MOLECULAR-CLONING AND ETHYLENE INDUCTION OF MESSENGER-RNA ENCODING A PHYTOALEXIN ELICITOR-RELEASING FACTOR, BETA-1,3-ENDOGLUCANASE, IN SOYBEAN

Soybean (Glycine max) β-1,3-endoglucanase (EC 3.2.1.39) is involved in one of the earliest plant-pathogen interactions that may lead to active disease resistance by releasing elicitor-active carbohydrates from the cell walls of fungal pathogens. Ethylene induced β-1,3-endoglucanase activity to 2- to 3-fold higher levels in cotyledons of soybean seedlings. A specific polyclonal antiserum raised against purified soybean β-1,3-endoglucanase was used to immunoprecipitate in vitro translation products, demonstrating that ethylene induction increased translatable β-1,3-endoglucanase mRNA. Several cDNA clones for the endoglucanase gene were obtained by antibody screening of a λ-gt11 expression library prepared from soybean cotyledons. Hybrid-select translation experiments indicated that the cloned cDNA encoded a 36-kilodalton precursor protein product that was specifically immunoprecipitated with β-1,3-endoglucanase antiserum. Escherichia coli cells expressing the cloned cDNA also synthesized an immunologically positive protein. Nucleotide sequence of three independent clones revealed a single uninterrupted open reading frame of 1041 nucleotides, corresponding to a polypeptide of 347 residue long. The primary amino acid sequence of β-1,3-endoglucanase as deduced from the nucleotide sequence was confirmed by direct amino acid sequencing of trypsin digests of the glucanase. The soybean β-1,3-endoglucanase exhibited 53% amino acid homology to a β-1,3-glucanase cloned from cultured tobacco cells and 48% homology to a β-(1,3-1,4)-glucanase from barley. Utilizing the largest cloned cDNA (pEG488) as a hybridization probe, it was found that the increase in translatable β-1,3-endoglucanase mRNA seen upon ethylene treatment of soybean seedlings was due to 50- to 100-fold increase in steady state mRNA levels, indicating that ethylene regulates gene expression of this enzyme important in disease resistance at the level of gene transcription.