Calcyclin-binding protein-promoted degradation of MdFRUCTOKINASE2 regulates sugar homeostasis in apple

Apple vacuolar sugar transporters regulated by MdDREB2A enhance drought resistance by promoting accumulation of soluble sugars and activating ABA signaling

Soluble sugars are not only an important contributor to fruit quality, but also serve as the osmotic regulators in response to abiotic stresses. Early drought stress promotes sugar accumulation, while specific sugar transporters govern the cellular distribution of the sugars. Here, we show that apple plantlets accumulate soluble sugars in leaf tissues under drought stress. Transcriptional profiling of stressed and control plantlets revealed differential expression of several plasma membrane-or vacuolar membrane-localized sugar transporter genes. Among these, four previously identified vacuolar sugar transporter (VST) genes (MdERDL6-1, MdERDL6-2, MdTST1, and MdTST2) showed higher expression under drought, suggesting their roles in response to drought stress. Promoter cis-elements analyses, yeast one-hybrid, and dual-luciferase tests confirmed that the drought-induced transcription factor MdDREB2A could promote the expression of MdERDL6-1/-2 and MdTST1/2 by binding to their promoter regions. Moreover, overexpressing of each of these four MdVSTs alone in transgenic apple or Arabidopsis plants accumulated more soluble sugars and abscisic acid (ABA), and enhanced drought resistance. Furthermore, apple plants overexpressing MdERDL6-1 also showed reduced water potential, facilitated stomatal closure, and reactive oxygen species scavenging under drought conditions compared to control plants. Overall, our results suggest a potential strategy to enhance drought resistance and sugar accumulation in fruits through manipulating the genes involved in vacuolar sugar transport.

An insertion in the promoter of a malate dehydrogenase gene regulates malic acid content in apple fruit

Malic acid is an important flavor determinant in apple (Malus × domestica Borkh.) fruit. One known variation controlling malic acid is the A/G single nucleotide polymorphism in an aluminum-activated malate transporter gene (MdMa1). Nevertheless, there are still differences in malic acid content in apple varieties with the same Ma1 genotype (Ma1/Ma1 homozygous), such as 'Honeycrisp' (high malic acid content) and 'Qinguan' (low malic acid content), indicating that other loci may influence malic acid and fruit acidity. Here, the F1 (Filial 1) hybrid generation of 'Honeycrisp' × 'Qinguan' was used to analyze quantitative trait loci for malic acid content. A major locus (Ma7) was identified on chromosome 13. Within this locus, a malate dehydrogenase gene, MDH1 (MdMa7), was the best candidate for further study. Subcellular localization suggested that MdMa7 encodes a cytosolic protein. Overexpression and RNA interference of MdMa7 in apple fruit increased and decreased malic acid content, respectively. An insertion/deletion (indel) in the MdMa7 promoter was found to affect MdMa7 expression and malic acid content in both hybrids and other cultivated varieties. The insertion and deletion genotypes were designated as MA7 and ma7, respectively. The transcription factor MdbHLH74 was found to stimulate MdMa7 expression in the MA7 genotype but not in the ma7 genotype. Transient transformation of fruit showed that MdbHLH74 affected MdMa7 expression and malic acid content in 'Gala' (MA7/MA7) but not in 'Fuji' (ma7/ma7). Our results indicated that genetic variation in the MdMa7 (MDH1) promoter alters the binding ability of the transcription factor MdbHLH74, which alters MdMa7 (MDH1) transcription and the malic acid content in apple fruit, especially in Ma1/Ma1 homozygous accessions.

How abiotic stresses trigger sugar signaling to modulate leaf senescence?

