Mannitol Synthesis in Higher Plants

Abstract

Mannitol is a major photosynthetic product in many algae and higher plants. Photosynthetic pulse and pulse-chase 14C-radiolabeling studies with the mannitol-synthesizing species, celery (Apium graveolens L.) and privet (Ligustrum vulgare L.), showed that mannose 6-phosphate (M6P) and mannitol 1-phosphate were among the early photosynthetic products. A NADPH-dependent M6P reductase was detected in these species (representing two different higher plant families), and the enzyme was purified to apparent homogeneity (68-fold with a 22% yield) and characterized from celery leaf extracts. The celery enzyme had a monomeric molecular mass, estimated from mobilities on sodium dodecyl sulfate-polyacrylamide gels, of 35 kilodaltons. The isoelectric point was pH 4.9; the apparent Km (M6P) was 15.8 millimolar, but the apparent Km (mannitol 1-phosphate) averaged threefold higher; pH optima were 7.5 with M6P/NADPH and 8.5 with mannitol 1-phosphate/NADP as substrates. Substrate and cofactor requirements were quite specific. NADH did not substitute for NADPH, and there was no detectable activity with fructose 6-phosphate, glucose 6-phosphate, fructose 1-phosphate, mannose 1-phosphate, mannose, or mannitol. NAD only partially substituted for NADP. Mg2+, Ca2+, Zn2+, and fructose-2,6-bisphosphate had no apparent effects on the purified enzyme's activity. In vivo radiolabeling results and the enzyme's kinetics, specificity, and distribution (in two-plant families) all suggest that NADPHdependent M6P reductase plays an important role in mannitol biosynthesis in higher plants.Sugar alcohols (acyclic polyols or alditols) are obtained when the aldo or keto group of a sugar is reduced to a hydroxyl. Mannitol, the most frequently occurring sugar alcohol in plants, is particularly abundant in algae and has been detected in at least 70 higher plant families. It is a major carbohydrate in many members of some dicot families, e.g. the Scrophulariaceae, Oleaceae, Rubiaceae, and Apiaceae (2). Until recently, however, little information has been available on mannitol's role in higher plants (16,17 that it is an early photosynthetic product (27, 29) and present in phloem tissue or phloem exudates of celery (family Apiaceae) (9) and species in many other families, e.g. the Oleaceae (30). Other physiological roles have been proposed, including osmoregulation, storage and recycling of reducing power, and service as a compatible solute (16,17), but very little is known of mannitol metabolism in higher plants. A M6PR2 has been reported as being located in the cytosol of mesophyll protoplasts from celery (27). Preliminary labeling data derived from celery and privet were responsible for the initial assays for reductase activity with M6P and mannitol 1-P as substrates.Here we demonstrate the formation of M6P and mannitol 1-P as early photosynthetic products in celery and privet (family Oleaceae). We also report evidence for the role and importance ofM6PR and its characteristics in mannitol biosynthesis in celery. MATERIALS AND METHODS Plant Mate...

Exogenous application of mannitol and thiourea regulates plant growth and oxidative stress responses in salt-stressed maize (Zea maysL.)

“…It constitutes as much as 50% of the translocated photoassimilates (Loester et al 1992). So, the more increasing effect of mannitol as compared to that of thiourea might be due to that it is a source of C and energy for plant growth.…”

“…Its proportion is about 50% of the total translocated photoassimilates (Loester et al 1992). Although mannitol plays an important role in osmotic adjustment, it acts as an antioxidant to scavenge of hydroxyl radicals (OH ) (Shen et al 1997;Srivastava et al 2010).…”

Gas Exchange and Carbon Partitioning in the Leaves of Celery (Apium graveolens L.) at Various Levels of Root Zone Salinity

“…M6PR was assayed according to Loescher et al (1992). Assays were run at at least two different extract volumes; further assays were performed if linearity with extract volume was poor (i.e.…”

Carbon Fluxes in Mature Peach Leaves

“…celery (5,13), less is known about carbon partitioning in sorbitol-synthesizing species. Since the labeling study of Redgwell and Bieleski (20) and the purification of aldose-6-P reductase (EC 1.1.1.200) from loquat (10) and apple leaves (18), the precursor for sorbitol biosynthesis in mature leaves is known to be glucose-6-P.…”