MdPR4, a pathogenesis-related protein in apple, is involved in chitin recognition and resistance response to apple replant disease pathogens.

To maximize breeding and exploitation of disease resistance traits for managing apple replant disease (ARD), it is of great importance to understand the mechanisms of apple root resistance. Currently, little is known about the functions of the specific genes that confer resistance traits in apple root. In this study, molecular, biochemical, and genetic approaches allowed an in-depth understanding of the role of the MdPR4 gene in the defense response of apple root. The MdPR4 encoding gene showed upregulation following ARD pathogen inoculation in our previous transcriptome data. Subcellular localization analyses revealed that MdPR4 is localized on the plasma membrane, endoplasmic reticulum, and apoplast, which is mainly determined by its signal peptide. Molecular docking analysis between MdPR4 protein with chitin molecule and in vitro MdPR4 chitin affinity assay proved its chitin-binding ability, which provided evidence for its role in chitin-mediated immune responses. Purified MdPR4 protein and MdPR4 overexpressed apple callus inhibited spore germination and mycelial growth of ARD-related Fusarium spp. pathogens. These data support the conclusion that MdPR4 is a chitin-binding protein in apple vegetative tissues that may play an important role in defense activation in response to ARD pathogen infection.

Major Latex Protein MdMLP423 Negatively Regulates Defense against Fungal Infections in Apple.

Major latex proteins (MLPs) play critical roles in plants defense and stress responses. However, the roles of MLPs from apple (Malus × domestica) have not been clearly identified. In this study, we focused on the biological role of MdMLP423, which had been previously characterized as a potential pathogenesis-related gene. Phylogenetic analysis and conserved domain analysis indicated that MdMLP423 is a protein with a 'Gly-rich loop' (GXGGXG) domain belonging to the Bet v_1 subfamily. Gene expression profiles showed that MdMLP423 is mainly expressed in flowers. In addition, the expression of MdMLP423 was significantly inhibited by Botryosphaeria berengeriana f. sp. piricola (BB) and Alternaria alternata apple pathotype (AAAP) infections. Apple calli overexpressing MdMLP423 had lower expression of resistance-related genes, and were more sensitive to infection with BB and AAAP compared with non-transgenic calli. RNA-seq analysis of MdMLP423-overexpressing calli and non-transgenic calli indicated that MdMLP423 regulated the expression of a number of differentially expressed genes (DEGs) and transcription factors, including genes involved in phytohormone signaling pathways, cell wall reinforcement, and genes encoding the defense-related proteins, AP2-EREBP, WRKY, MYB, NAC, Zinc finger protein, and ABI3. Taken together, our results demonstrate that MdMLP423 negatively regulates apple resistance to BB and AAAP infections by inhibiting the expression of defense- and stress-related genes and transcription factors.

Genome-wide identification and expression analysis of major latex protein (MLP) family genes in the apple (Malus domestica Borkh.) genome.

Major latex protein/ripening-related proteins (MLP/RRP) subfamily are a class of proteins that play crucial roles in response to defense and stress response. However, their biological function is still not clear, the identification and characterization will provide essential information for understanding their roles. Here, we carried out a genome-wide evolutionary characteristics and gene expression analysis of the MLP family in apple (Malus domestica, Borkh.). A total of 36 MdMLP genes were screened in apple genome. They were uneven located on 5 chromosomes, where were mainly arranged in tandem clusters, and the phylogenetic analysis put forward further views on the evolutionary relationship and putative functions among the genes. The conserved motifs showed that the MLP proteins which contained motif 1 had the potential function, and tissue-specific expression analysis showed that apple MLP members had diverse biological roles. Furthermore, the results showed seven of the MdMLPs that harbored cis-acting regulatory elements in response to defense and stress, and our expression data proved that they were involved in biotic stresses. The present study provides new views to the evolution and regulation of MdMLP genes, which represent objectives of future research and incorporate in resistance-related molecular breeding projects.

An Integrated Transcriptome and Proteome Analysis Reveals New Insights into Russeting of Bagging and Non-Bagging "Golden Delicious" Apple.

Apple skin russeting naturally occurs in many varieties, particularly in "Golden Delicious" and its pedigree, and is regarded as a non-invasive physiological disorder partly caused by excessive deposition of lignin. However, the understanding of its molecular mechanism is still limited. In this study, we used iTRAQ (isobaric tags for relative and absolute quantitation) and RNA-seq to detect the changes in the expression levels of genes and proteins in three developmental stages of russeting formation, in russeted (non-bagging) and non-russeted (bagging) skin of "Golden Delicious" apple. 2856 differentially expressed genes and 942 differentially expressed proteins in the comparison groups were detected at the transcript level and protein level, respectively. A correlation analysis of the transcriptomics and proteomics data revealed that four genes (MD03G1059200, MD08G1009200, MD17G1092400, and MD17G1225100) involved in lignin biosynthesis are significant changed during apple russeting formation. Additionally, 92 transcription factors, including 4 LIM transcription factors, may be involved in apple russeting formation. Among them, one LIM transcription factor (MD15G1068200) was capable of binding to the PAL-box like (CCACTTGAGTAC) element, which indicated it was potentially involved in lignin biosynthesis. This study will provide further views on the molecular mechanisms controlling apple russeting formation.