The Sugar Transporter MST1 Is Involved in Colonization of Rhizosphere and Rhizoplane by Metarhizium robertsii

Over 90% of all vascular plant species develop an intimate symbiosis with fungi, which has an enormous impact on terrestrial ecosystems. It is widely recognized that plant-symbiotic fungi are supported by photosynthates, but little is known about the mechanisms for fungi to utilize plant-derived carbon sources.

New Strategies for Efficient Expression of Heterologous Sugar Transporters in Saccharomyces cerevisiae

: In our previous work we had developed an hxt-null Saccharomyces cerevisiae strain displaying high xylose reductase, xylitol dehydrogenase and xylulokinase activities that proved to be useful as a chassis strain to study new xylose transporters, as SsXUT1 from Scheffersomyces stipitis. Spathaspora passalidarum and Spathaspora arborariae have in their genomes genes with high sequence similarity (78-80%) to SsXUT1. To characterize these putative transporter genes (SpXUT1 and SaXUT1, respectively) they were expressed in the same chassis strain as SsXUT1. Surprisingly, the cloned genes could not restore the ability to grow in several monosaccharides tested, although the strains expressing the SsXUT1 and SpXUT1 permeases, after growth on maltose, showed the presence of 14C-glucose and 14C-xylose transport activity. An important feature of these permeases is that SsXUT1 lacks lysine residues in its N-terminal domain with high-confidence ubiquitinylation potential, and has only one at the C-terminal domain, while the SpXUT1 transporter had several of such residues at its C-terminal domain. When the SpXUT1 gene was cloned in a truncated version lacking such lysine residues, the permease allowed grow on glucose or xylose, and even promoted xylose fermentation by the hxt-null strain. In another approach, we deleted two arrestins known to be involved in sugar transporter ubiquitinylation and endocytosis (ROD1 and ROG3), but only the rog3Δ strain allowed modest growth on these sugars. Taken together, these results suggest that to allow efficient sugar transporter expression in S. cerevisiae the lysines involved in transporter endocytosis should be removed from the sequence of the permease.

A Strategy for the Production and Molecular Validation of Agrobacterium-Mediated Intragenic Octoploid Strawberry

Intragenesis is an all-native engineering technology for crop improvement. Using an intragenic strategy to bring genes from wild species to cultivated strawberry could expand the genetic variability. A robust regeneration protocol was developed for the strawberry cv. ‘Shanghai Angel’ by optimizing the dose of Thidiazuron and identifying the most suitable explants. The expression cassette was assembled with all DNA fragments from F. vesca, harboring a sugar transporter gene FvSTP8 driven by a fruit-specific FvKnox promoter. Transformed strawberry was developed through an Agrobacterium-mediated strategy without any selectable markers. Other than PCR selection, probe-based duplex droplet digital PCR (ddPCR) was performed to determine the T-DNA insert. Four independent transformed shoots were obtained with a maximum of 5.3% efficiency. Two lines were confirmed to be chimeras, while the other two were complete transformants with six and 11 copies of the intragene, respectively. The presence of a vector backbone beyond the T-DNA in these transformants indicated that intragenic strawberries were not obtained. The current work optimized the procedures for producing transformed strawberry without antibiotic selection, and accurately determined the insertion copies by ddPCR in the strawberry genome for the first time. These strategies might be promising for the engineering of ‘Shanghai Angel’ and other cultivars to improve agronomic traits.

Rice SUT and SWEET Transporters

Sugar transporters play important or even indispensable roles in sugar translocation among adjacent cells in the plant. They are mainly composed of sucrose–proton symporter SUT family members and SWEET family members. In rice, 5 and 21 members are identified in these transporter families, and some of their physiological functions have been characterized on the basis of gene knockout or knockdown strategies. Existing evidence shows that most SUT members play indispensable roles, while many SWEET members are seemingly not so critical in plant growth and development regarding whether their mutants display an aberrant phenotype or not. Generally, the expressions of SUT and SWEET genes focus on the leaf, stem, and grain that represent the source, transport, and sink organs where carbohydrate production, allocation, and storage take place. Rice SUT and SWEET also play roles in both biotic and abiotic stress responses in addition to plant growth and development. At present, these sugar transporter gene regulation mechanisms are largely unclear. In this review, we compare the expressional profiles of these sugar transporter genes on the basis of chip data and elaborate their research advances. Some suggestions concerning future investigation are also proposed.

Loss of The Phloem-Expressed Sugar Transporter VST1 Reduces Aphid Performance In Watermelon

Aphids can damage plants through sugar withdrawal by hijacking sugar metabolism and transport genes to increase their sugar sucking ability. Blocking plant diseases and pest access to nutrients has emerged as an exciting strategy for improving disease and insect resistance in plants. Our previous work identified a shift in the localization of the vacuolar sugar transporter (VST1) that contributes to sucrose (Suc) and glucose (Glc) unloading in the phloem of sweet watermelons. In this study, the potential role of VST1 in the response to aphid infestation was investigated. Loss of VST1 function adversely impacted aphid settling and honeydew production. The vst1 mutant displayed less aphid-induced hydrogen peroxide accumulation and cell death than wild-type (WT) plants. Additionally, the expression of the VST1 gene was induced by aphids. The mutation of VST1 reduced Suc and Glc accumulation in the phloem, indicating that Suc and Glc are important carbohydrate substrates in phloem sap that are transported by VST1 to support aphid propagation and infestation. Taken together, our results demonstrated that the mutation of VST1 by genome editing can decrease aphid performance in watermelon by blocking the sugar supply obtained from phloem sap.