QTL Mapping of Root Architectural Traits in Apple Rootstocks

Grafting allows uncoupling the selection of scion (fruit quality) and rootstock traits (dwarfing, precocity, and productivity) for breeding and development of new cultivars.  Among apple rootstock traits, root specific traits have been largely ignored during the selection process because of the difficulty in the evaluation of such traits.  Recently we noticed that several high performance apple rootstocks that shared common parentage displayed unusually prominent fine root formations.  The same group of rootstocks also exhibits tolerance to replant disease. Such fine root formations might be involved in better soil profile exploration and improved tree nutrition.  We utilized a segregating population made up of 186 individuals that shared parentage (Ottawa 3 and Robusta 5) with these high performance rootstocks to investigate the inheritance of this trait.  We collected replicated quantitative data on in 2008 and 2009 on several tree architecture traits as well as root architecture using a visual scoring method in 2008 for preliminary analysis and a more rigorous method in 2009 that utilized the WinRhizo^TM image analysis software to quantify root mass, root length, and preponderance of fine roots in the individuals in the population.  A 530 point molecular linkage map spanning all 17 linkage groups of apple and made with the individuals of the same population had been already developed.  This quantitative data were used in combination with the molecular marker linkage map to scan the genome using interval and MQM mapping with MapQTL 6 software.  We discovered several significant loci modulating all root architecture traits analyzed.  In particular we discovered a large QTL on linkage group 11 of apple that explained ~40% of the genetic variation with the  interval mapping method.  This data is being utilized to fine map and clone this genetic factor for further characterization.  Furthermore we are developing data on a number of nutrition and disease resistance related traits to investigate possible correlation with the improved performance of apple rootstocks.