Abstract
Hexokinase (HXK) is a sugar-phosphorylating enzyme involved in sugar-sensing. It has recently been shown that HXK in guard cells mediates stomatal closure and coordinates photosynthesis with transpiration in the annual species tomato and Arabidopsis. To examine the role of HXK in the control of the stomatal movement of perennial plants, we generated citrus plants that express Arabidopsis HXK1 (AtHXK1) under KST1, a guard cell-specific promoter. The expression of KST1 in the guard cells of citrus plants has been verified using GFP as a reporter gene. The expression of AtHXK1 in the guard cells of citrus reduced stomatal conductance and transpiration with no negative effect on the rate of photosynthesis, leading to increased water-use efficiency. The effects of light intensity and humidity on stomatal behavior were examined in rooted leaves of the citrus plants. The optimal intensity of photosynthetically active radiation and lower humidity enhanced stomatal closure of AtHXK1-expressing leaves, supporting the role of sugar in the regulation of citrus stomata. These results suggest that HXK coordinates photosynthesis and transpiration and stimulates stomatal closure not only in annual species, but also in perennial species.
Highlights
Stomata, formed by two guard cells, open at dawn to allow the atmospheric carbon dioxide (CO2) needed for photosynthesis to enter the leaf, at the cost of extensive transpirational water loss
No expression was observed in any other tissues or plant parts such as mesophyll cells (Figure 1A6,9) or roots, indicating that the KST1 promoter might be an efficient tool for driving guard cell expression in citrus plants
The observed stomatal-closure effect was stronger in citrus plants expressing AtHXK1 in their guard cells (GCHXK plants) and the transpiration rate of intact GCHXK plants was lower than that observed in WT plants primarily in the middle of the day when light intensity and sugar production are probably high (Figure 4)
Summary
Stomata, formed by two guard cells, open at dawn to allow the atmospheric carbon dioxide (CO2) needed for photosynthesis to enter the leaf, at the cost of extensive transpirational water loss. When carbon fixation and utilization are less efficient, the stomata close to reduce the loss of water via transpiration (Assmann, 1993). Stomata open in response to increases in the osmolarity of the guard cells. Stomata close when the osmolarity of the guard cells is reduced and the water exits the guard cells. At the start of the previous century, the prevailing paradigm was that sugars generated from starch degradation in guard cells at dawn are the primary osmolytes that open stomata (Lloyd, 1908). The discovery that K+ ions, Cl− ions and malate ions are the primary osmolytes that open
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