Hexokinase mediates stomatal closure

Abstract

SUMMARYStomata, composed of two guard cells, are the gates whose controlled movement allows the plant to balance the demand for CO 2 for photosynthesis with the loss of water through transpiration. Increased guardcell osmolarity leads to the opening of the stomata and decreased osmolarity causes the stomata to close. The role of sugars in the regulation of stomata is not yet clear. In this study, we examined the role of hexokinase (HXK), a sugar-phosphorylating enzyme involved in sugar-sensing, in guard cells and its effect on stomatal aperture. We show here that increased expression of HXK in guard cells accelerates stomatal closure. We further show that this closure is induced by sugar and is mediated by abscisic acid. These findings support the existence of a feedback-inhibition mechanism that is mediated by a product of photosynthesis, namely sucrose. When the rate of sucrose production exceeds the rate at which sucrose is loaded into the phloem, the surplus sucrose is carried toward the stomata by the transpiration stream and stimulates stomatal closure via HXK, thereby preventing the loss of precious water.

Acclimation to Fluctuating Light Impacts the Rapidity of Response and Diurnal Rhythm of Stomatal Conductance

“…This has also been described previously by transpiration (Lu et al, 1997;Outlaw 2003;Kang et al, 2007;Kelly et al, 2013;Lawson et al, 426 2014;Daloso et al, 2016). However, no significant acclimation of the slow decrease in A and 427 g s through the day by growth light intensity and pattern was observed in this experiment.…”

Transitioning to the Next Phase: The Role of Sugar Signaling throughout the Plant Life Cycle

“…Importantly, it was shown that catalytically inactive HXK1 mutant forms still have a signaling function, supporting the view that the role of HKX1 in signaling high sugar availability is independent of its role in Glc metabolism (Moore et al, 2003). More recent work has shown that HXK1 also mediates stomatal closure, thereby contributing to the feedback effect of sugar accumulation on photosynthesis (Kelly et al, 2012;Kelly et al, 2013).…”

Diurnal Variation in Gas Exchange: The Balance between Carbon Fixation and Water Loss

“…Initially proposed by Outlaw and coworkers (Outlaw and Manchester, 1979;Lu et al, 1995Lu et al, , 1997Ewert et al, 2000;Outlaw and De Vlieghere-He, 2001;Kang et al, 2007), Suc generated by mesophyll photosynthesis is uploaded to the phloem and transported away from sources to sinks driven by transpiration (Outlaw and De Vlieghere-He, 2001). Excess Suc (when photosynthesis is high) is carried toward the stomata by the apoplast, stimulating stomatal closure either through some signaling mechanism or by acting as an osmoticum (Lu et al, 1997;Outlaw, 2003;Kang et al, 2007;Kelly et al, 2013). Such a process could only occur over longer time scales, as high rates of photosynthesis are not associated with low g s ; however, decreases in g s often are seen toward the end of the diurnal period, despite environmental conditions being similar to morning conditions .…”

“…Over the diurnal period, a number of species display a decrease in g s and A that is not driven by decreases in light intensity or the temporal response of g s (Mott and Parkhurst, 1991;Allen and Pearcy, 2000;Mencuccini et al, 2000;Moriana et al, 2002;Dodd et al, 2006;de Dios et al, 2012), but the exact mechanism for this requires further investigation. As discussed earlier, sugar accumulation due to high A is believed to provide a long-term photosynthetic feedback on g s (Lu et al, 1995(Lu et al, , 1997Outlaw, 2003;Kang et al, 2007;Kelly et al, 2013), which also needs to be taken into account when considering the incorporation of temporal responses into models of stomatal behavior. Noe and Giersch (2004) proposed a model based on the assumption that the pool of sugars, resulting from the difference between the rate of sugar production by photosynthesis and their rate of export, increasingly inhibited A over the diurnal period.…”