Genome-wide Identification and Expression Analysis of Apple ASR Gene Family in Response t­­o Alternaria alternata f. Sp. Mali

The ABA/water stress/ripening-induced (ASR) gene family exists universally in higher plants, and many genes are up-regulated during periods of environmental stresses, and fruit ripening. Although a considerable amount of research has been performed investigating ASR gene responses to abiotic stresses, relatively little is known about their roles in response to biotic stresses. In this report, we identified five ASR genes in apple (Malus x domestica) and explored their phylogenetic relationship, duplication events, and selective pressure. Five apple ASR genes (MdASR) were divided into two clades based on phylogenetic analysis. Species-specific duplication was detected in MdASR genes. Leaves of ‘Golden Delicious’ and ‘Starking’ were infected with Aternaria. alternata f. sp. mali, which causes apple blotch disease, and examined for the expression of the ASR genes in lesion areas during the first 72 h after inoculation. Expression of all five MdASR genes was up-regulated, but only MdASR1 was up-regulated at all sampling time with a highest expression level up to eight times in 'Golden Delicious' compared to ‘Starking’. The activities of stress-related enzymes, peroxidase (POD), superoxide dismutase (SOD), catalase (CAT), phenylalanine ammonia lyase (PAL), and polyphenoloxidase (PPO) and the content of malondialdehyde (MDA) were also measured. As the physiological indexes for disease resistance, the maximum activity of these enzymes and MDA content was measured in different stages of disease developement in different cultivars. Our research suggests that MdASR1 is very likely involved in biotic stress responses in apple.