• Five QTLs associated with weeping traits on chromosome 7 were identified by BSR-seq.

Weeping species are used both as ornamental plants and for breeding dwarf plant types. However, exploration of casual genes controlling weeping traits is rather limited. Here, we identified individuals with contrasting phenotypes from an F₁ bi-parental mapping population of Prunus mume which was developed from a cross between the upright cultivar ‘Liuban’ and the weeping cultivar ‘Fentai Chuizhi’. Bulked segregant RNA sequencing was used and five QTLs on Chromosome 7 were identified. The Pm024074 (PmUGT72B3) allele, belonging to the UDP-glycosyltransferase superfamily containing the coniferyl-alcohol glucosyltransferase domain, was identified in a genomic region overlapping with a previously identified QTL, and had a synonymous transition of T⁶⁶ (upright) to C (weeping) in the coding sequence and a 470-bp deletion in the promoter region. Pm024074 had exceptionally high expression in buds and stems of weeping P. mume. Weighted correlation network analysis indicates that genes neighboring Pm024074 were significantly associated with plant architecture. In addition, a reliable single nucleotide polymorphism marker was developed based on the variation in the Pm024074 gene, providing precise marker-assisted breeding for weeping traits. This study provides insights into the genetic mechanism governing the weeping trait in P. mume, and indicates potential applications for the manipulation of tree architecture.

• Metabolites of fresh tea shoots at harvest were profiled.

Metabolites, especially secondary metabolites, are very important in the adaption of tea plants and the quality of tea products. Here, we focus on the seasonal variation in metabolites of fresh tea shoots and their regulatory mechanism at the transcriptional level. The metabolic profiles of fresh tea shoots of 10 tea accessions collected in spring, summer, and autumn were analyzed using ultra-performance liquid chromatography coupled with quadrupole-obitrap mass spectrometry. We focused on the metabolites and key genes in the phenylpropanoid/flavonoid pathway integrated with transcriptome analysis. Multivariate statistical analysis indicates that metabolites were distinctly different with seasonal alternation. Flavonoids, amino acids, organic acids and alkaloids were the predominant metabolites. Levels of most key genes and downstream compounds in the flavonoid pathway were lowest in spring but the catechin quality index was highest in spring. The regulatory pathway was explored by constructing a metabolite correlation network and a weighted gene co-expression network.

• The contents of anthocyanin and AsA in red-flesh apples are higher than that in non-red-flesh apples.

Ascorbic acid (AsA, vitamin C) is involved in the regulation of many aspects of plant growth and development. It is an essential micronutrient for humans and can prevent scurvy, maintain the health of gums and blood vessels, reduce the level of plasma cholesterol and enhance the immune systen. Apple cultivars Orin and Guanghui were crossed to obtain a group of hybrid offspring with and without red flesh in the course of assessing apple germplasm resources. Unexpectedly, the red-flesh apples had higher AsA contents than other apples. Further studies showed that the anthocyanin biosynthetic regulator MdMYB1 directly activates the expression of dehydroascorbate reductase gene MdDHAR, thus promoting the activity of the DHAR enzyme and the accumulation of AsA. This finding reveals the mechanism leading to high AsA levels in red-flesh apples and suggests a new idea to cultivate red-flesh apples with high AsA contents and produce AsA efficiently and without pollution.

• A LEA family gene (PtrLEA7)was cloned from Poncirus trifoliata.

Late embryogenesis abundant (LEA) genes encode highly hydrophilic proteins that are essential in abiotic stress responses. However, most LEA genes in higher plants have not yet been investigated. This study identified an LEA family gene (PtrLEA7) from Poncirus trifoliata and studied its function in drought tolerance. The full-length coding sequence of PtrLEA7 was 420 bp encoding a protein of 139 amino acids. Phylogenetic analysis shows that PtrLEA7 protein belongs to the LEA_4 subfamily. Expression profiling by qPCR found that PtrLEA7 was strongly induced by dehydration, cold and ABA treatments, and slightly induced by salt stress. Subcellular localization reveals that PtrLEA7 protein was located in both cytoplasm and nucleus. To investigate its function, transgenic plants of both tobacco and Poncirus trifoliata overexpressing PtrLEA7 were obtained. Stress tolerance assays show that overexpression lines had enhanced dehydration and drought tolerance compared with wild type plants, indicating that PtrLEA7 positively regulates drought tolerance. In addition, transgenic plants had much higher expression levels of three antioxidant enzyme genes (CAT, SOD and POD) and significantly increased catalase enzyme activity, accompanied by reduced reactive oxygen species accumulation in comparison with wild type plants. Collectively, this study demonstrates that PtrLEA7 can confer enhanced drought tolerance partially via enhancing antioxidant capacity.

