title: Raffinose in chloroplasts is synthesized in the cytosol and transported across the chloroplast envelope.
abstract: In chloroplasts, several water-soluble carbohydrates have been suggested to act as stress protectants. The trisaccharide raffinose (alpha-1,6-galactosyl sucrose) is such a carbohydrate but has received little attention. We here demonstrate by compartmentation analysis of leaf mesophyll protoplasts that raffinose is clearly (to about 20%) present in chloroplasts of cold-treated common bugle (Ajuga reptans L.), spinach (Spinacia oleracea L.) and Arabidopsis [Arabidopsis thaliana (L.) Heynh.] plants. The two dedicated enzymes needed for raffinose synthesis, galactinol synthase and raffinose synthase, were found to be extra-chloroplastic (probably cytosolic) in location, suggesting that the chloroplast envelope contains a raffinose transporter. Uptake experiments with isolated Ajuga and Arabidopsis chloroplasts clearly demonstrated that raffinose is indeed transported across the chloroplast envelope by a raffinose transporter, probably actively. Raffinose uptake into Ajuga chloroplasts was a saturable process with apparent K(m) and v(max) values of 27.8 mM and 3.3 micromol mg(-1) Chl min(-1), respectively.
authors.affiliation: Molekulare Pflanzenphysiologie, Institut für Pflanzenbiologie, Universität Zürich, Zollikerstrasse 107, 8008 Zürich, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Schneider
authors.forename: Felix
authors.initials: F
authors.lastname: Keller
mesh_terms: D031328:Ajuga / Q000378:metabolism; D017360:Arabidopsis / Q000378:metabolism; D001693:Biological Transport, Active; D002736:Chloroplasts / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism*; D005700:Galactosyltransferases / Q000378:metabolism; D011523:Protoplasts / Q000378:metabolism; D011887:Raffinose / Q000096:biosynthesis*; D018724:Spinacia oleracea / Q000378:metabolism
chemical_list: D005700:Galactosyltransferases; C096645:inositol 1-alpha-galactosyltransferase; C468749:raffinose synthase; D011887:Raffinose