Tonoplast monosaccharide transporters (TMTs) play important roles in vacuolar sugar accumulation in plants. In this study, six TMT genes (PbTMT1-6) were identified in the Pyrus bretschneideri genome database, and their expression profiles were correlated with soluble sugar contents during the pear (P. bretschneideri cv. Ya Li) fruit development process. Subsequently, PbTMT4 was identified as a strong contributor to fructose, glucose and sucrose accumulation in fructescence of pears. Heterologous expression of PbTMT4, in the hexose transporter-deficient yeast strain EBY.VW4000, facilitated growth in media containing low levels of glucose, fructose, sucrose or sorbitol. In addition, PbTMT4-transformed tomato plants flowered and bore fruit significantly earlier than wild-type (WT) plants, and glucose and fructose levels in mature tomatoes were increased by about 32 and 21% compared with those in WT plants. However, no obvious alterations in sucrose content, plant height and weight per fruit were observed. Finally, subcellular localization experiments in transformed Arabidopsis plants showed that PbTMT4 is localized to tonoplast vesicles of protoplasts. These preliminary results suggest that PbTMT4 participates in vacuolar accumulation of sugars, and thus affects plant growth and development.

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The K transporter/high-affinity K/K uptake (KT/HAK/KUP) family, as one of the largest K transporter families in higher plants, plays an essential role in plant growth, mineral element absorption, salt stress tolerance, and other physiological processes. However, little is known about this family in pear (Pyrus). Here, we identified 20 K transporter genes in pear (P. bretschneideri) using genome-wide analysis. Their gene structure, chromosomal distribution, conserved motifs, phylogenetics, duplication events, and expression patterns were also examined. The results of phylogenetic analysis showed that PbrKT/HAK/KUP genes were clustered into three major groups (Groups I-III). Among the 20 PbrKT/HAK/KUP genes, 18 were mapped to nine chromosomes and two to scaffolds. Four WGD/segmental gene pairs were identified, indicating that WGD/segmental duplication may have contributed to the expansion of the KT/HAK/KUP family in pear. Among the four pairs of WGD/segmentally duplicated genes, both members of three pairs had been subjected to purifying selection, whereas the fourth pair had been subjected to positive selection. Furthermore, phenotypic experiments showed that the growth of pear seedlings was affected by potassium deficiency treatment. Expression patterns of 20 PbrKT/HAK/KUP genes in roots were further assayed with qRT-PCR. PbrHAK1 and PbrHAK12/16 were significantly expressed in response to K deficiency, suggesting that these genes are crucial for K uptake in pear, especially under the condition of K starvation. Our results provide a foundation for further study on the function of KT/HAK/KUP genes in pear.

Sucrose accumulation is one of the important factors that determine fruit enlargement and quality. Evaluation of the sugar profile of 105 pear cultivars revealed low- and high-sucrose types of pear fruits. To better understand the molecular mechanisms governing the sucrose content of pear fruits, this study performed transcriptome analysis during fruit development using low-sucrose ‘Korla’ fragrant pear and high-sucrose ‘Hosui’ pear, and a coexpression module uniquely associated with the control of high sucrose accumulation was identified by weighted gene coexpression network analysis (WGCNA). These results suggested that there are seven candidate genes encoding key enzymes [fructokinase (FRK), glucose-6-phosphate isomerase (PGI), sucrose phosphate synthase (SPS), and sucrose synthase (SUS)] involved in sucrose biosynthesis and several transcription factors whose expression patterns correlate with those of genes associated with sucrose biosynthesis. This correlation was confirmed by linear regression analysis between predicted gene expression and sucrose content in different pear cultivars during fruit development. This study provides insight into the molecular mechanism underlying differences in sucrose content across pear cultivars and presents candidate structural genes and transcription factors that could play important roles in regulating carbohydrate partitioning and sucrose accumulation.

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