Seed dormancy transition is a vital developmental process for seedling propagation and agricultural production. The process is precisely regulated by diverse endogenous genetic factors and environmental cues. Callery pear (Pyrus calleryana Decne) is an important rootstock species that requires cold stratification to break seed dormancy, but the mechanisms underlying pear seed dormancy release are not yet fully understood. Here, we analyzed the transcriptome profiles at three different stages of cold stratification in callery pear seeds using RNA sequencing combined with phytohormone and sugar content measurements. Significant alterations in hormone contents and carbohydrate metabolism were observed and reflected the dormancy status of the seeds. The expressions of genes related to plant hormone metabolism and signaling transduction, including indole-3-acetic acid (IAA) biosynthesis (ASAs, TSA, NITs, YUC, and AAO) genes as well as several abscisic acid (ABA) and gibberellic acid (GA) catabolism and signaling transduction genes (CYP707As, GA2ox, and DELLAs), were consistent with endogenous hormone changes. We further found that several genes involved in cytokinin (CTK), ethylene (ETH), brassionolide (BR), and jasmonic acid (JA) metabolism and signaling transduction were differentially expressed and integrated in pear seed dormancy release. In accordance with changes in starch and soluble sugar contents, the genes associated with starch and sucrose metabolism were significantly up-regulated during seed dormancy release progression. Furthermore, the expression levels of genes involved in lipid metabolism pathways were also up-regulated. Finally, 447 transcription factor (TF) genes (including ERF, bHLH, bZIP, NAC, WRKY, and MYB genes) were observed to be differentially expressed during seed cold stratification and might relate to pear seed dormancy release. Our results suggest that the mechanism underlying pear seed dormancy release is a complex, transcriptionally regulated process involving hormones, sugars, lipids, and TFs.

Watercore is a physiological disorder that commonly occurs in sand pear cultivars. The typical symptom of watercore tissue is transparency, and it is often accompanied by browning, breakdown and a bitter taste during fruit ripening. To better understand the molecular mechanisms of watercore affecting fruit quality, this study performed transcriptome and metabolome analyses on watercore pulp from “Akibae” fruit 125 days after flowering. The present study found that the “Akibae” pear watercore pulp contained higher sorbitol and sucrose than healthy fruit. Moreover, the structure of the cell wall was destroyed, and the content of pectin, cellulose and hemicellulose was significantly decreased. In addition, the content of ethanol and acetaldehyde was significantly increased, and the content of polyphenol was significantly decreased. Watercore induced up-regulated expression levels of sorbitol synthesis-related (sorbitol-6-phosphate dehydrogenase, S6PDH) and sucrose synthesis-related genes (sucrose synthesis, SS), whereas it inhibited the expression of sorbitol decomposition-related genes (sorbitol dehydrogenase, SDH) and sorbitol transport genes (sorbitol transporter, SOT). Watercore also strongly induced increased expression levels of cell wall-degrading enzymes (polygalactosidase, PG; ellulase, CX; pectin methylesterase, PME), as well as ethanol synthesis-related (alcohol dehydrogenase, ADH), acetaldehyde synthesis-related (pyruvate decarboxylase, PDC) and polyphenol decomposition-related genes (polyphenol oxidase, PPO). Moreover, the genes that are involved in ethylene (1-aminocyclopropane- 1-carboxylate oxidase, ACO; 1-aminocyclopropane- 1-carboxylate synthase, ACS) and abscisic acid (short-chain alcohol dehydrogenase, SDR; aldehyde oxidase, AAO) synthesis were significantly up-regulated. In addition, the bitter tasting amino acids, alkaloids and polyphenols were significantly increased in watercore tissue. Above all, these findings suggested that the metabolic disorder of sorbitol and sucrose can lead to an increase in plant hormones (abscisic acid and ethylene) and anaerobic respiration, resulting in aggravated fruit rot and the formation of bitter substances.

Grafting is a crucial horticultural propagation technique that plays a vital role in citrus production and research. Selecting compatible rootstock–scion combinations is essential for achieving high yields and superior fruit quality in citrus cultivation. This paper reviews recent advances in the physiological and molecular mechanisms involved in rootstock–scion interactions in citrus, with a focus on (1) commonly used rootstocks, (2) graft compatibility, (3) signal molecule transport at the graft union, and (4) the effects of rootstock–scion interactions on citrus growth, nutrient absorption, fruit quality, and responses to both biotic and abiotic stresses. Additionally, we prospected the future research direction and practical applications of rootstock–scion interactions.

