Effects of Cycloheximide and Actinomycin D on Glycosidase Activities in the Cotyledons of Legume Seeds Following Imbibition

Role of desiccation in the termination of expression of genes for storage proteins

“…Although mature seed is metabohcally quiescent, resumption of metabolism during germination commences within minutes of the introduction of water to the dry seed It is generally acknowledged that early metabolic events utilize and depend upon components present within the dry seed, which are replaced as normal turnover events proceed during germination For example, protein synthesis during very early hydration of mature seed utilizes some of the conserved mRNA (which persists during desiccation and is stored in the mature dry seed) (reviewed in Bewley and Black, 1985, Kermode, 1990, Lane, 1991 However, within a few hours new mRNAs are synthesized and, as the conserved ones are degraded, protein synthesis, leading to a completion of germination, probably becomes increasingly dependent on the newly synthesized messages Some of the newly synthesized mRNAs may code for the same proteins as conserved messages, whereas others are for different products, perhaps proteins essential for the commencement of radicle elongation and/or differentiation during early seedling formation (Bewley and Black, 1985, Lalonde and Bewley, 1986, Datta et al, 1987 However, such proteins remain to be identified Gene expression during germination and early seedling growth shows precise temporal and spatial regulation (Datta et al, 1987, Allen et al, 1988, Harada et al, 1988, Kermode, 1990and references therein, Comai et al, 1992 Regulatory controls over the synthesis of post-germinative proteins (e g those involved in reserve mobilization) appear to differ among species, and have one or more components, l e transcnptional (where de not o synthesis occurs after germination), translational (where stored messages are accumulated during development, but not translated until seedling growth) and post-translational (where mRNA and protein are present during development, but enzyme activity is not detected until post-germinative growth) (Fervert et al, 1980, Marcus and Rodaway, 1982, Harada et al, 1988, Krishna and Murray, 1988 The ability of the seed to synthesize mRNAs for developmental proteins is retained during drying, although the rates at which their genes are transcribed decline with increasing water loss It is upon rehydration that developmental genes are no longer transcribed, and only then is the germinative and post-germinative programme of gene expression initiated (Comai and Harada, 1990) Simple 'on-off' models of gene expression during development and germination are oversimplistic, genes also appear to be subject to an 'up/down' modulation (Weintraub andIzant, 1984, Dure, 1985) A residual pattern of developmental protein synthesis may continue for several hours during germination prior to the expression of a pattern unique to germination and growt...…”