Apple NPR1 homologs and their alternative splicing forms may contribute to SA and disease responses

The non-expressor of pathogenesis-related genes 1 (NPR1) plays essential roles in the salicylic acid (SA) signal pathway and in systemic acquired resistance (SAR) responses. Although a genome-wide analysis of NPR1 gene family has been conducted in some plant species, little is known about these genes in apple (Malus spp.). In this study, eight NPR1 homologs were identified within the apple genome and 12 different transcripts were cloned by the reverse transcription-polymerase chain reaction. Based on these sequences, the gene structures and sequence alignment of the apple NPR1 homologs were analyzed. Phylogenetic analysis showed that apple NPR1 homologs could be classified into three groups as in Arabidopsis. Expression analysis demonstrated that NPR1 homologs showed different expression patterns in various tissues of apple. Under the induction of SA and MeJA, the transcription levels of some members were upregulated in leaves. Meantime, some NPR1 genes also showed significantly different expression levels between "Pacific Rose" and Malus baccata after inoculation with Marssonina coronaria. With the most similarity on amino acid sequence and expression pattern, MdNPR1 may function as a key regulator in SAR-like AtNPR1. These results suggested that the NPR1 genes may play an important role in plant immune responses, and their alternative splicing may contribute to disease resistance. Our study provides essential information about the NPR1 homologs in apple and contributes to the understanding of NPR1 homologs functions in other plants.