Ectopic maltase alleviates dwarf phenotype and improves plant frost tolerance of maltose transporter mutants

Degradation of FATTY ACID EXPORT PROTEIN1 by RHOMBOID-LIKE PROTEASE11 contributes to cold tolerance in Arabidopsis

Plants need to acclimate to different stresses to optimize growth under unfavorable conditions. In Arabidopsis (Arabidopsis thaliana), the abundance of the chloroplast envelope protein FATTY ACID EXPORT PROTEIN1 (FAX1) decreases after the onset of low temperatures. However, how FAX1 degradation occurs and whether altered FAX1 abundance contributes to cold tolerance in plants remains unclear. The rapid cold-induced increase in RHOMBOID-LIKE PROTEASE11 (RBL11) transcript levels, the physical interaction of RBL11 with FAX1, the specific FAX1 degradation after RBL11 expression, and the absence of cold-induced FAX1 degradation in rbl11 loss-of-function mutants suggest that this enzyme is responsible for FAX1 degradation. Proteomic analyses showed that rbl11 mutants have higher levels of FAX1 and other proteins involved in membrane lipid homeostasis, suggesting that RBL11 is a key element in the remodeling of membrane properties during cold conditions. Consequently, in the cold, rbl11 mutants show a shift in lipid biosynthesis toward the eukaryotic pathway, which coincides with impaired cold tolerance. To test whether cold sensitivity is due to increased FAX1 levels, we analyzed FAX1 overexpressors. The rbl11 mutants and FAX1 overexpressor lines show superimposable phenotypic defects upon exposure to cold temperatures. Our re-sults show that the cold-induced degradation of FAX1 by RBL11 is critical for Arabidop-sis to survive cold and freezing periods.

The novel chloroplast glucose transporter pGlcT2 affects adaptation to extended light periods

Intracellular sugar compartmentation is critical in plant development and acclimation to challenging environmental conditions. Sugar transport proteins are present in plasma membranes and in membranes of organelles such as vacuoles, the Golgi apparatus, and plastids. However, there may exist other transport proteins with uncharacterized roles in sugar compartmentation. Here we report one such, a novel transporter of the Monosaccharide Transporter Family (MSF), the closest phylogenetic homolog of which is the chloroplast-localized glucose transporter pGlcT and that we therefore term plastidic glucose transporter 2 (pGlcT2). We show, using gene-complemented glucose uptake deficiency of an Escherichia coli ptsG/manXYZ mutant strain and biochemical characterization, that this protein specifically facilitates glucose transport, whereas other sugars do not serve as substrates. In addition, we demonstrate pGlcT2-GFP localized to the chloroplast envelope, and that pGlcT2 is mainly produced in seedlings and in the rosette center of mature Arabidopsis plants. Therefore, in conjunction with molecular and metabolic data, we propose pGlcT2 acts as a glucose importer that can limit cytosolic glucose availability in developing pGlcT2-overexpressing seedlings. Finally, we show both overexpression and deletion of pGlcT2 resulted in impaired growth efficiency under long day and continuous light conditions, suggesting pGlcT2 contributes to a release of glucose derived from starch mobilization late in the light phase. Together, these data indicate the facilitator pGlcT2 changes the direction in which it transports glucose during plant development and suggest the activity of pGlcT2 must be controlled spatially and temporarily in order to prevent developmental defects during adaptation to periods of extended light.

Cytosolic disproportionating enzyme2 is essential for pollen germination and pollen tube elongation in rice

Degradation of starch accumulated in pollen provides energy and cellular materials for pollen germination and pollen tube elongation. Little is known about the function of cytosolic disproportionating enzyme2 (DPE2) in rice (Oryza sativa). Here, we obtained several DPE2 knockout mutant (dpe2) lines via genomic editing and found that the mutants grew and developed normally but with greatly reduced seed-setting rates. Reciprocal crosses between dpe2 and wild-type plants demonstrated that the mutant was male sterile. In vitro and in vivo examinations revealed that the pollen of the dpe2 mutant developed and matured normally but was defective in germination and elongation. DPE2 deficiency increased maltose content in pollen, whereas it reduced the levels of starch, glucose, fructose, and adenosine triphosphate (ATP). Exogenous supply of glucose or ATP to the germination medium partially rescued the pollen germination defects of dpe2. The expression of cytosolic phosphorylase2 (Pho2) increased significantly in dpe2 pollen. Knockout of Pho2 resulted in a semi-sterile phenotype. We failed to obtain homozygous dpe2 pho2 double mutant lines. Our results demonstrate that maltose catalyzed by DPE2 to glucose is the main energy source for pollen germination and pollen tube elongation, while Pho2 might partially compensate for deficiency of DPE2.

Chloroplasts are key players to cope with light and temperature stress

Under natural environmental conditions, changes in light intensity and temperature are closely interwoven, and of all organelles, only chloroplasts react strongly upon alterations of these two parameters. We review increasing evidence indicating that changes in chloroplast metabolism are critical for the comprehensive cellular answer in a challenging environment. This cellular answer starts with rapid modifications of thylakoid-located processes, followed by modifications in the stroma and transport activities across the chloroplast envelope. We propose that the 'modulators' involved contribute to plant stress tolerance and that deciphering of their characteristics is essential to understand 'acclimation'. Especially in times of climatic changes, we must gain knowledge on physiological reactions that might become instrumental for directed breeding strategies aiming to develop stress-tolerant crop plants.

Temperature-induced dynamics of plant carbohydrate metabolism

Carbohydrates are direct products of photosynthetic CO₂ assimilation. Within a changing temperature regime, both photosynthesis and carbohydrate metabolism need tight regulation to prevent irreversible damage of plant tissue and to sustain energy metabolism, growth and development. Due to climate change, plants are and will be exposed to both long-term and short-term temperature changes with increasing amplitude. Particularly sudden fluctuations, which might comprise a large temperature amplitude from low to high temperature, pose a challenge for plants from the cellular to the ecosystem level. A detailed understanding of fundamental regulatory processes, which link photosynthesis and carbohydrate metabolism under such fluctuating environmental conditions, is essential for an estimate of climate change consequences. Further, understanding these processes is important for biotechnological application, breeding and engineering. Environmental light and temperature regimes are sensed by a molecular network that comprises photoreceptors and molecular components of the circadian clock. Photosynthetic efficiency and plant productivity then critically depend on enzymatic regulation and regulatory circuits connecting plant cells with their environment and re-stabilising photosynthetic efficiency and carbohydrate metabolism after temperature-induced deflection. This review summarises and integrates current knowledge about re-stabilisation of photosynthesis and carbohydrate metabolism after perturbation by changing temperature (heat and cold).