Genome-Wide Identification, Structure Characterization, and Expression Pattern Profiling of the Aquaporin Gene Family in Betula pendula.

Aquaporin water channels (AQPs) constitute a large family of transmembrane proteins present throughout all kingdoms of life. They play key roles in the flux of water and many solutes across the membranes. The AQP diversity, protein features, and biological functions of silver birch are still unknown. A genome analysis of Betula pendula identified 33 putative genes encoding full-length AQP sequences (BpeAQPs). They are grouped into five subfamilies, representing ten plasma membrane intrinsic proteins (PIPs), eight tonoplast intrinsic proteins (TIPs), eight NOD26-like intrinsic proteins (NIPs), four X intrinsic proteins (XIPs), and three small basic intrinsic proteins (SIPs). The BpeAQP gene structure is conserved within each subfamily, with exon numbers ranging from one to five. The predictions of the aromatic/arginine selectivity filter (ar/R), Froger's positions, specificity-determining positions, and 2D and 3D biochemical properties indicate noticeable transport specificities to various non-aqueous substrates between members and/or subfamilies. Nevertheless, overall, the BpePIPs display mostly hydrophilic ar/R selective filter and lining-pore residues, whereas the BpeTIP, BpeNIP, BpeSIP, and BpeXIP subfamilies mostly contain hydrophobic permeation signatures. Transcriptional expression analyses indicate that 23 BpeAQP genes are transcribed, including five organ-related expressions. Surprisingly, no significant transcriptional expression is monitored in leaves in response to cold stress (6 °C), although interesting trends can be distinguished and will be discussed, notably in relation to the plasticity of this pioneer species, B. pendula. The current study presents the first detailed genome-wide analysis of the AQP gene family in a Betulaceae species, and our results lay a foundation for a better understanding of the specific functions of the BpeAQP genes in the responses of the silver birch trees to cold stress.

Integrated drought responses of black poplar: how important is phenotypic plasticity?

Climate change is expected to increase drought frequency and intensity which will threaten plant growth and survival. In such fluctuating environments, perennial plants respond with hydraulic and biomass adjustments, resulting in either tolerant or avoidant strategies. Plants' response to stress relies on their phenotypic plasticity. The goal of this study was to explore physiology of young Populus nigra in the context of a time-limited and progressive water deficit in regard to their growth and stress response strategies. Fourteen French 1-year-old black poplar genotypes, geographically contrasted, were subjected to withholding water during 8 days until severe water stress. Water fluxes (i.e. leaf water potentials and stomatal conductance) were analyzed together with growth (i.e. radial and longitudinal branch growth, leaf senescence and leaf production). Phenotypic plasticity was calculated for each trait and response strategies to drought were deciphered for each genotype. Black poplar genotypes permanently were dealing with a continuum of adjusted water fluxes and growth between two extreme strategies, tolerance and avoidance. Branch growth, leaf number and leaf hydraulic potential traits had contrasted plasticities, allowing genotype characterization. The most tolerant genotype to water deficit, which maintained growth, had the lowest global phenotypic plasticity. Conversely, the most sensitive and avoidant genotype ceased growth until the season's end, had the highest plasticity level. All the remaining black poplar genotypes were close to avoidance with average levels of traits plasticity. These results underpinned the role of plasticity in black poplar response to drought and calls for its wider use into research on plants' responses to stress.

Sucrose (JrSUT1) and hexose (JrHT1 and JrHT2) transporters in walnut xylem parenchyma cells: their potential role in early events of growth resumption.

