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.

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.

The fruit softening directly impacts its storage life, transportability, and customer acceptance. Auxin plays a key role during fruit ripening, but the underlying mechanisms of how auxin regulates fruit softening remain unclear. In this study, we investigated the regulatory roles of auxin on berry cell wall degradation during grape (Vitis vinifera L.) softening. During grape berry development, berry firmness and auxin content are both firstly increasing and then decreasing, and peaks occur at 4-6 WAFB (weeks after full blooming). Exogenous NAA (α-naphthalene acetic acid, a synthetic auxin) treatment inhibits berry softening by delaying propectin, cellulose and hemicellulose degradation, which maintains cell wall integrity in the grape flesh. Weighted gene co-expression network analysis (WGCNA) shows that VvLBD13 correlated with VvARF19 could be a key gene in this delaying of berry softening, which involved in auxin signal transduction and cell wall degradation metabolism. Over-expression and transient over-expression of VvLBD13 in tomato or in grape berry indicate that VvLBD13 accelerates hemicellulose degradation by binding the promoters of VvXTH10 (the xyloglucan endotransglucosylase/hydrolase 10) and VvEXPLA1 (expansion-like A1), which results in rapid softening after veraison. Collectively, this research furnishes an exhaustive understanding of the auxin- driven regulatory mechanisms of grape berry softening.