Analysis of celery (Apium graveolens) mannitol dehydrogenase (Mtd) promoter regulation in Arabidopsis suggests roles for MTD in key environmental andmetabolic responses

Of the growing list of promising genes for plant improvement, some of the m ost versatile appear to be those involved in sugar alcohol metabolism. Mann itol, one of the best characterized sugar alcohols, is a significant photos ynthetic product in many higher plants. The roles of mannitol as both a met abolite and an osmoprotectant in celery (Apium graveolens) are well documen ted. However, there is growing evidence that `metabolites' can also have ke y roles in other environmental and developmental responses in plants. For i nstance, in addition to its other properties, mannitol is an antioxidant an d may have significant roles in plant-pathogen interactions. The mannitol c atabolic enzyme mannitol dehydrogenase (MTD) is a prime modulator of mannit ol accumulation in plants. Because the complex regulation of MTD is central to the balanced integration of mannitol metabolism in celery, its study is crucial in clarifying the physiological role(s) of mannitol metabolism in environmental and metabolic responses. In this study we used transformed Ar abidopsis to analyze the multiple environmental and metabolic responses of the Mtd promoter. Our data show that all previously described changes in Mt d RNA accumulation in celery cells mirrored changes in Mtd transcription in Arabidopsis. These include up-regulation by salicylic acid, hexokinase-med iated sugar down-regulation, and down-regulation by salt, osmotic stress an d ABA. In contrast, the massive up-regulation of Mtd expression in the vasc ular tissues of salt-stressed Arabidopsis roots suggests a possible role fo r MTD in mannitol translocation and unloading and its interrelation with su gar metabolism.