Valsa mali PR1-like protein modulates an apple valine-glutamine protein to suppress JA signaling-mediated immunity.

Apple Valsa canker (AVC) is a devastating disease of apple (Malus × domestica), caused by Valsa mali (Vm). The Cysteine-rich secretory protein, Antigen 5, and Pathogenesis-related protein 1 (CAP) superfamily protein PATHOGENESIS-RELATED PROTEIN 1-LIKE PROTEIN c (VmPR1c) plays an important role in the pathogenicity of Vm. However, the mechanisms through which it exerts its virulence function in Vm-apple interactions remain unclear. In this study, we identified an apple valine-glutamine (VQ)-motif-containing protein, MdVQ29, as a VmPR1c target protein. MdVQ29-overexpressing transgenic apple plants showed substantially enhanced AVC resistance as compared with the wild type. MdVQ29 interacted with the transcription factor MdWRKY23, which was further shown to bind to the promoter of the jasmonic acid (JA) signaling-related gene CORONATINE INSENSITIVE 1 (MdCOI1) and activate its expression to activate the JA signaling pathway. Disease evaluation in lesion areas on infected leaves showed that MdVQ29 positively modulated apple resistance in a MdWRKY23-dependent manner. Furthermore, MdVQ29 promoted the transcriptional activity of MdWRKY23 toward MdCOI1. In addition, VmPR1c suppressed the MdVQ29-enhanced transcriptional activation activity of MdWRKY23 by promoting the degradation of MdVQ29 and inhibiting MdVQ29 expression and the MdVQ29-MdWRKY23 interaction, thereby interfering with the JA signaling pathway and facilitating Vm infection. Overall, our results demonstrate that VmPR1c targets MdVQ29 to manipulate the JA signaling pathway to regulate immunity. Thus, this study provides an important theoretical basis and guidance for mining and utilizing disease-resistance genetic resources for genetically improving apples.

Global hypermethylation of the N6-methyladenosine RNA modification associated with apple heterografting.

Grafting can facilitate better scion performance and is widely used in plants. Numerous studies have studied the involvement of mRNAs, small RNAs, and epigenetic regulations in the grafting process. However, it remains unclear whether the mRNA N6-methyladenosine (m6A) modification participates in the apple (Malus x domestica Borkh.) grafting process. Here, we decoded the landscape of m6A modification profiles in 'Golden delicious' (a cultivar, Gd) and Malus prunifolia 'Fupingqiuzi' (a unique rootstock with resistance to environmental stresses, Mp), as well as their heterografted and self-grafted plants. Interestingly, global hypermethylation of m6A occurred in both heterografted scion and rootstock compared with their self-grafting controls. Gene Ontology (GO) term enrichment analysis showed that grafting-induced differentially m6A-modified genes were mainly involved in RNA processing, epigenetic regulation, stress response, and development. Differentially m6A-modified genes harboring expression alterations were mainly involved in various stress responses and fatty acid metabolism. Furthermore, grafting-induced mobile mRNAs with m6A and gene expression alterations mainly participated in ABA synthesis and transport (e.g. carotenoid cleavage dioxygenase 1 [CCD1] and ATP-binding cassette G22 [ABCG22]) and abiotic and biotic stress responses, which might contribute to the better performance of heterografted plants. Additionally, the DNA methylome analysis also demonstrated the DNA methylation alterations during grafting. Downregulated expression of m6A methyltransferase gene MdMTA (ortholog of METTL3) in apples induced the global m6A hypomethylation and distinctly activated the expression level of DNA demethylase gene MdROS1 (REPRESSOR OF SILENCING 1) showing the possible association between m6A and 5mC methylation in apples. Our results reveal the m6A modification profiles in the apple grafting process and enhance our understanding of the m6A regulatory mechanism in plant biological processes.

The MdWRKY31 transcription factor binds to the MdRAV1 promoter to mediate ABA sensitivity.

The phytohormone abscisic acid (ABA) is a major element involved in apple (Malus domestica) production because of its role in seed germination and early seedling development. The WRKY family, which is one of the largest families of transcription factors, plays an important role in ABA signaling in plants. However, the underlying molecular mechanisms of WRKY-mediated ABA sensitivity in apple are poorly understood. A genome-wide transcriptome analysis indicated that MdWRKY31 is a key factor induced by ABA. Quantitative real-time PCR showed that MdWRKY31 is induced by ABA in response to PEG4000, which is used to simulate drought. As a transcription factor, MdWRKY31 is localized in the nucleus. Ectopic expression of MdWRKY31 in Arabidopsis and Nicotiana benthamiana enhanced plant sensitivity to ABA. Overexpression of MdWRKY31 in apple roots and apple calli increased sensitivity to ABA, whereas repression of MdWRKY31 reduced sensitivity to ABA in the roots of 'Royal Gala'. Electrophoretic mobility shift assays, chromatin immunoprecipitation PCR, and yeast one-hybrid assays indicated that MdWRKY31 directly binds to the promoter of MdRAV1. Expression analyses of transgenic apple calli containing MdWRKY31 and pMdRAV1::GUS implied that MdWRKY31 represses the expression of MdRAV1. We also found that MdRAV1 binds directly to the promoters of MdABI3 and MdABI4 and repressed their expression. Our findings reveal a new important regulatory mechanism of MdWRKY31-MdRAV1-MdABIs in the ABA signaling pathway in apple.