Deciphering genetic and physiological determinants of alternate bearing in apple tree

UMR AGAP - équipe AFEF - Architecture et fonctionnement des espèces fruitièresUMR AGAP - équipe AFEF - Architecture et fonctionnement des espèces fruitièresPromoting 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 the collaborative German-French “AlternApp” project, we are investigating 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. Based on previous information on genetic determinism obtained on an apple segregating population and first candidate genes (CGs), we are (i) enlarging the genetic basis considered in genetic studies, (ii) examining the respective role of nutritional competition between reproductive and vegetative growth and hormonal control of FI and (iii) enlarging the list of CGs by RNAseq. New segregating populations are under study for FI and bearing behavior in order to identify genomic regions associated to regular phenotypes. Phenotypes as well as genotypes and years effects are being quantified by specific statistical methods. Apical meristem tissues have been 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. CGs expression profiles, hormonal (especially GA) and carbohydrate contents are also being studied in these tissues. 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