•MdSIZ1 RNAi transgenic apple trees are drought tolerance than wild type—GL-3.

Drought stress typically causes heavy losses in apple production and uncovering the mechanisms by which apple tolerates drought stress is important in apple breeding. MdSIZ1 is a SUMO (small ubiquitin-like modifier) E3 ligase that promotes SUMO binding to substrate proteins. Here, we demonstrate that MdSIZ1 in apple has a negative relationship with drought tolerance. MdSIZ1 RNAi transgenic apple trees had a higher survival rate after drought stress. During drought stress they had higher leaf water potential, reduced ion leakage, lower H₂O₂ and malondialdehyde contents, and higher catalase activity. In addition, MdSIZ1 RNAi transgenic plants had a higher net photosynthetic rate during the latter period of drought stress. Finally, the transgenic apple trees also altered expression levels of some microRNAs in response to drought stress. Taken together, these results indicate that apple MdSIZ1 negatively regulates drought stress by enhancing leaf water-holding capacity and antioxidant enzyme activity.

• D1 turnover plays a more important role than xanthophyll cycle in photoprotection under sub-high temperature and high light (HH) conditions in tomato.

D1 protein turnover and the xanthophyll cycle (XC) are important photoprotective mechanisms in plants that operate under adverse conditions. Here, streptomycin sulfate (SM) and dithiothreitol (DTT) were used in tomato plants as inhibitors of D1 protein turnover and XC to elucidate their photoprotective impacts under sub-high temperature and high light conditions (HH, 35°C, 1000 µmol·m^-2·s^-1). SM and DTT treatments significantly reduced the net photosynthetic rate, apparent quantum efficiency, maximum photochemical efficiency, and potential activity of photosystem II, leading to photoinhibition and a decline in plant biomass under HH. The increase in reactive oxygen species levels resulted in thylakoid membrane lipid peroxidation. In addition, there were increased non-photochemical quenching and decreased chlorophyll pigments in SM and DTT application, causing an inhibition of D1 protein production at both transcriptional and translational levels. Overall, inhibition of D1 turnover caused greater photoinhibition than XC inhibition. Additionally, the recovery levels of most photosynthesis indicators in DTT-treated plants were higher than in SM-treated plants. These findings support the view that D1 turnover has a more important role than XC in photoprotection in tomato under HH conditions.

• A novel netted-cracking fruit phenotype was discovered in tomato introgression line IL4-4.

Fruit cracking is a major disorder that affects the integrity of fruit and reduces the commercial value of tomato and other fleshy fruit. Here, we have found a novel fruit ‘netted-cracking’ (FNC) phenotype in tomato introgression line IL4-4 which is present in neither the donor parent (LA0716) nor the receptor parent (M82). An F₂ population was generated by crossing IL4-4 with M82 to genetically characterize the FNC gene and this showed that a single dominant gene determined fruit netted-cracking. Further map-based cloning narrowed down the FNC locus to a 230 kb region on chromosome 4. Sequencing and annotation analysis show that FNC (Solyc04 g082540) was the most likely candidate gene. Functional characterization of FNC by overexpressing FNC^AC and FNC^IL4-4 resulted in the fruit netted-cracking phenotype, suggesting that the FNC transcript level results in the functional gain of fruit netted-cracking. These findings were further confirmed by FNC ortholog in netted-cracking pepper and melon, indicating a common regulatory mechanism in different plant species. Furthermore, cytoplasm and nucleus-localized FNC indicates increased expression of genes involved in suberin, lignin, lipid transport and cell wall metabolism. These findings provide novel genetic insights into fruit netted-cracking and offer a way to promote molecular improvement toward cracking resistant cultivars.

• Herbivory and mechanical wounding elicited electrical signals.

