The external application of acetic acid (AA) has been shown to improve drought survival in plants, such as Arabidopsis, rice, maize, wheat, rapeseed and cassava, and the application of AA also increased drought tolerance in perennial woody apple (Malus domestica) plants. An understanding of AA-induced drought tolerance in apple plants at the molecular level will contribute to the development of technology that can be used to enhance drought tolerance. In this study, the morphological, physiological and transcriptomic responses to drought stress were analyzed in apple plants after watering without AA (CK), watering with AA (AA), drought treatment (D) and drought treatment with AA (DA). The results suggested that the AA-treated apple plants had a higher tolerance to drought than water-treated plants. Higher levels of chlorophyll and carotenoids were found under the DA conditions than under D stress. The levels of abscisic acid (ABA), jasmonic acid (JA) and methyl jasmonate were increased in AA-treated apple plants. Transcriptomic profiling indicated the key biological pathways involved in metabolic processes, mitogen-activated protein kinase (MAPK) signaling, plant hormone signal transduction and the biosynthesis of secondary metabolites in response to different drought conditions. The 9-cis-epoxycarotenoid dioxygenase, (9S,13S)-cis-oxophytodienoic acid reductase, allene oxide synthase, allene oxide cyclase and lipoxygenase genes participate in the synthase of ABA and JA under drought and AA treatments. Collectively, the results showed that external application of AA enhanced drought tolerance in apple plants by influencing the ABA- and JA-induced MAPK signaling pathways. These data indicated that the application of AA in plants is beneficial for enhancing drought tolerance and decreasing growth inhibition in agricultural fields.

Background: Leaf colour mutations are universally expressed at the seedling stage and are ideal materials for exploring the chlorophyll biosynthesis pathway, carotenoid metabolism and the flavonoid biosynthesis pathway in plants. Results: In this research, we analysed the different degrees of albinism in apple (Malus domestica) seedlings, including white-leaf mutants (WM), piebald leaf mutants (PM), light-green leaf mutants (LM) and normal leaves (NL) using bisulfite sequencing (BS-seq) and RNA sequencing (RNA-seq). There were 61,755, 79,824, and 74,899 differentially methylated regions (DMRs) and 7566, 3660, and 3546 differentially expressed genes (DEGs) identified in the WM/NL, PM/NL and LM/NL comparisons, respectively. Conclusion: The analysis of the methylome and transcriptome showed that 9 DMR-associated DEGs were involved in the carotenoid metabolism and flavonoid biosynthesis pathway. The expression of different transcription factors (TFs) may also influence the chlorophyll biosynthesis pathway, carotenoid metabolism and the flavonoid biosynthesis pathway in apple leaf mutants. This study provides a new method for understanding the differences in the formation of apple seedlings with different degrees of albinism.

Potassium (K) is one of the most important macronutrients for plant development and growth. The influence mechanism of different potassium stresses on the molecular regulation and metabolites of apple remains largely unknown. In this research, physiological, transcriptome, and metabolite analyses were compared under different K conditions in apple seedlings. The results showed that K deficiency and excess conditions influenced apple phenotypic characteristics, soil plant analytical development (SPAD) values, and photosynthesis. Hydrogen peroxide (H₂O₂) content, peroxidase (POD) activity, catalase (CAT) activity, abscisic acid (ABA) content, and indoleacetic acid (IAA) content were regulated by different K stresses. Transcriptome analysis indicated that there were 2,409 and 778 differentially expressed genes (DEGs) in apple leaves and roots under K deficiency conditions in addition to 1,393 and 1,205 DEGs in apple leaves and roots under potassium excess conditions, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment showed that the DEGs were involved in flavonoid biosynthesis, photosynthesis, and plant hormone signal transduction metabolite biosynthetic processes in response to different K conditions. There were 527 and 166 differential metabolites (DMAs) in leaves and roots under low-K stress as well as 228 and 150 DMAs in apple leaves and roots under high-K stress, respectively. Apple plants regulate carbon metabolism and the flavonoid pathway to respond to low-K and high-K stresses. This study provides a basis for understanding the metabolic processes underlying different K responses and provides a foundation to improve the utilization efficiency of K in apples.

Leaf color mutations in plants serve as valuable resources for investigating metabolic pathways related to biosynthesis of chlorophyll, flavonoid, and carotenoid. In this study, a comprehensive analysis of the physiological characteristics, transcriptomes, and metabolomic profiles of a mutant of Malus hupehensis  var. pingyiensis  with red leaves (RM) and normal plant (CK) with green leaves was performed. A total of 283 differentially abundant metabolites (DAMs) and 9,822 differentially expressed genes (DEGs) were identified between RM and CK. Compared with the CK, RM presented a reduced chlorophyll content and elevated levels of carotenoids, flavonoids, and anthocyanins. KEGG pathway enrichment analysis revealed that the DAMs and DEGs were associated mainly with the metabolism of chlorophyll, carotenoids, and flavonoids, with the anthocyanidin biosynthesis pathway showing the greatest enrichment. Several key anthocyanin biosynthetic genes, including chalcone isomerase ( CHI ), dihydroflavonol 4-reductase ( DFR ), UDP-Glucose:Flavonoid 3-O-Glucosyltransferase ( UFGT ), and UDP-glycosyltransferases ( UGT ), were upregulated in the red leaves, in line with a significant increase in cyanidin 3-O-glucoside (C3G) and pelargonidin 3-O-glucoside (pg3G). Furthermore, RNA sequencing (RNA-seq) analysis revealed upregulation of MdMYB10 and MdUFGT . Yeast one-hybrid (Y1H), chromatin immunoprecipitation (ChIP), and dual-luciferase reporter (LUC) assays demonstrated that MdMYB10 bound to the MdUFGT promoter fragment and subsequently upregulated its expression, leading to the accumulation of C3G and pg3G in RM. Our findings provide new insights into the regulatory mechanisms of leaf coloration and offer a foundation for the development of new cultivars with alteration of leaf color.