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The testa of higher plant seeds protects the embryo against adverse environmental conditions. Its role is assumed mainly by controlling germination through dormancy imposition and by limiting the detrimental activity of physical and biological agents during seed storage. To analyze the function of the testa in the model plant Arabidopsis, we compared mutants affected in testa pigmentation and/or structure for dormancy, germination, and storability. The seeds of most mutants exhibited reduced dormancy. Moreover, unlike wild-type testas, mutant testas were permeable to tetrazolium salts. These altered dormancy and tetrazolium uptake properties were related to defects in the pigmentation of the endothelium and its neighboring crushed parenchymatic layers, as determined by vanillin staining and microscopic observations. Structural aberrations such as missing layers or a modified epidermal layer in specific mutants also affected dormancy levels and permeability to tetrazolium. Both structural and pigmentation mutants deteriorated faster than the wild types during natural aging at room temperature, with structural mutants being the most strongly affected.

The mechanisms imposing a gibberellin (GA) requirement to promote the germination of dormant and non-dormant Arabidopsis seeds were analyzed using the GA-deficient mutant ga1, several seed coat pigmentation and structure mutants, and the abscisic acid (ABA)-deficient mutant aba1. Testa mutants, which exhibit reduced seed dormancy, were not resistant to GA biosynthesis inhibitors such as tetcyclacis and paclobutrazol, contrarily to what was found before for other non-dormant mutants in Arabidopsis. However, testa mutants were more sensitive to exogenous GAs than the wild-types in the presence of the inhibitors or when transferred to a GA-deficient background. The germination capacity of the ga1-1 mutant could be integrally restored, without the help of exogenous GAs, by removing the envelopes or by transferring the mutation to a tt background (tt4 and ttg1). The double mutants still required light and chilling for dormancy breaking, which may indicate that both agents can have an effect independently of GA biosynthesis. The ABA biosynthesis inhibitor norflurazon was partially efficient in releasing the dormancy of wild-type and mutant seeds. These results suggest that GAs are required to overcome the germination constraints imposed both by the seed coat and ABA-related embryo dormancy.

It is widely believed that desert annual plants maintain between-year seed banks, yet few field studies actually have measured the proportion of the viable seed bank that remains dormant through a season. Dormancy and germination fractions were quantified for a guild of winter annuals on a creosote flat in the Sonoran Desert for three years. Predictions from two types of theoretical models applicable to temporally variable environments were examined: (1) the evolution of life history traits promoting persistence in the face of temporal variation and (2) the role of temporal variation in mediating species coexistence. The density of ungerminated seeds was estimated by collecting soil samples after germination, but prior to new seed set. Seedlings were followed in nearby plots to estimate the density of germinated seedlings and their reproductive success. Long-term data collected from permanent plots over a 10-yr period were used to calculate temporal variation in reproductive success for each species. Species with higher temporal variation in reproductive success had lower germination fractions and smaller seeds, consistent with the theory that seed dormancy and large seed size are partially substitutable bet-hedging strategies. The data also suggested that this system possesses traits that are necessary for temporal variation to promote coexistence. First, between-year seed banks, necessary to buffer populations in unfavorable years, were documented for 17 species. Second, there was a strong tendency for year-to-year variation in germination fractions to vary among species. Finally, plants germinated more in years of higher reproductive success. We discuss how a correlation between germination and reproductive success enhances the role of temporal variance in success hierarchies in promoting species coexistence.

In the gibberellin (GA) biosynthesis pathway, 20-oxidase catalyzes the oxidation and elimination of carbon-20 to give rise to C19-GAs. All bioactive GAs are C19-GAs. We have overexpressed a cDNA encoding 20-oxidase isolated from Arabidopsis seedlings in transgenic Arabidopsis plants. These transgenic plants display a phenotype that may be attributed to the overproduction of GA. The phenotype includes a longer hypocotyl, lighter-green leaves, increased stem elongation, earlier flowering, and decreased seed dormancy. However, the fertility of the transgenic plants is not affected. Increased levels of endogenous GA1, GA9, and GA20 were detected in seedlings of the transgenic line examined. GA4 which is thought to be the predominantly active GA in Arabidopsis, was not present at increased levels in this line. These results suggest that the overexpression of this 20-oxidase increases the levels of some endogenous GAs in transgenic seedlings, which causes the GA-overproduction phenotype