Electrical signals commonly occur in plants in response to various environmental changes and have a dominant function in plant acclimation. The transduction of wound-elicited electrical signals in the model plant species Arabidopsis has been characterized but the characteristics of electrical signal transduction in response to herbivory or wounding in crop species remain unknown. Here, the features of electrical signals elicited by insect herbivory and wounding in tomato were investigated. Unlike those in Arabidopsis, wounding tomato leaves did not cause leaf-to-leaf electrical signal transduction. In contrast, electrical signals elicited in response to petiole wounding were stronger and more strongly transduced. Leaflet wounding also activated electrical signal transduction and jasmonic acid (JA) signaling within the whole compound leaf. It was also demonstrated that tomato glutamate receptor-like 3.3 (GLR3.3) and GLR3.5 mediated leaflet-to-leaflet electrical signal transduction. Herbivory-induced JA accumulation and Helicoverpa armigera resistance were reduced in glr3.3/3.5 plants. This work reveals the nature of electrical signal transduction in tomato and emphasizes the key roles of GLR3.3 and GLR3.5 in electrical signal transduction and JA signaling activation.

• In addition to triploid progeny, tetraploid hybrids derived from the fertilization of 2n megagametophytes are frequently regenerated from 2x × 4x crosses that utilize ‘Orah’ mandarin as the female parent.

Seedless fruits are desirable in the citrus fresh fruit market. Triploid production via diploid × tetraploid interploidy crosses is thought to be the most efficient and widely-used strategy for the breeding of seedless citrus. Although ‘Orah’ mandarin has desirable organoleptic qualities, seeds in the fruits weaken its market competitiveness. To produce new seedless cultivars that are similar to ‘Orah’ mandarin, we performed three 2x × 4x crosses using ‘Orah’ mandarin as the seed parent to regenerate triploid plantlets. A total of 182 triploid and 36 tetraploid plantlets were obtained. By analyzing their genetic origins using nine novel single nucleotide polymorphism (SNP) markers, all of the triploids and tetraploids derived from these three crosses were proven to be hybrids. Also, we demonstrated that 2n megagametophyte formation in ‘Orah’ mandarin result in tetraploid production in these three interploidy crosses. These tetraploid plantlets were genotyped using eight pericentromeric SNP markers and nine centromere distal SNP markers. Based on the genotypes of the 2n megagametophytes, the parental heterozygosity rates in 16 SNP loci and all 2n megagametophytes were less than 50%, indicating that second division restitution was the mechanism underlying 2n megagametophyte formation at both the population and individual levels. These triploid hybrids enrich the germplasm available for seedless breeding. Moreover, the tetraploid hybrids are valuable as parents for ploidy breeding for the production of seedless citrus fruits.

• The dynamic interplay between phytohormones plays an important part in climacteric fruit ripening.

Fruit ripening is a complex developmental process made up of genetically programmed physiological and biochemical activities. It culminates in desirable changes in the structural and textural properties and is governed by a complex regulatory network. Much is known about ethylene, one of the most important metabolites promoting the ripening of climacteric fruits. However, the dynamic interplay between phytohormones also plays an important part. Additional regulatory factors such as transcription factors (TFs) and epigenetic modifications also play vital role in the regulation of climacteric fruit ripening. Here, we review and evaluate the complex regulatory network comprising interactions between hormones and the action of TFs and epigenetic modifications during climacteric fruit ripening.

• China is the largest producer of pomelo globally.

Pomelo is a member of the genus Citrus that is a key contributor to the breeding of modern citrus cultivars. China is the largest producer of pomelo and one of the top five pomelo exporting countries. Pomelos from Thailand are also well-known for their excellent quality and flavor and are ranked in the top ten export countries. This review introduces pomelo planting locations and conditions in China and Thailand. The characteristics and qualities of some commercial pomelo cultivars in China and Thailand are summarized to introduce them to international consumers and to document their similarities and dissimilarities. Data on bioactive compounds and antioxidant capacity are also included for most Chinese and Thai pomelos to highlight how they differ in this aspect because consumers are increasingly interested in healthier foods. In addition, the sensory perception in terms of aroma, flavor, texture and taste attributes and consumer perspective and preferences are discussed.

The results presented in “The anthocyanin biosynthetic regulator MdMYB1 positively regulates ascorbic acid biosynthesis in apple” (An et al., this issue) provide evidence for a new mechanism for the elevation of ascorbate concentration in apple. Using a red-fleshed apple breeding population, the authors show how the anthocyanin-regulating MYB transcription factor, MdMYB1, also increases ascorbate concentrations by directly activating transcription of the dehydroascorbate reductase gene MdDHAR. This gene recycles oxidized ascorbate back to ascorbate, leading to elevated concentrations of vitamin C. These red-fleshed apples have enhanced concentrations of both anthocyanins and ascorbate, both of which are appealing traits for the development of healthier apples.