Mannose-6-Phosphate Reductase, a Key Enzyme in Photoassimilate Partitioning, Is Abundant and Located in the Cytosol of Photosynthetically Active Cells of Celery (Apium graveolens L.) Source Leaves

Abstract

991 64-4238 (V.R.F.)Mannitol, a major photosynthetic product and transport carbohydrate in many plants, accounts for approximately 50% of the carbon fixed by celery (Apium graveolens 1.) leaves. Previous subfractionation studies of celery leaves indicated that the enzymes for mannitol synthesis were located in the cytosol, but these data are inconsistent with that published for the sites of sugar alcohol synthesis in other families and taxa, including apple (Malus) and a brown alga (fucus). Using antibodies to a key synthetic enzyme, NADPH-dependent mannose-6-phosphate reductase (MCPR), and immunocytochemical techniques, we have resolved both the intercellular and intracellular sites of mannitol synthesis. In leaves, MCPR was found only in cells containing ribulose-l,5-bisphosphate carboxylase/oxygenase. MCPR was almost exclusively cytosolic in these cells, with the nucleus being the only organelle to show labeling. The key step in transport carbohydrate biosynthesis that is catalyzed by MCPR displays no apparent preferential association with vascular tissues or with the bundle sheath. These results show that MCPR and, thus, mannitol synthesis are closely associated with the distribution of photosynthetic carbon metabolism in celery leaves. The principal role of MCPR is, therefore, in the assimilation of carbon being exported from the chloroplast, and i t seems unlikely that this enzyme plays even an indirect role in phloem loading of mannitol.The acyclic sugar alcohols (polyols) are second only to the sugars as principal products of carbon assimilation and account for an estimated 30% of global primary production (Bieleski, 1982). In a11 polyol-producing higher plants studied to date, Suc is also a major photoassimilate. In some species, sucrosyl-oligosaccharides (eg. raffinose) are found in the phloem sap in addition to the polyol and SUC. The nature of primary production and transport in higher plant polyol producers raises fundamental questions as to why such an apparently complex system has evolved when Suc alone performs adequately in many other species. Questions are also raised as to how carbon partitioning into polyols and other primary products is regulated at a biochemical, cellular, and tissue level. Celery (Apium graveolens L.) is a fine model in which to study such questions, because mature leaves

Methyl Jasmonate-Induced Ethylene Production Is Responsible for Conifer Phloem Defense Responses and Reprogramming of Stem Cambial Zone for Traumatic Resin Duct Formation

“…Our results show that the enzyme is present in the cytoplasm and the nucleus but not in other compartments. Nuclear localization in plants and animal cells has been shown for a number of different types of enzymes, including some in the glycolytic and gluconeogenic pathways (Yanez et al, 2003), Man-6-P reductase (Everard et al, 1993), plant hemoglobin (Seregelyes et al, 2000), apyrase (Shibata et al, 2002), lipoxygenase (Brock et al, 2001), and glutathione peroxidase (Orbea et al, 2000). Ethyleneresponsive transcription factors are present in the plant nucleus (Johnson and Ecker, 1998; Ohta et al, Figure 9.…”

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

“…The protocols followed Everard et al (1993a). For quantification of M6PR, Coomassie blue R-stained gels were laminated between two sheets of cellulose and the amount of M6PR was determined by densitometry using a Molecular Dynamics model 300A computing densitometer (Molecular Dynamics, Inc., Sunnyvale CA).…”

“…Celery (Apium graveolens L.) not only produces mannitol as a primary photosynthetic product , but it also has a halophilic ancestry (Bailey, 1942) and displays substantial salt tolerance and elevated levels of mannitol under salt stress Everard et al, 1993b;Stoop and Pharr, 1994). In addition, mannitol synthesis has been proposed as a supplemental mechanism to dissipate reducing power (NADPH) accumulated during the light reactions of photosynthesis (Loescher, 1987).…”

Sugar Alcohols, Salt Stress, and Fungal Resistance: Polyols—Multifunctional Plant Protection?

“…This enzyme catalyzes the conversion of mannose-6-P to mannitol 1-P, which is then dephosphorylated by a phosphatase to form mannitol (Loescher et al, 1992;Rumpho et al, 1983). Mannitol synthesis in celery occurs mainly in mature, source leaves where M6PR is localized in the cytosol of green palisade and spongy parenchyma tissues and bundle-sheath cells (Everard et al, 1993;Loescher et al, 1995). M6PR activity, and thus mannitol synthesis, is developmentally as well as light regulated, with high activities observed in mature leaves, but no detectable activities in sink tissues (such as roots and unstressed, immature leaves) (Everard et al, 1993and Stoop and Pharr 1994.…”

“…Mannitol synthesis in celery occurs mainly in mature, source leaves where M6PR is localized in the cytosol of green palisade and spongy parenchyma tissues and bundle-sheath cells (Everard et al, 1993;Loescher et al, 1995). M6PR activity, and thus mannitol synthesis, is developmentally as well as light regulated, with high activities observed in mature leaves, but no detectable activities in sink tissues (such as roots and unstressed, immature leaves) (Everard et al, 1993and Stoop and Pharr 1994. In extracts from mature leaves of celery plants irrigated with increasing concentrations of NaCl (up to 0.3 M) there is little or no change in the amount of M6PR protein (Everard et al, 1994).…”