In temperate woody species, the vegetative growth period is characterized by active physiological events (e.g., bud break), which require an adequate supply of soluble sugars imported in the xylem sap stream. One-year-old shoots of walnut (Juglans regia L. cv. 'Franquette') trees, which have an acrotonic branching pattern (only apical and distal vegetative buds burst), were used to study the regulation of xylem sugar transporters in relation to bud break. At the end of April (beginning of bud break), a higher xylem sap sucrose concentration and a higher active sucrose uptake by xylem parenchyma cells were found in the apical portion (bearing buds able to burst) than in the basal portion (bearing buds unable to burst) of the sample shoots. At the same time, xylem parenchyma cells of the apical portion of the shoots exhibited greater amounts of both transcripts and proteins of JrSUT1 (Juglans regia putative sucrose transporter 1) than those of the basal stem segment. Conversely, no pronounced difference was found for putative hexose transporters JrHT1 and JrHT2 (Juglans regia hexose transporters 1 and 2). These findings demonstrate the high capacity of bursting vegetative buds to import sucrose. Immunological analysis revealed that sucrose transporters were localized in all parenchyma cells of the xylem, including vessel-associated cells, which are highly specialized in nutrient exchange. Taken together, our results indicate that xylem parenchyma sucrose transporters make a greater contribution than hexose transporters to the imported carbon supply of bursting vegetative buds.

Spatial activity and expression of plasma membrane H+-ATPase in stem xylem of walnut during dormancy and growth resumption.

Plasma membrane H+-ATPase (PM H+-ATPase) plays a key role in nutrient transport, stress responses and growth. To evaluate proton motive force differences between apical and basal parts of acrotonic 1-year-old shoots of walnut (Juglans regia L. cv 'Franquette') trees, spatial and seasonal changes in PM H+-ATPase were studied in mature xylem tissues. During both the dormancy and growth resumption periods, and in both the apical and basal parts of the stem, PM H+-ATPase activity showed positive correlations with the amount of immunodetectable protein. In spring, at the time of growth resumption, higher activities and immunoreactivities of PM H+-ATPase were found in the apical part of the stem than in the basal part of the stem. In spring, the decrease in xylem sugar concentration reflected the high sugar uptake rate. Our data suggest that PM H+-ATPase plays a major role in the uptake of carbohydrates from xylem vessels during growth resumption. These results are discussed in the context of the acrotonic tendency of walnut shoots.

JrSUT1, a putative xylem sucrose transporter, could mediate sucrose influx into xylem parenchyma cells and be up-regulated by freeze-thaw cycles over the autumn-winter period in walnut tree (Juglans regia L.).

Sucrose has been reported to play multiple roles in the winter biology of temperate woody species. However, no report on the molecular basis of sucrose transport in xylem tissue has yet been made. In the walnut tree, it is demonstrated that during the autumn-winter period, active absorption of sucrose from xylem vessels to parenchyma cells (sucrose influx) is much higher when samplings were taken shortly after a period of freezing temperatures. Here, the question of whether this increased sucrose influx mirrors a regulation of sucrose transporters in xylem tissue was tested. A putative sucrose transporter cDNA (JrSUT1: Juglans regia sucrose transporter 1) was isolated. Over the autumn-winter period, JrSUT1 transcripts and respective proteins were present in xylem parenchyma cells and highly detected when samplings were performed shortly after a freeze-thaw cycle. This up-regulation of JrSUT1 level was confirmed in controlled conditions and was not obtained in bark. Immunolocalization studies showed that JrSUT1 and plasma membrane H+ -ATPase (JrAHA) were colocalized to vessel-associated cells (VACs), which control solute exchanges between parenchyma cells and xylem vessels. We propose that JrSUT1 could be involved in the retrieval of sucrose from xylem vessel. All these data are discussed with respect to the winter biology of the walnut tree.

Temperature effects on xylem sap osmolarity in walnut trees: evidence for a vitalistic model of winter embolism repair.

