CLASS-I BETA-1,3-GLUCANASES IN THE ENDOSPERM OF TOBACCO DURING GERMINATION

Rupture of the seed coat and rupture of the endosperm are separate eve nts in the germination of Nicotiana tabacum L. cv Havana 425 seeds. Tr eatment with 10(-5) M abscisic acid (ABA) did not appreciably affect s eed-coat rupture but greatly delayed subsequent endosperm rupture by m ore than 100 h and resulted in the formation of a novel structure cons isting of the enlarging radicle with a sheath of greatly elongated end osperm tissue. Therefore, ABA appears to act primarily by delaying end osperm rupture and radicle emergence. Measurements of beta-1,3-glucana se activity, antigen content, and mRNA accumulation together with repo rter gene experiments showed that induction of class I beta-1,3-glucan ase genes begins just prior to the onset of endosperm rupture but afte r the completion of seed-coat rupture. This induction was localized ex clusively in the micropylar region of the endosperm, where the radicle will penetrate. ABA treatment markedly inhibited the rate of beta-1,3 -glucanase accumulation but did not delay the onset of induction. Inde pendent of the ABA concentration used, onset of endosperm rupture was correlated with the same beta-1,3-glucanase content/seed. These result s suggest that ABA-sensitive class I beta-1,3-glucanases promote radic le penetration of the endosperm, which is a key limiting step in tobac co seed germination.