Galactose as an Inhibitor of the Expansion of Root Cells

The inhibition of the growth of cultured tomato roots by galactose is due to an inhibition of cell expansion. Galactose is rapidly absorbed during the first 8 h following application and the full inhibitory effect on extension growth of the roots is exerted within the first 24 h. At a concentration of 0·05 per cent or less (50 per cent inhibition occurs at 0·035 per cent) the galactose is not toxic and growth continues for 7 days at the partially inhibited rate. The simultaneous presence of glucose reduces galactose uptake but significant galactose uptake continues at sites insensitive to a high concentration of external glucose. In presence of an appropriate level of glucose, although galactose uptake proceeds, the growth inhibitory effect of the galactose is fully reversed. Galactose reduces the content in the cell walls of the α-cellulose fraction and during feeding with (I-14C) galactose all the cell wall fractions become labelled. The α-cellulose fraction then yields galactose of high specific activity. Glucose inhibits the incorporation of carbon from galactose into the α-cellulose fraction and galactose inhibits the incorporation into this fraction of the carbon of sucrose. The hypothesis is developed that galactose inhibits cell expansion by a disruption of cellulose synthesis which involves a direct incorporation of the externally applied galactose into the a-cellulose fraction of the cell walls.