Expression of Vp1 and water channel proteins during seed germination

Abstract

Germination of seeds from individual seed capsules ofMesembryanthemum crystallinum (common ice plant) is spread out over time with some seeds germinating within 1 d (early, E) and others germinating up to more than 4 weeks after imbibition (late, L). L-seeds are characterized by a lack of expression of Cdc2-related transcripts and an increase of Vp1-transcripts after water uptake, while Cdc2-related transcripts increase early and Vp1 decline early in E-seeds. Maintenance of Vp1 transcription, which can be disrupted by abolishing translation activity, seems to be at the basis of prolonged dormancy in L-seeds. We have in addition characterized the expression of several MIP (water channel) proteins during germination and in organs of adult plants. Using probes specific for individual ice plant Mip, we observed differences during germination that are not exclusively due to water uptake. Mip transcripts increase before Lseeds begin to germinate. Gene-specific probes indicate that the expression of all Mip is high in germinating seedlings, but differences in expression exist in the root, hypocotyl and cotyledon. In adult plants, all Mip-transcripts are expressed at a significantly lower rate than in seedlings, and organ-specific expression of individual Mip transcripts is observed. Their expression, measured by MIP-specific antibodies, indicates developmental specificity of MIP in different organs and highest amounts in actively growing tissues.Key-words: Mesembryanthemum crystallinum; gene and protein expression; germination; seed dormancy; VP1; water channels.Abbreviations: ABA, abscisic acid; Cdc2, transcript for a cellcycle related protein; Mip/MIP, transcript/protein for aquaporins (AQP) or water channels (WCH); Vp1/VP1, transcript/protein for transcriptional activator VP1. INTRODUCTIONIn Mesembryanthemum crystallinum (common ice plant), germination of imbibed seeds from individual seed capsules is spread over a time of more than 4 weeks. This adaptation provides an advantage for survival in an environment characterized by infrequent and unpredictable water supply and a short rainy season (Bloom & Troughton 1979;Cushman & Bohnert 1997;Adams et al. 1998). We were interested in the basis of differences in dormancy maintenance and mechanism(s) underlying the differential germination capability of ice plant seeds.Maintaining dormancy is an active process, requiring gene expression and biochemical activities in dormant seeds (Larondelle et al. 1987;Bewley & Black 1994). Prolonged dormancy of seeds is a common phenomenon with agricultural importance. The mechanisms that maintain dormancy are largely unknown, but dormancy is broken over time when environmental factors cause a decline of endogenous inhibitors. Among the factors that maintain or break dormancy, either low or high temperatures or changes in temperature and phytohormone composition play important roles (Bewley & Black 1994;Nicolas, Nicolas & Rodriguez 1996). Continued activity in the seeds is most clearly shown by the breaking of dormancy following the inhi...

The expression of a Vp1-like gene and seed dormancy in Mesembryanthemum crystallinum.

“…This is different from observations in A. thaliana, corn and rice, where Vp1 transcripts are associated with seed maturation and are low or not detectable in germinating seeds (Giraudat et al, 1992, Hattori et al, 1994, McCarty et al, 1991, Parcy et al, 1994. In contrast to the dormant seeds, Mvp1 transcripts disappear from early-germinating seeds within a few days following imbibition but persists longer in late-germinating seeds (Fukuhara et al, 1999). Here, we characterize the ice plant Mvp1 transcript.…”

“…uptake, such as those encoding cdc2 and water channel proteins, and studied their expression. While late-germinating seeds took up water and expressed transcripts for water channel proteins, transcripts indicative of cell cycle progression and cell division were expressed much later than in early-germinating seeds (Fukuhara et al, 1999) (and data not shown).…”

“…This work started from an observation indicating different classes of seeds in a single seed capsule of the common ice plant (M. crystallinum) (Fukuhara et al, 1999). We observed seeds that were lightly colored by betalaines and these seeds germinated early (E seeds), often within the first day following imbibition.…”

“…Within single seed capsules, we observed stratification, with higher numbers of lightly colored, early germinating seeds towards the tip of the capsule, while seeds at the base were increasingly more pigmented. The latter invariably germinated late (Fukuhara et al, 1999). We were interested in searching for genes that distinguished the two phenotypes.…”

“…Seeds with light coloration, which are shed first, germinate immediately after imbibition, i.e., approximately 80% within two days, and subpopulations of pigmented seeds germinate later, with an average 50% germination within 10 to12 days. A small population of seeds may germinate only after 30 to 40 days (Fukuhara et al, 1999). Since dormancy and uneven germination pose problems during screening of EMS-generated mutant populations, we were interested in studying the basis of the dormancy phenotype.…”