Genetic mechanisms underlying alternate cropping in apple (Malus x domestica)

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Promoting an environmentally safe and economically competitive fruit tree industry in Europe is challenging because fruit production is fragile in front of global warming and is prone to alternate bearing, which is defined as a high cropping year followed by a low (or nil) cropping. Cropping irregularity is managed so far by thinning methods, chemical, mechanical or manual. But several reasons (public opinion, European laws, and economical constraints) lead to the reduction of chemical use and production costs to ensure the sustainability of fruit production in Europe. Fruit production has also to face two major climatic threats: first frost damage in spring which kills flowers and subsequently suppresses cropping and enhances alternate bearing and second high temperature and drought impacts during summer which can perturb floral induction (FI). Moreover, because of their long life, breeding for new fruit tree selections is slow. For all these reasons, food security and the survival of fruit growers in Europe will require new competiveness based on innovative and sustainable solutions. In AlternApp, we will investigate new avenues on the genetic and environmental control of FI to open perspectives for innovative material selection. In apple tree, FI, the time at which meristems become committed to develop flowers, is the critical developmental stage for the intensity of alternate bearing in a given year. The existence of a genetic determinism, suspected since a long time, has been recently demonstrated in an apple segregating population. First candidate genes (CGs) were revealed which support the assumption of hormonal control of FI, involving developing fruits. But the role of nutritional competition between reproductive and vegetative growth, must also be considered. These two assumptions will be examined by genetic and genomics approaches. Two segregating populations will be studied for FI and bearing behavior, in order to identify genomic regions associated to regular phenotypes. Phenotypes, as well as genotypes, years and climatic effects will be quantified by specific statistical developments. Transcriptome of varieties contrasted in their bearing behavior and artificially set into high or low cropping conditions will be explored by New Generation Sequencing Technology (NGS) to identify new CGs and allelic variation of interest. Apical meristem tissues will be collected on shoots which are micro-climatically, morphologically and physiologically characterized in order to correlate transcriptome with the heterogeneity of meristem flowering behaviors within the trees. By this project, we expect new and high standing results on FI in apple tree in relation to their alternate bearing behavior and more applied results linked to the discovery of allelic variation in key genes that could be used in breeding programs.