Beta-1,3:1,4-glucan synthase activity in rice seedlings under water

Abstract

Background and aims: The metabolism of beta-1,3 : 1,4-glucan regulates the mechanical properties of cell walls, and thereby changes the elongation growth of Poaceae plants. A previous study has shown that elongation growth of rice coleoptiles under water is enhanced by increased activity of beta-1,3 : 1,4-glucan hydrolases; however, the involvement of beta-1,3 : 1,4-glucan synthase activity in elongation growth under water has not yet been clarified.

Methods: The beta-1,3 : 1,4-glucan synthase activity in a microsomal fraction prepared from rice seedlings grown under water was compared with that from control seedlings grown in air. The change under water in the relative expression level of CslF6, a major isoform of the beta-1,3 : 1,4-glucan synthase genes, was examined by quantitative reverse-transcriptase PCR.

Key results: The level of beta-1,3 : 1,4-glucan synthase activity in submerged seedlings decreased to less than 40 % of that of the control seedlings and was accompanied by a significant reduction in the amount of beta-1,3 : 1,4-glucan in the cell walls. Under water, the expression of CslF6 was reduced to less than 20 % of the unsubmerged control. Bubble aeration partially restored both beta-1,3 : 1,4-glucan synthase activity and the expression of CslF6 under water, correlating with suppression of the submergence-induced elongation growth of coleoptiles.

Conclusions: Submergence down-regulates the expression of the CslF6 gene, leading to a decreased level of beta-1,3 : 1,4-glucan synthase activity. Together with the increased activity of beta-1,3 : 1,4-glucan hydrolases, the decreased activity of beta-1,3 : 1,4-glucan synthase contributes to the decrease in the amount of beta-1,3 : 1,4-glucan in the cell walls under water. The suppression of beta-1,3 : 1,4-glucan synthesis under water may be mainly due to oxygen depletion.

Fig. 1.

Detection of β-1,3 : 1,4-glucan synthase activity in rice seedlings grown in air. (A) Length of rice coleoptiles at various growth stages. (B) β-1,3 : 1,4-glucan synthase activity in microsomes prepared from the rice seedlings grown in air was assayed using a reaction mixture containing UDP-[¹⁴C]Glc as the substrate. The diagnostic oligosaccharides G4G3G and G4G4G3G are indicated, released from the β-1,3 : 1,4-glucan product by the action of lichenase followed by paper chromatographic separation. (C) β-1,3 : 1,4-glucan synthase activities per mg protein and per g fresh weight, determined based on the radioactivity of G4G3G and G4G4G3G. Values are means ± s.e of triplicate assays.

Fig. 2.

Effects of submergence on β-1,3 : 1,4-glucan synthase activity. (A) Length of coleoptiles before the submergence treatment (initial), grown for 12 h under water without aeration (submergence), with aeration (submergence + aeration) and in air (control). (B) β-1,3 : 1,4-glucan synthase activities per mg protein and per g fresh weight in the seedlings. Values are means ± s.e. of triplicate assays.

Fig. 3.

Change in relative amount of CslF6 mRNA under water. The relative amount of CslF6 mRNA to ACT1 mRNAs in the seedlings was measured by quantitative RT-PCR. Treatments are as described in Fig. 2. Values are means ± s.e. of triplicate assays.