SEQUENCE-ANALYSIS OF A MANNITOL DEHYDROGENASE CDNA FROM PLANTS REVEALS A FUNCTION FOR THE PATHOGENESIS-RELATED PROTEIN ELI3

Mannitol is the most abundant sugar alcohol in nature, occurring in ba cteria, fungi, lichens, and many species of vascular plants. Celery (A pium graveolens L.), a plant that forms mannitol photosynthetically, h as high photosynthetic rates thought to result from intrinsic differen ces in the biosynthesis of hexitols vs. sugars, Celery also exhibits h igh salt tolerance due to the function of mannitol as an osmoprotectan t. A mannitol catabolic enzyme that oxidizes mannitol to mannose (mann itol dehydrogenase, MTD) has been identified, In celery plants, MTD ac tivity and tissue mannitol concentration are inversely related, MTD pr ovides the initial step by which translocated mannitol is committed to central metabolism and, by regulating mannitol pool size, is importan t in regulating salt tolerance at the cellular level, We have now isol ated, sequenced, and characterized a Mtd cDNA from celery, Analyses sh owed that Mtd RNA was more abundant in cells grown on mannitol and les s abundant in salt-stressed cells, A protein database search revealed that the previously described ELI3 pathogenesis-related proteins from parsley and Arabidopsis are MTDs, Treatment of celery cells with salic ylic acid resulted in increased MTD activity and RNA, Increased MTD ac tivity results in an increased ability to utilize mannitol, Among othe r effects, this may provide an additional source of carbon and energy for response to pathogen attack. These responses of the primary enzyme controlling mannitol pool size reflect the importance of mannitol met abolism in plant responses to divergent types of environmental stress.