ALUMINUM IONS INDUCE OAT PROTOPLASTS TO PRODUCE AN EXTRACELLULAR (1-]3)BETA-D-GLUCAN

Aluminum chloride induced mesophyll protoplasts of oat (Avena sativa) to produce an extracellular polysaccharide (EPS). EPS induced by AlCl3 appeared identical to that produced in response to the phytotoxin victorin (JD Walton, ED Earle [1985] Planta 165: 407-415). Al ions at 1 millimolar were toxic to protoplasts, but maximum EPS production occurred at a sublethal concentration of 200 micromolar, assayed at pH 6.0. As measured by incorporation of [14C]glucose, AlCl3 stimulated EPS production 10- to 15-fold. Pretreatment of protoplasts with cycloheximide prevented EPS production but not cell death in response to AlCl3, indicating that protein synthesis was necessary for EPS production but not for the phytotoxicity of Al ions. The trivalent salts of Y, Yb, Gd, and In also induced EPS production but those of Sc, Fe, Ga, Cr, and La did not. Mesophyll protoplasts from an acid-soil tolerant oat cultivar, Coker 83-23, produced less EPS in response to AlCl3 than the acid-soil sensitive cultivar Fla 501. EPS was also produced by wheat (Triticum aestivum) and barley (Hordeum vulgare) protoplasts in response to AlCl3. An Al-tolerant cultivar of wheat, Atlas, produced less EPS than an Al-sensitive cultivar, Scout, but an Al-tolerant cultivar of barley, Dayton, produced more than the Al-sensitive cultivar Kearney. Therefore, production of EPS by protoplasts in response to Al ions did not appear to be related to Al ion tolerance at the level of whole plants. EPS fluoresced in the presence of Calcofluor and Sirofluor and was degraded by purified laminarinase [(1→3)β-D-glucanase] but not pectinase (polygalacturonase). EPS was composed solely of glucose in 1→ 3 linkages; hence it is a (1→3)β-D-glucan (callose).