Altered stomatal dynamics in ascorbate oxidase over-expressing tobacco plants suggest a role for dehydroascorbate signalling

Control of stomatal aperture is of paramount importance forplant adaptation to the surrounding environment. Here, we reporton several parameters related to stomatal dynamics and performancein transgenic tobacco plants (Nicotiana tabacum L., cv. Xanthi)over-expressing cucumber ascorbate oxidase (AO), a cell wall-localizedenzyme of uncertain biological function that oxidizes ascorbicacid (AA) to monodehydroascorbic acid which dismutates yieldingAA and dehydroascorbic acid (DHA). In comparison to WT plants,leaves of AO over-expressing plants exhibited reduced stomatalconductance (due to partial stomatal closure), higher watercontent, and reduced rates of water loss on detachment. Transgenicplants also exhibited elevated levels of hydrogen peroxide anda decline in hydrogen peroxide-scavenging enzyme activity. LeafABA content was also higher in AO over-expressing plants. Treatmentof epidermal strips with either 1 mM DHA or 100 µM hydrogenperoxide resulted in rapid stomatal closure in WT plants, butnot in AO-over-expressing plants. This suggests that signalperception and/or transduction associated with stomatal closureis altered by AO over-expression. These data support a specificrole for cell wall-localized AA in the perception of environmentalcues, and suggest that DHA acts as a regulator of stomatal dynamics. Key words: ABA, apoplast, ascorbic acid, ascorbate oxidase, dehydroascorbic acid (DHA), hydrogen peroxide, Nicotiana tabacum L., cv. Xanthi, stomata, transgenic plants, water stressReceived 26 September 2007; Revised 11 December 2007 Accepted 12 December 2007