Developmental and environmental induction of Lea and LeaA mRNAs and the postabscission program during embryo culture

The major programs of gene expression during late embryogenesis are the muturation or reserve accumulation program and, after ovule abscission, the postabscission program that is composed largely of Lea and LeaA mRNAs that probably encode desiccation protectants. There are diverse opinions about the developmental regulators of these programs. Several candidates are evaluated here by measuring, in cultured embryos, the accumulation kinetics of cloned mRNAs specifically expressed in the normal maturation, postabscission, or germination programs of cotton. Maturation-stage embryos both terminate the maturation program and induce the postabscission program after excision and culture, just as they do later in the plant after ovule abscission. However, they also induce simultaneously the germination program and are thus different from any normal stage of embryo development or germination. The developmental induction of the postabscission program in culture does not require exogenous abscisic acid, but its expression is enhanced by precocious desiccation or culture on abscisic acid or high osmoticum, probably by an environmentally responsive mechanism that normally operates during germination. Normal desiccation does not control any of these programs because the embryo acquires all of the characteristics of a mature embryo before it desiccates. These and other results suggest regulation of normal embryogenesis by a maternal maturation factor, a postabscission factor, and the postabscission program.