Shoot branching is an important factor that influences the architecture of apple trees and cytokinin is known to promote axillary bud outgrowth. The cultivar ‘Fuji’, which is grown on ~75% of the apple-producing area in China, exhibits poor natural branching. The TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) family genes BRANCHED1/2 (BRC1/2) are involved in integrating diverse factors that function locally to inhibit shoot branching; however, the molecular mechanism underlying the cytokinin-mediated promotion of branching that involves the repression of BRC1/2 remains unclear. In this study, we found that apple WUSCHEL2 (MdWUS2), which interacts with the co-repressor TOPLESS-RELATED9 (MdTPR9), is activated by cytokinin and regulates branching by inhibiting the activity of MdTCP12 (a BRC2 homolog). Overexpressing MdWUS2 in Arabidopsis or Nicotiana benthamiana resulted in enhanced branching. Overexpression of MdTCP12 inhibited axillary bud outgrowth in Arabidopsis, indicating that it contributes to the regulation of branching. In addition, we found that MdWUS2 interacted with MdTCP12 in vivo and in vitro and suppressed the ability of MdTCP12 to activate the transcription of its target gene, HOMEOBOX PROTEIN 53b (MdHB53b). Our results therefore suggest that MdWUS2 is involved in the cytokinin-mediated inhibition of MdTCP12 that controls bud outgrowth, and hence provide new insights into the regulation of shoot branching by cytokinin.

Floral fragrance is an important trait that contributes to the ornamental properties and pollination of crabapple. However, research on the physiological and molecular biology of the floral volatile compounds of crabapple is rarely reported. In this study, metabolomic and transcriptomic analyses of the floral volatile compounds of standard Malus robusta flowers (Mr), and progeny with strongly and weakly fragrant flowers (SF and WF, respectively), were conducted. Fifty-six floral volatile compounds were detected in the plant materials, mainly comprising phenylpropane/benzene ring-type compounds, fatty acid derivatives, and terpene compounds. The volatile contents were significantly increased before the early flowering stage (ES), and the contents of SF flowers were twice those of WF and Mr flowers. Odor activity values were determined for known fragrant volatiles and 10–11 key fragrant volatiles were identified at the ES. The predominant fragrant volatiles were methyl benzoate, linalool, leaf acetate, and methyl anthranilate. In the petals, stamens, pistil, and calyx of SF flowers, 26 volatiles were detected at the ES, among which phenylpropane/benzene ring-type compounds were the main components accounting for more than 75% of the total volatile content. Functional analysis of transcriptome data revealed that the phenylpropanoid biosynthesis pathway was significantly enriched in SF flowers. By conducting combined analyses between volatiles and differentially expressed genes, transcripts of six floral scent-related genes were identified and were associated with the contents of the key fragrant volatiles, and other 23 genes were potentially correlated with the key volatile compounds. The results reveal possible mechanisms for the emission of strong fragrance by SF flowers, and provide a foundation for improvement of the floral fragrance and development of new crabapple cultivars.

Apple scar skin viroid (ASSVd) can infect apple trees and cause scar skin symptoms. However, the associated physiological mechanisms are unclear in young saplings. In this study, ASSVd-infected and control ‘Odysso’ and ‘Tonami’ apple saplings were examined to clarify the effects of ASSVd on apple tree growth and physiological characteristics as well as the leaf metabolome. The results indicated that leaf ASSVd contents increased significantly after grafting and remained high in the second year. Leaf size, tree height, stem diameter, branch length, and leaf photosynthetic efficiency decreased significantly in viroid-infected saplings. In response to the ASSVd infection, the chlorophyll a and b contents decreased significantly in ‘Odysso’, but were unchanged in ‘Tonami’. Moreover, the N, P, K, Fe, Mn, and Ca contents decreased significantly in the leaves of viroid-infected ‘Odysso’ or ‘Tonami’. Similarly, the CAT and POD contents decreased significantly in the viroid-infected saplings, but the SOD content increased in the viroid-infected ‘Tonami’ saplings. A total of 15 and 40 differentially abundant metabolites were respectively identified in the metabolome analyses of ‘Odysso’ and ‘Tonami’ leaves. Specifically, in the viroid-infected ‘Odysso’ and ‘Tonami’ samples, the L-2-aminobutyric acid, 6″-O-malonyldaidzin, and D-xylose contents increased, while the coumarin content decreased. These metabolites are related to the biosynthesis of isoflavonoids and phenylpropanoids as well as the metabolism of carbohydrates and amino acids. These results imply that ASSVd affects apple sapling growth by affecting physiological characteristics and metabolism of apple leaves. The study data may be useful for future investigations on the physiological mechanisms underlying apple tree responses to ASSVd.