In perennial plants, freeze-thaw cycles during the winter months can induce the formation of air bubbles in xylem vessels, leading to changes in their hydraulic conductivity. Refilling of embolized xylem vessels requires an osmotic force that is created by the accumulation of soluble sugars in the vessels. Low water potential leads to water movement from the parenchyma cells into the xylem vessels. The water flux gives rise to a positive pressure essential for the recovery of xylem hydraulic conductivity. We investigated the possible role of plasma membrane aquaporins in winter embolism recovery in walnut (Juglans regia). First, we established that xylem parenchyma starch is converted to sucrose in the winter months. Then, from a xylem-derived cDNA library, we isolated two PIP2 aquaporin genes (JrPIP2,1 and JrPIP2,2) that encode nearly identical proteins. The water channel activity of the JrPIP2,1 protein was demonstrated by its expression in Xenopus laevis oocytes. The expression of the two PIP2 isoforms was investigated throughout the autumn-winter period. In the winter period, high levels of PIP2 mRNA and corresponding protein occurred simultaneously with the rise in sucrose. Furthermore, immunolocalization studies in the winter period show that PIP2 aquaporins were mainly localized in vessel-associated cells, which play a major role in controlling solute flux between parenchyma cells and xylem vessels. Taken together, our data suggest that PIP2 aquaporins could play a role in water transport between xylem parenchyma cells and embolized vessels.Winter embolism, the generation of air bubbles in xylem vessels induced by freeze-thaw cycles, often leads to a loss of hydraulic conductivity of the vessels (Cochard and Tyree, 1990; Améglio et al., 2001; Ewers et al., 2001). Vulnerability to winter embolism is related to the anatomy and vessel diameter of woody plants (Cochard and Tyree, 1990) and affects the ability of plants to survive cold climates and the geographic distribution of species (Tyree and Cochard, 1996; Pockman and Sperry, 1997; Lemoine et al., 1999).Detailed physiological studies of the responses of temperate woody plants to winter embolism have been made. Plants minimize the impact of winter embolism by replacing embolized vessels by new functional vessels every year and/or by refilling embolized vessels by generating positive xylem pressures (Holbrook and Zwieniecki, 1999; Tyree et al., 1999; Améglio et al., 2002). Although making new vessels is common to all the plants that exhibit secondary growth, the generation of xylem pressures has only been reported in a few species such as maple (Acer pseudoplatanus; O'Malley and Milburn, 1983; Tyree, 1983; Sperry et al., 1987 Sperry et al., , 1994, grapevine (Vitis vinifera; Sperry et al., 1987), birch (Betula alleghaniensis) (Sperry et al., 1994; Zhu et al., 2000), and walnut (Juglans regia; Améglio et al., 1995Améglio et al., , 2001 Ewers et al., 2001).In walnut trees, depending on the temperature, two types of positive xylem pressures have been ...
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.
Brassica napus L. (oilseed rape) is an important crop plant characterised by low nitrogen (N) use efficiency. This is mainly due to a weak N recycling from leaves that is related to incomplete protein degradation. Assuming that protease inhibitors are involved throughout protein mobilisation, the goal of this study was to determine their role in the control of N mobilisation associated with leaf senescence. Results showed that a 19-kDa polypeptide exhibiting trypsin inhibitor (TI) activity presented an increased gradient from the older to the younger leaves. According to the SAG12/Cab gene expression profile, which is an indicator of leaf senescence, mature leaves of nitrate-deprived plants presented an earlier initiation of senescence and a decrease in protein concentration when compared with nitrate-replete plants. This coincided with disappearance of both TI activity and a reduction in the transcript level of the BnD22 gene (encoding a protein sharing homology with Künitz protease inhibitor). In young leaves of N-deprived plants, initiation of senescence was delayed; soluble protein concentration was maintained while both TI activity and BnD22 transcripts were high. This indicates that in oilseed rape growing under nitrate deprivation, the more efficient N recycling from mature leaves contributes to the maintenance of growth in young leaves. The data suggest a significant role for protease inhibitors in the regulation of proteolytic processes associated with N mobilisation during leaf senescence.
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