Plant Physiol, March 2003, Vol. 131, pp. 1496-1507
Carbon Export from Arbuscular Mycorrhizal Roots Involves
the Translocation of Carbohydrate as well as Lipid
Berta
Bago,1
Philip E.
Pfeffer,
Jehad
Abubaker,
Jeongwon
Jun,
James W.
Allen,
Janine
Brouillette,
David D.
Douds,
Peter J.
Lammers, and
Yair
Shachar-Hill*
Eastern Regional Research Center (U.S. Department of
Agriculture/Agricultural Research Service), Wyndmoor, Pennsylvania
19038 (B.B., P.E.P., J.B., D.D.D.); Department of Chemistry and
Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003 (J.A., J.J., J.W.A., P.J.L., Y.S.-H.); and Plant Biology Department,
Michigan State University, East Lansing, Michigan 48823 (J.W.A.,
Y.S.-H.)
Arbuscular mycorrhizal (AM) fungi take up photosynthetically
fixed carbon from plant roots and translocate it to their external mycelium. Previous experiments have shown that fungal lipid synthesized from carbohydrate in the root is one form of exported carbon. In this
study, an analysis of the labeling in storage and structural carbohydrates after 13C1 glucose was provided
to AM roots shows that this is not the only pathway for the flow of
carbon from the intraradical to the extraradical mycelium (ERM).
Labeling patterns in glycogen, chitin, and trehalose during the
development of the symbiosis are consistent with a significant flux of
exported glycogen. The identification, among expressed genes, of
putative sequences for glycogen synthase, glycogen branching enzyme,
chitin synthase, and for the first enzyme in chitin synthesis
(glutamine fructose-6-phosphate aminotransferase) is reported. The
results of quantifying glycogen synthase gene expression within
mycorrhizal roots, germinating spores, and ERM are consistent with
labeling observations using 13C-labeled acetate and
glycerol, both of which indicate that glycogen is synthesized by the
fungus in germinating spores and during symbiosis. Implications of the
labeling analyses and gene sequences for the regulation of carbohydrate
metabolism are discussed, and a 4-fold role for glycogen in the AM
symbiosis is proposed: sequestration of hexose taken from the host,
long-term storage in spores, translocation from intraradical mycelium
to ERM, and buffering of intracellular hexose levels throughout the
life cycle.
1
Present address: CIDE, Cami de la Marjal s.n.,
46470-Albal, Valencia, Spain.
*
Corresponding author; e-mail yairhill{at}msu.edu;
fax 240-352-8021.
© 2003 American Society of Plant Biologists
