Japanese apricot (Prunus mume) is remarkably valuable for its high ornamental and economic importance due to its distinctive features. Low temperature is a serious environmental constraint for this species, restricting its cultivation and dispersal in the north of China. To address this issue, breeding requires an understanding of the molecular mechanisms underlying responses to cold stress. We examined the leaf physiological and transcriptome profile by RNA sequencing in ‘Bungo’ scion cultivar grafted onto Prunus mume (cold-sensitive) and Prunus armeniaca (cold-tolerant) rootstocks at 4 °C for 0, 6, and 24 h. Our results revealed that the increased MDA concentration in the leaves of P. mume cultivar (cold-sensitive) suggests that cold stress might cause oxidative damage and increased sensitivity. Moreover, the cold-tolerant cultivar (P. armeniaca) considerably enhances the enzyme activities (i.e., SOD, POD, and CAT), as well as osmo-protectants (soluble sugars and proline) compared with sensitive cultivar, which helps plants to withstand oxidative damage caused by cold stress. Additionally, differentially expressed genes were shown to be enriched in plant hormone signal transduction, ribosome, MAPK signaling, and circadian rhythm pathway. After 24 h of cold stress, genes related to PYL4, histidine kinase 1, SAUR36, bHLH130, bHLH123, TIFY 6B-like, WRKY 40, WRKY 57, and 60S acidic ribosomal protein P1 were differentially expressed, implying that these DEGs involved in multiple pathways are involved in cold tolerance in Japanese apricot. This study improved our current understanding of the mechanism of cold tolerance in Japanese apricot, and the findings could be utilized for other related fruit species.

Background: Japanese apricot (Prunus mume Sieb. et Zucc.) is popular for both ornamental and processing value, fruit color affects the processing quality, and red pigmentation is the most obvious phenotype associated with fruit color variation in Japanese apricot, mutations in structural genes in the anthocyanin pathway can disrupt the red pigmentation, while the formation mechanism of the red color trait in Japanese apricot is still unclear. Results: One SNP marker (PmuSNP_27) located within PmUFGT3 gene coding region was found highly polymorphic among 44 different fruit skin color cultivars and relative to anthocyanin biosynthesis in Japanese apricot. Meantime, critical mutations were identified in two alleles of PmUFGT3 in the green-skinned type is inactivated by seven nonsense mutations in the coding region, which leads to seven amino acid substitution, resulting in an inactive UFGT enzyme. Overexpression of the PmUFGT3 allele from red-skinned Japanese apricot in green-skinned fruit lines resulted in greater anthocyanin accumulation in fruit skin. Expression of same allele in an Arabidopsis T-DNA mutant deficient in anthocyanidin activity the accumulation of anthocyanins. In addition, using site-directed mutagenesis, we created a single-base substitution mutation (G to T) of PmUFGT3 isolated from green-skinned cultivar, which caused an E to D amino acid substitution and restored the function of the inactive allele of PmUFGT3 from a green-skinned individual. Conclusion: This study confirms the function of PmUFGT3, and provides insight into the mechanism underlying fruit color determination in Japanese apricot, and possible approaches towards genetic engineering of fruit color.

Self-incompatibility (SI) is a complex mechanism that prevents plants from self-fertilizing to preserve and promote genetic variability. The angiosperm species have developed two different SI systems, the sporophytic (SSI) and the gametophytic (GSI) systems. SI is a significant impediment to steady fruit production in fruit tree species of the Rosaceae. In Rosaceae, GSI is genetically regulated via a single locus, named the ‘S-locus’, which includes a minimum of two polymorphic and relatively intercorrelated S genes: a pistil-expressed S-RNase gene and several pollen-expressed SFBB (S-locus F-Box Brothers) or SFB (S haplotype-specific F-box protein). This necessitates the interaction of S-RNases with the male determinants. Although genetic and molecular analyses of S genes have shown that mutations in both pistils and pollen-specific components induce self-compatibility in many species and cultivars, other genes or molecules outside the S-locus can co-participate in the male gamete rejection in GSI. However, we highlight and synthesize the most recent knowledge on different mechanisms of GSI in Rosaceae in this current review.

