The impact of mannose and other carbon sources on the elongation and diameter of the primary root of Arabidopsis thaliana

To determine to what extent plant growth and morphology are sensitive to perturbed carbon metabolism, we grew Arabidopsis thaliana L. (Heynh.) seedlings for 10 d in the presence of various carbon compounds and measured the length and diameter of the primary root. Compounds fell into three groups based on their effect on root length: group one supported about as much elongation as sucrose; group two supported about the same elongation as occurred in the absence of sugar; and group three reduced or even eliminated root growth. No compound changed the diameter of the root notably, although there was a weak, positive correlation between root diameter and elongation. To investigate the inhibition of root elongation by mannose, we transplanted seedlings on to test media and measured primary root growth over the subsequent 2 d. Mannose scarcely changed root diameter, in contrast to 2-deoxyglucose, which caused marked swelling, similar in extent to that caused by tunicamycin. Mannose inhibited elongation rate by 90% within 24 h and required a further 2 d to reduce the elongation rate to zero, with the saturating dose being 30 mM in the presence of 3% sucrose and 0.3 mM in its absence. By contrast, cell production rate was little affected over the first 2 d of treatment. The inhibition of elongation by mannose was not reproduced by two sugar analogs that cannot be phosphorylated on carbon six, was not affected by manipulating phosphate levels in the medium, and was largely prevented by simultaneous treatment with either 30 mM mannoheptulose, 1 mM glucose, or 56 mM fructose. These results suggest that mannose inhibits root elongation via hexokinase-mediated sugar signaling.