Sugar treatment inhibits IAA-induced expression of endo-1,3:1,4-β-glucanase EI transcripts in barley coleoptile segments

Abstract

The degradation of 1,3:1,4-beta-glucan by glucanases is believed to be critical for auxin-induced elongation in Gramineae coleoptile. In the present study, we reinvestigated the relationship between auxin-induced elongation and gene expression of glucanases upon treatment of coleoptile segments with sugars. Gene expression of exo-beta-1,3:1,4-glucanase ExoII was not affected by treatment with IAA and/or sucrose. In contrast, levels of endo-beta-1,3:1,4-glucanase EI transcripts increased in response to IAA treatment, which was negated by the addition of glucose or sucrose, although the addition of sucrose or glucose did not suppress IAA-induced elongation. Sugar composition analysis of the hemicellulosic fraction revealed that the addition of glucose suppressed the IAA-induced reduction of beta-glucan. In the coleoptile segments that were starved by pre-incubation in water, the IAA-induced accumulation of EI mRNA was accelerated, as compared with the non-starved segments, which suggests that the level of carbon source in the cytoplasm regulates EI expression. Moreover, in the basal region of coleoptiles, where IAA treatment does not induce elongation growth, high levels of EI transcripts were observed in the presence and absence of IAA treatment. These results strongly demonstrated that the expressions of exo- and endo-beta-glucanase genes are not directly involved in the IAA-induced loosening of cell walls associated with elongation and also suggests that cell walls may degrade 1,3:1,4-beta-glucan to provide glucose as an energy source for cell elongation.