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Journal of Experimental Botany, Vol 50, 1613-1618, Copyright © 1999 by Oxford University Press

ARTICLES

Galactose stimulation of carbon import into roots is confined to the Poaceae

M Thorpe, E MacRae, M Minchin and C Edwards
Horticulture and Food Research Institute of New Zealand, Private Bag 3123, Hamilton, New Zealand; Horticulture and Food Research Institute of New Zealand, Private Bag 92169, Auckland, New Zealand; Corresponding author; Fax: +64 7 858 4700; E-mail: MThorpe@hort.cri.nz

Galactose applied to barley roots causes a transient promotion of carbon import into the roots, followed by growth inhibition and a decline in carbon import. In this study the ubiquitous nature of the promotion of carbon import, and whether the response occurs primarily in the cell wall or in the cell, was investigated. 11C movement into roots was measured across a range of monocotyledons and dicotyledons in response to exposing the root environment to 20 mM galactose. Only members of the Poaceae showed a transient increase in carbon import similar to that previously reported in barley. All other species showed a decline, similar to that recorded for other sugars examined in barley. Addition of D-galactono-1,4-lactone (a galactose analogue) to barley roots showed no transient increase in carbon import. After removal of the lactone, the roots responded to galactose with an increase in carbon import. Unlike other plants, members of the Poaceae have cell wall polysaccharides containing very low amounts of galactose, and low levels of UDP-galactose (glucose) epimerase. During cell expansion the walls transiently contain a 1-3, 1-4 glucan which requires UDP-glucose as a precursor. It is proposed that the galactose challenge causes elongating Poaceae cells to be temporarily starved of UDP-glucose, and that phloem import is therefore stimulated.Keywords: UDP-glucose, epimerase, root elongation, growth inhibition, -galactosidase, 11C.
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