Dormancy is an adaptive strategy in plants to survive under unfavorable climatic conditions during winter. In temperate regions, most fruit trees need exposure to a certain period of low temperatures to overcome endodormancy. After endodormancy release, exposure to warm temperatures is needed to flower (ecodormancy). Chilling and heat requirements are genetically determined and, therefore, are specific for each species and cultivar. The lack of sufficient winter chilling can cause failures in flowering and fruiting, thereby compromising yield. Thus, the knowledge of the chilling and heat requirements is essential to optimize cultivar selection for different edaphoclimatic conditions. However, the lack of phenological or biological markers linked to the dormant and forcing periods makes it difficult to establish the end of endodormancy. This has led to indirect estimates that are usually not valid in different agroclimatic conditions. The increasing number of milder winters caused by climatic change and the continuous release of new cultivars emphasize the necessity of a proper biological marker linked to the endo- to ecodormancy transition for an accurate estimation of the agroclimatic requirements (AR) of each cultivar. In this work, male meiosis is evaluated as a biomarker to determine endodormancy release and to estimate both chilling and heat requirements in apricot. For this purpose, pollen development was characterized histochemically in 20 cultivars over 8 years, and the developmental stages were related to dormancy. Results were compared to three approaches that indirectly estimate the breaking of dormancy: an experimental methodology by evaluating bud growth in shoots collected periodically throughout the winter months and transferred to forcing chambers over 3 years, and two statistical approaches that relate seasonal temperatures and blooming dates in a series of 11–20 years by correlation and partial least square regression. The results disclose that male meiosis is a possible biomarker to determine the end of endodormancy and estimate AR in apricot.

We examined whether spectrally active phytochrome A (PhyA) and phytochrome B (PhyB) play specific roles in the induction of seed germination in Arabidopsis thaliana (L.) Heynh., using PhyA- and PhyB-null mutants, fre1-1 (A. Nagatani, J.W. Reed, J. Chory [1993] Plant Physiol 102: 269-277) and hy3-Bo64 (J. Reed, P. Nagpal, D.S. Poole, M. Furuya, J. Chory [1993] Plant Cell 5: 147-157). When dormant seeds of each genotype imbibed in the dark on aqueous agar plates, the hy3 (phyB) mutant did not germinate, whereas the fre1 (phyA) mutant germinated at a rate of 50 to 60%, and the wild type (WT) germinated at a rate of 60 to 70%. By contrast, seeds of all genotypes germinated to nearly 100% when plated in continuous irradiation with white or red light. When plated in continuous far-red light, however, frequencies of seed germination of the WT and the fre1 and hy3 mutants averaged 14, nearly 0, and 47%, respectively, suggesting that PhyB in the red-absorbing form prevents PhyA-dependent germination under continuous far-red light. When irradiated briefly with red or far-red light after imbibition for 1 h, a typical photoreversible effect on seed germination was observed in the fre1 mutant and the WT but not in the hy3 mutant. in contrast, when allowed to imbibe in the dark for 24 to 48 h and exposed to red light, the seed germination frequencies of the hy3 mutant were more than 40%. Immunoblot analyses of the mutant seeds showed that PhyB apoprotein accumulated in dormant seeds of the WT and the fre1 mutant as much as in the seeds that had imbibed. In contrast, PhyA apoprotein, although detected in etiolated seedlings grown in the dark for 5 d, was not detectable in the dormant seeds of the WT and the hy3 mutant. The above physiological and immunochemical evidence indicates that PhyB in the far-red-absorbing form was stored in the Arabidopsis seeds and resulted in germination in the dark. Hence, PhyA does not play any role in dark germinati

Molecular studies of late embryogenesis and seed development have emphasized differential gene expression as a means of identifying discrete stages of embryogenesis. Little has been done to identify factors that regulate the length of a given developmental stage or the degree of overlap between adjacent developmental programs. We designed a genetic screen to identify mutations that disrupt late embryo development in Arabidopsis without loss of hormonal responses. One such mutation, fusca3 (fus3), alters late embryo functions, such as the establishment of dormancy and desiccation tolerance, and reduces storage protein levels. fus3 cotyledons bear trichomes, and their ultrastructure is similar to that of leaf primordia. Immature fus3 embryos enter germinative development, and the shoot apical meristems develop leaf primordia before seed desiccation begins. The cotyledons resemble leaf primordia, yet retain some cotyledon characteristics; thus, cotyledon- and leaf-specific functions are expressed simultaneously. Together, these observations are consistent with a heterochronic interpretation of the fus3 mutation