Watercore is a physiological disorder which often occurs on the pear fruit and the excessive accumulation of sorbitol in fruit intercellular space is considered to be an important cause of watercore. Our previous studies found that the metabolic disorder of sugars may lead to hypoxia stress and disturb respiration, resulting in aggravated fruit rot and the formation of bitter substances. However, the further changes of respiration and the fruit response mechanism are not well understood. A comprehensive transcriptome analysis of ‘Akibae’ pear watercore fruit was performed in this study. The transcriptome results revealed the hypoxia stress significantly induced the expression of several key enzymes in the TCA cycle and may lead to the accumulation of succinic acid in watercore fruit. The glycolytic pathway was also significantly enhanced in watercore fruit. Moreover, the γ-aminobutyric acid (GABA) synthesis related genes, glutamate decarboxylase (GAD) genes and polyamine oxidase (PAO) genes, which associated with the GABA shunt and the polyamine degradation pathway were significantly upregulated. In addition, the PpGAD1 transcript level increased significantly along with the increase of GAD activity and GABA content in the watercore fruit. Above all, these findings suggested that the hypoxic response was marked by a significant increase of the hypoxia-inducible metabolites succinic acid and GABA and that PpGAD1 may play a key role in response to watercore by controlling the GABA synthesis.

Postharvest ripening of sand pear fruit leads to quality deterioration, including changes in texture, flavor, and fruit color. Salicylic acid (SA), an important defense-related hormone, delays fruit ripening and maintains fruit quality, but the underling mechanism remains unclear. Herein, we evaluated the efficacy of SA in delaying the ripening process of Pyrus pyrifolia cv. ’Hosui’ pear fruit, as evidenced by the reduction in fruit weight loss, inhibition of firmness loss, cell wall degradation and soluble sugars, and retention of total phenols. Based on comparative transcriptomic data, a total of 3837 and 1387 differentially expressed genes (DEGs) were identified during room-temperature storage of control fruit and between SA-treated and control fruit, respectively. Further KEGG analysis revealed that the DEGs were mainly implicated in plant hormone signal transduction, starch and sugar metabolism, and cell wall modification. Moreover, exogenous SA treatment also altered the expression of many transcription factor (TF) families, including those in the ethylene-responsive factor (ERF), NAM, ATAF, CUC (NAC), basic helix-loop-helix (bHLH), basic leucine zipper (bZIP), and v-myb avian myeloblastosis viral oncogene homolog (MYB) families. Together, the results offer important insights into the role of SA-responsive genes in controlling fruit ripening in sand pears.

Watercore is a physiological disorder which often occurs in Rosaceae fruits, and it causes hypoxia stress, promoting fruit decay. γ-aminobutyric acid (GABA) was reported as being involved in different abiotic stresses, and glutamate decarboxylase (GAD) is the key enzyme of GABA synthesis in plants. Our previous transcriptome analysis found that PpGAD2 was significantly induced in watercore fruit; however, the mechanism through which PpGAD2 regulates watercore-induced hypoxia stress resistance in pears is unclear. The present study found that the fruit pulp ethanol, malondialdehyde (MDA) and H₂O₂ content was significantly inhibited by exogenous GABA. The transcript abundance of PpGAD2 was significantly higher than that of other PpGADs in watercore fruit or healthy fruit. Tissue expression showed that the content of PpGAD2 in mature fruit was higher than in young fruit. Moreover, subcellular localization showed that PpGAD2 was located in the cytoplasm. Transient overexpression assays suggested that PpGAD2 had a role in GABA synthesis. Several CML (calmodulin–like) genes were also significantly increased in watercore fruit. Moreover, PpWRKY53 was significantly induced in watercore fruit, and the GUS activity assay showed that PpWRKY53 can significantly increase the activity of the PpGAD2 promoter. Taken together, these results demonstrate that PpGAD2 played an important role in GABA synthesis to increase plants’ resistance to hypoxia stress, and its activity may be affected by PpWRKY53 and several watercore-induced CML genes.