We studied the effect of temperature on the carbohydrate status of parenchyma cells during winter in relation to the efflux and influx of sugars between parenchyma cells and xylem vessels in 1-year-old twigs of walnut (Juglans regia L.). The mechanism of sugar transfer between contact cells and vessels was also investigated. We obtained new insights into the possible osmotic role of sugars, particularly sucrose, in stem pressure formation and winter embolism repair. Accumulation of sucrose in the xylem sap during winter was mainly influenced by: (1) abundant conversion of starch to sucrose in the symplast at low temperatures; (2) sucrose efflux into the apoplast at low temperatures (1 degrees C); and (3) inefficient sugar uptake at low temperatures, although efficient sugar uptake occurred at 15 degrees C. We hypothesize that a diethyl pyrocarbonate (DEPC)-sensitive protein mediates facilitated diffusion of sucrose from parenchyma cells to xylem vessels (efflux) in walnut. We discuss the possible occurrence of active H+-sucrose symports and the coexistence of both influx and efflux processes in walnut in winter and the modulation of the relative importance of these flows by temperature.

Trophic control of bud break in peach (Prunus persica) trees: a possible role of hexoses.

Vegetative buds of peach (Prunus persica L. Batsch.) trees act as strong sinks and their bud break capacity can be profoundly affected by carbohydrate availability during the rest period (November-February). Analysis of xylem sap revealed seasonal changes in concentrations of sorbitol and hexoses (glucose and fructose). Sorbitol concentrations decreased and hexose concentrations increased with increasing bud break capacity. Sucrose concentration in xylem sap increased significantly but remained low. To clarify their respective roles in the early events of bud break, carbohydrate concentrations and uptake rates, and activities of NAD-dependent sorbitol dehydrogenase (SDH), sorbitol oxidase (SOX) and cell wall invertase (CWI) were determined in meristematic tissues, cushion tissues and stem segments. Only CWI activity increased in meristematic tissues shortly before bud break. In buds displaying high bud break capacity (during January and February), concentrations of sorbitol and sucrose in meristematic tissues were almost unchanged, paralleling their low rates of uptake and utilization by meristematic tissues, and indicating that sorbitol and sucrose play a negligible role in the bud break process. Hexose concentrations in meristematic tissues and glucose imported by meristematic tissues correlated positively with bud break capacity, suggesting that hexoses are involved in the early events of bud break. These findings were confirmed by data for buds that were unable to break because they had been collected from trees deprived of cold. We therefore conclude that hexoses are of greater importance than sorbitol or sucrose in the early events of bud break in peach trees.

Sorbitol uptake is regulated by glucose through the hexokinase pathway in vegetative peach-tree buds.

In peach trees (Prunus persica L. Batsch cv. Redhaven), sorbitol is a primary photosynthetic product and may play an important role in the budbreak process. Surprisingly, before budbreak (from January to early March), the concentration of sorbitol in the xylem sap decreases, while that of hexoses (glucose and fructose) increases. The aim of this work was to study the control of sorbitol uptake into vegetative buds by hexoses. Sorbitol uptake was selectively inhibited by hexoses at low and physiological concentrations and this effect was both reversible and concentration-dependent. In addition, the active uptake of sorbitol significantly declined in the plasma membrane vesicles-enriched fraction purified from glucose-treated vegetative buds, suggesting that the inhibitory action of glucose was at the membrane level. Finally, among several glucose analogues tested, only hexokinase substrates (2-deoxyglucose and mannose) were able to mimic the glucose effect, which was completely blocked by the hexokinase inhibitor mannoheptulose. These results represent the first steps towards a better understanding of polyol transport control in plants.

Winter variation in xylem sap pH of walnut trees: involvement of plasma membrane H+-ATPase of vessel-associated cells.

We studied seasonal variation in xylem sap pH of Juglans regia L. Our main objectives were to (1) test the effect of temperature on seasonal changes in xylem sap pH and (2) study the involvement of plasma membrane H+-ATPase of vessel-associated cells in the control of sap pH. For this purpose, orchard-grown trees were compared with trees grown in a heated (> or = 15 degrees C) greenhouse. During autumn, sap pH was not directly influenced by temperature. A seasonal change in H+-ATPase activity resulting from seasonal variation in the amount of protein was measured in orchard-grown trees, whereas no significant seasonal changes were recorded in greenhouse-grown trees. Our data suggest that H+-ATPase does not regulate xylem sap pH directly by donating protons to the xylem, but by facilitating secondary active H+/sugar transport, among other mechanisms.