Plants have evolved the adaptive capacity to mitigate the negative effect of external adversities at chemical, molecular, cellular, and physiological levels. This capacity is conferred by triggering the coordinated action of internal regulatory factors, in which sugars play an essential role in the regulating chloroplast degradation and leaf senescence under various stresses. In this review, we summarize the recent findings on the senescent-associated changes in carbohydrate metabolism and its relation to chlorophyl degradation, oxidative damage, photosynthesis inhibition, programmed cell death (PCD), and sink-source relation as affected by abiotic stresses. The action of sugar signaling in regulating the initiation and progression of leaf senescence under abiotic stresses involves interactions with various plant hormones, reactive oxygen species (ROS) burst, and protein kinases. This discussion aims to elucidate the complex regulatory network and molecular mechanisms that underline sugar-induced leaf senescence in response to various abiotic stresses. The imperative role of sugar signaling in regulating plant stress responses potentially enables the production of crop plants with modified sugar metabolism. This, in turn, may facilitate the engineering of plants with improved stress responses, optimal life span and higher yield achievement.

Transcription factor PbNAC71 regulates xylem and vessel development to control plant height

Dwarfism is an important agronomic trait in fruit breeding programs. However, the germplasm resources required to generate dwarf pear (Pyrus spp.) varieties are limited. Moreover, the mechanisms underlying dwarfism remain unclear. In this study, "Yunnan" quince (Cydonia oblonga Mill.) had a dwarfing effect on "Zaosu" pear. Additionally, the dwarfism-related NAC transcription factor gene PbNAC71 was isolated from pear trees comprising "Zaosu" (scion) grafted onto "Yunnan" quince (rootstock). Transgenic Nicotiana benthamiana and pear OHF-333 (Pyrus communis) plants overexpressing PbNAC71 exhibited dwarfism, with a substantially smaller xylem and vessel area relative to the wild-type controls. Yeast one-hybrid, dual-luciferase, chromatin immunoprecipitation-qPCR, and electrophoretic mobility shift assays indicated that PbNAC71 downregulates PbWalls are thin 1 expression by binding to NAC-binding elements in its promoter. Yeast two-hybrid assays showed that PbNAC71 interacts with the E3 ubiquitin ligase PbRING finger protein 217 (PbRNF217). Furthermore, PbRNF217 promotes the ubiquitin-mediated degradation of PbNAC71 by the 26S proteasome, thereby regulating plant height as well as xylem and vessel development. Our findings reveal a mechanism underlying pear dwarfism and expand our understanding of the molecular basis of dwarfism in woody plants.

Comparative transcriptome analysis identifies candidate genes related to sucrose accumulation in longan (Dimocarpus longan Lour.) pulp

Sucrose content is one of the important factors to determine longan fruit flavor quality. To gain deep insight of molecular mechanism on sucrose accumulation in longan, we conducted comparative transcriptomic analysis between low sucrose content longan cultivar 'Qingkebaoyuan' and high sucrose content cultivar 'Songfengben'. A total of 12,350 unique differentially expressed genes (DEGs) were detected across various development stages and different varieties, including hexokinase (HK) and sucrose-phosphate synthase (SPS), which are intricately linked to soluble sugar accumulation and metabolism. Weighted gene co-expression network analysis (WGCNA) identified magenta module, including DlSPS gene, was significantly positively correlated with sucrose content. Furthermore, transient expression unveiled DlSPS gene play crucial role in sucrose accumulation. Moreover, 5 transcription factors (MYB, ERF, bHLH, C2H2, and NAC) were potentially involved in DlSPS regulation. Our findings provide clues for sucrose metabolism, and lay the foundation for longan breeding in the future.

