ABA regulation of calcium-related genes and bitter pit in apple

Calcium deficiency disorders in fruit, such as bitter pit (BP), have puzzled researchers for years, and little is known regarding the mechanisms involved in the development of these physiophaties. Previous studies have suggested abscisic acid (ABA) as an alternative treatment to influence calcium allocation to the fruit, by regulating stomatal conductance and transpiration in leaves. We explored the feasibility of this approach in 'Super chief', an apple cultivar susceptible to BP, with the specific purpose to elucidate the mechanisms underpinning calcium partitioning and its resulting availability in the fruit cells through field trials and molecular analysis. Our findings reveal that foliar spraying with ABA, carried out four times during the season (starting from 66 DAFB) can be effective in apple bitter pit prevention, via a preferential supply of Ca2+ to fruit. Higher amounts of Ca2+ have been detected in treated apples at harvest and, consistently, the transcriptional analysis evidenced, in the same samples, the down-regulation of three genes involved in Ca2+ partitioning (MdACA8, MdCAX and MdCDPK). These data are in agreement with the hypothesis that BP prevention could be associated to the increased Ca2+ concentration in the apoplast and a coordinated regulation of specific Ca2+-related genes.