of branches due to fruit bearing at tips. To determine the genetic cause of weeping growth habit in Malus, a pooled genome sequencing-based approach was taken using an F₁ population segregating for the weeping trait. Through a detailed analysis of segregation types of DNA variants, four genomic regions were revealed of a significant association with weeping, including a major locus Weeping (W) on chromosome 13 and others on chromosomes 10 (W2), 16 (W3) and 5 (W4). Examining the putative function and expression profile of the genes annotated under the W region, a strong candidate gene MDP0000254069 (MD13G1122400) encoding a LAZY1-like protein was identified, designated MdLAZY1. Apple transgenic lines under-expressing the MdLAZY1 grow leaves downward while those over-expressing the gene grow leaves with normal angles at young stage. Currently, an extensive effort to characterize MdLAZY1 is underway.