Salicylic acid (SA) is known to be an efficient elicitor of secondary metabolism in plants. Arbutin, a characteristic phenolic glycoside found in ‘Yuluxiang’ pear (Pyrus bretschneideri Rehder × Pyrus sinkiangensis Yu), is widely used in lightening agents, in addition to cough, anti-inflammatory, and anti-microbial remedies, among other applications. However, research into the synthesis of arbutin in pear is limited. This study aimed to clarify the effect of exogenous SA on the arbutin content of pear using HPLC and investigate the mechanism for arbutin accumulation using RNA-Seq analysis. HPLC revealed that SA increased the arbutin contents of leaf, fruit, and callus in pear and demonstrated that the effect of SA is concentration and time dependent. RNA-Seq analysis of pear callus treated with or without SA revealed 380 differentially expressed genes (DEGs), 335 of which were up-regulated. According to a KEGG database analysis, the highest number of genes were annotated for phenylpropane biosynthesis. Overall, 21 DEGs were found to be involved in the synthesis of hydroquinone and UDP-glucose, which are substrates of arbutin synthesis. It is noteworthy that the expression levels of three up-regulated genes (Pbr006844.1, Pbr021064.1 and Pbr021069.1) related to hydroquinone glycosyltransferase were induced by SA and hydroquinone. Furthermore, transient overexpression of PbUGT72B1 (Pbr021069.1) increased the arbutin content in pear callus. These data explain the regulation of gene transcription associated with the promotive effect of SA on arbutin biosynthesis in pear, thus providing a theoretical foundation for enhancing the arbutin content of fruit through genetic engineering.

The responses of Ca²⁺ signaling mechanism to salt stress in pears are poorly understood. In this study, we investigated the difference in the Ca²⁺ signal responses to NaCl stress in two pear species (Pyrus betulaefolia Bunge. and P. bretschneideri Rehd. cv. ‘Xuehua’). Pear protoplasts were treated with 100 mM NaCl alone and NaCl combined with various chemical agents before changes in intracellular Ca²⁺ levels and the expression of Ca²⁺ sensor-related genes were analyzed. NaCl stress caused elevated Ca²⁺ levels in protoplasts labeled with the calcium indicator Fluo-3/AM. The Ca²⁺ signal increased earlier in P. betulaefolia than in the ‘Xuehua’ cultivar. The cytoplasmic Ca²⁺ bursts induced by NaCl stress were significantly constrained by the chemical agents supplied to both species. The plasma-membrane Ca²⁺ channel inhibitor LaCl₃ and extracellular Ca²⁺ chelator EGTA had greater inhibitory effects than the intracellular Ca²⁺ chelator BAPTA/AM. Under NaCl stress, upregulation of PbCBL10 and genes in the classes PbCDPK and PbCIPK occurred more quickly in P. betulaefolia than the ‘Xuehua’ cultivar. In conclusion, NaCl stress stimulates Ca²⁺ signaling in pear cells, the ions for which are taken from extracellular and intracellular Ca²⁺ stores. Furthermore, the accumulation of cytoplasmic Ca²⁺ mediates early expression of Ca²⁺ sensor-related genes, especially PbCDPK1. Thus the response of Ca²⁺ signal plays pivotal roles, which enhance the salt tolerance of pear.

Rootstocks largely determine the tree architecture of the grafted scions, significantly affects yield, suitability for mechanical harvesting, and planting pattern of apple orchards. It is thus important to reveal the mechanisms behind the rootstocks influence on the tree architecture of scions in apple trees. This study analyzed the grafting survival rate, the physiological parameters including plant growth, photosynthesis and nutrient accumulation in the apple variety ‘Harlikar’ with eight apple rootstocks. We also explored the mechanism of scion architecture formation using transcriptomics based on different scion/rootstock combinations. The results indicated that ‘Harlikar’ had the lowest grafting survival rate with rootstock ‘M26’, with less callus formed at the graft interface, foliage etiolation, and weak photosynthetic capacity. While ‘Harlikar’ had better affinities with ‘M9-T337’, ‘M9-Nic29’, ‘M9-Pajam2’, ‘B9’, ‘71-3-150’, ‘Qingzhen 2’, and ‘Malus baccata’. Among these, the highest plant height and the highest number of lateral branches were observed in ‘Harlikar’ with rootstock ‘Qingzhen 2’, they were 1.12-times and 2.0-times higher than ‘Harlikar’ with vigorous rootstock ‘M. baccata’, respectively. The highest accumulations of total nitrogen, total phosphorus, and total potassium in scions were observed in ‘Harlikar’/‘Qingzhen 2’, they were 2.22-times, 2.10-times, and 11.80-times higher than that in ‘Harlikar’/‘M. baccata’. The lowest plant height was observed in ‘Harlikar’/‘71-3-150’, only 50.47% of ‘Harlikar’/‘Qingzhen 2’ and 56.51% of ‘Harlikar’/‘M. baccata’, and the lowest internode length was observed in ‘Harlikar’/‘M9-Nic29’, only 60.76% of ‘Harlikar’/‘Qingzhen 2’ and 79.11% of ‘Harlikar’/‘M. baccata’. The transcriptome, weighted gene co-expression network and KEGG enrichment analyses revealed that, compared to ‘Harlikar’/‘M. baccata’, most differentially expressed genes screened from ‘Harlikar’/‘Qingzhen 2’, ‘Harlikar’/‘71-3-150’, and ‘Harlikar’/‘M9-Nic29’ were enriched in hormone signal transduction pathways. Specifically, auxin-repressed protein gene ARP, cytokinin synthesis related genes CKXs and CYP92A6, and brassinosteroid synthesis related gene CYP87A3 were involved in the dwarfing of ‘Harlikar’/‘71-3-150’ and ‘Harlikar’/‘M9-Nic29’. Cytokinin synthesis related gene ARR-A and abscisic acid-responsive element binding factor gene ABF were the key to increased branching in ‘Harlikar’/‘Qingzhen 2’. In addition, acid phosphatase genes ACPs, and serine/threonine-protein kinase genes PBLs were involved in the vegetative growth of scions in ‘Harlikar’/‘Qingzhen 2’ by affecting the absorption and utilization of nutrients. These results provide theoretical guidance for cultivating high-quality ‘Harlikar’ apple trees and elucidate the molecular mechanisms regulating plant height and lateral branch formation in apple.