The transcription factor MdBPC2 alters apple growth and promotes dwarfing by regulating auxin biosynthesis

Auxin plays important roles throughout plant growth and development. However, the mechanisms of auxin regulation of plant structure are poorly understood. In this study, we identified a transcription factor (TF) of the BARLEY B RECOMBINANT/BASIC PENTACYSTEINE (BBR/BPC) family in apple (Malus × domestica), MdBPC2. It was highly expressed in dwarfing rootstocks, and it negatively regulated auxin biosynthesis. Overexpression of MdBPC2 in apple decreased plant height, altered leaf morphology, and inhibited root system development. These phenotypes were due to reduced auxin levels and were restored reversed after exogenous indole acetic acid (IAA) treatment. Silencing of MdBPC2 alone had no obvious phenotypic effect, while silencing both Class I and Class II BPCs in apple significantly increased auxin content in plants. Biochemical analysis demonstrated that MdBPC2 directly bound to the GAGA-rich element in the promoters of the auxin synthesis genes MdYUC2a and MdYUC6b, inhibiting their transcription and reducing auxin accumulation in MdBPC2 overexpression lines. Further studies established that MdBPC2 interacted with the polycomb group (PcG) protein LIKE HETEROCHROMATIN PROTEIN 1 (LHP1) to inhibit MdYUC2a and MdYUC6b expression via methylation of histone 3 lysine 27 (H3K27me3). Silencing MdLHP1 reversed the negative effect of MdBPC2 on auxin accumulation. Our results reveal a dwarfing mechanism in perennial woody plants involving control of auxin biosynthesis by a BPC transcription factor, suggesting its use for genetic improvement of apple rootstock.

Enrichment of grape berries and tomato fruit with health-promoting tartaric acid by expression of the Vitis vinifera transketolase VvTK2 gene

Tartaric acid (TA) is a major non-fermentable plant soluble acid that abundantly occur in grapes and wines, imparting low pH and tart flavour to berries thereby regulating numerous quality attributes of wine, such as flavour, microbial stability, and aging potential. Evaluation of acidity in mature fruits of 21 wine grape (Vitis vinifera) varieties revealed significant variation between 'Beichun' and 'Gewürztraminer', which was correlated with TA content. RNA-seq analysis of fruits from the two cultivars at different developmental stages revealed that a transketolase gene, VvTK2, was significantly dominantly expressed in the high TA phenotype 'Beichun' variety. Subcellular localization assay showed that VvTK2 protein was located in the chloroplast. Virus-induced VvTK2 gene silencing significantly decreased the expression of 2-keto-L-gulonic acid reductase (Vv2-KGR) as well as L-idonate dehydrogenase (VvL-IdnDH3) and inhibited TA accumulation, while its transient over-expression in grape showed the opposite results. Heterologous VvTK2 over-expression in tomato demonstrated its obvious capacity to induce TA synthesis. Overall, these results highlights a novel role of VvTK2 in modulating TA biosynthesis, which could be an excellent strategy for future genetic improvement of grape flavour.

Exogenous gibberellin delays maturation in persimmon fruit through transcriptional activators and repressors

As the harvest season of most fruit is concentrated, fruit maturation manipulation is essential for the fresh fruit industry to prolong sales time. Gibberellin (GA), an important phytohormone necessary for plant growth and development, has also shown a substantial regulatory effect on fruit maturation; however, its regulatory mechanisms remain inconclusive. In this research, preharvest GA3 treatment effectively delayed fruit maturation in several persimmon (Diospyros kaki) cultivars. Among the proteins encoded by differentially expressed genes, 2 transcriptional activators (NAC TRANSCRIPTION FACTOR DkNAC24 and ETHYLENE RESPONSIVE FACTOR DkERF38) and a repressor (MYB-LIKE TRANSCRIPTION FACTOR DkMYB22) were direct regulators of GERANYLGERANYL DIPHOSPHATE SYNTHASE DkGGPS1, LYSINE HISTIDINE TRANSPORTER DkLHT1, and FRUCTOSE-BISPHOSPHATE ALDOLASE DkFBA1, respectively, resulting in the inhibition of carotenoid synthesis, outward transport of an ethylene precursor, and consumption of fructose and glucose. Thus, the present study not only provides a practical method to prolong the persimmon fruit maturation period in various cultivars but also provides insights into the regulatory mechanisms of GA on multiple aspects of fruit quality formation at the transcriptional regulation level.