Prunus zhengheensis, an extremely rare population of apricots, originated in warm South-East China and is an excellent material for genetic breeding. However, most apricots and two related species (P. sibirica, P. mandshurica) are found in the cold northern regions in China and the mechanism of their distribution is still unclear. In addition, the classification status of P. zhengheensis is controversial. Thus, we generated a high-quality haplotype-resolved genome for P. zhengheensis, exploring key genetic variations in its adaptation and the causes of phylogenetic incongruence. We found extensive phylogenetic discordances between the nuclear and organelle phylogenies of P. zhengheensis, which could be explained by incomplete lineage sorting. A 242.22-Mb pan-genome of the Armeniaca section was developed with 13 chromosomal genomes. Importantly, we identified a 566-bp insertion in the promoter of the HSFA1d gene in apricot and showed that the activity of the HSFA1d promoter increased under low temperatures. In addition, HSFA1d overexpression in Arabidopsis thaliana indicated that HSFA1d positively regulated plant growth under chilling. Therefore, we hypothesized that the insertion in the promoter of HSFA1d in apricot improved its low-temperature adaptation, allowing it to thrive in relatively cold locations. The findings help explain the weather adaptability of Armeniaca plants.

Japanese apricot (Prunus mume Sieb. et Zucc.) is among the most valued fruits and flowering plants in eastern Asia. However, few comparative studies have been conducted with respect to its agro-morphological and pomological traits, chloroplast (cp) genome sequences and plastid diversity. Therefore, a comparative study was, conducted to investigate the divergence and geographic distribution of ten Japanese apricot accessions from three Chinese provinces (Zhejiang, Jiangsu and Sichuan). Phenotypic characteristics of the evaluated accessions, such as leaf length, tip leaf length, flower diameter, anther number, fruit weight, longitudinal height, transversal height, lateral height, fruit stone weight, stone longitudinal height, stone transversal height, stone lateral heigh, titratable acid content and total soluble solids, varied significantly (p < 0.05) among the ten investigated accessions. On the other hand, most of the investigated accessions were statistically similar within the same province. Comparing the Cp genomes of P. mume accessions with those of the genus Prunus revealed a similarity in structure and composition with slight differences. “Bayes empirical Bayes” (BEB) analysis in Prunus species, including P. mume, revealed BEB in rps16, rps3, rpoC1(4*), rpl32, rpl16, rbcL, psbF, petB, ndhF, clpP and ccsA genes. The BEB value of the rpoC1 gene is higher than 0.95, indicating that it is potentially under positive selection. Interestingly, the accessions from the same province of origin had the same number of forward repeat sequences. Furthermore, all accessions from Zhejiang province had the same number of simple sequence repeats. Similarly, nucleotide deletion/insertion of the ycf1 sequence and the results of phylogenetic trees revealed that accessions were mainly clustered according to their province of origin. Our comparative study of agronomical traits, chloroplast composition, structure, nucleotide variability of cp genome and phylogeography in Japanese apricot accessions provides valuable information on their diversity and geographic distribution.

The Knotted1-like Homeobox gene is crucial for plant morphological development and growth. Physicochemical characteristics, phylogenetic relationships, chromosomal localization, cis-acting elements, and tissue-specific expression patterns of the 11 PmKNOX genes found in the Japanese apricot genome in this study were examined. Proteins of 11 PmKNOX were soluble proteins with isoelectric points between 4.29 and 6.53, molecular masses between 15.732 and 44.011 kDa, and amino acid counts between 140 and 430. The identified PmKNOX gene family was split into three subfamilies by jointly constructing the phylogenetic tree of KNOX proteins in Japanese apricot and Arabidopsis thaliana. Combined outcomes of the analyzed conserved motifs and gene structures of the 11 PmKNOX genes from the same subfamily displayed comparable gene structure and motif patterns. The 11 PmKNOX members were distributed across six chromosomes, while two sets of PmKNOX genes were found to be collinear. Analysis of the 2000 bp promoter upstream of the coding region of the PmKNOX gene revealed that most PmKNOX genes might be involved in the physiological metabolism, growth and development processes of plants. The PmKNOX gene expression profile revealed that these genes were expressed at varying levels in different tissues, and most of them were linked to the meristems of leaf and flower buds, suggesting that PmKNOX may be involved in plants’ apical meristems. In Arabidopsis thaliana, functional validation of PmKNAT2a and PmKNAT2b revealed that these two genes might be involved in regulating leaf and stem development. In addition to laying the groundwork for future research on the function of these genes, understanding the evolutionary relationships between members of the PmKNOX gene family provides opportunities for future breeding in Japanese apricots.