Arabidopisis seeds

          Arabidopsis thaliana, a small flowering annual dicotyledoneous plant, was discovered by Johannes Thal (hence, thaliana) in the Harz mountains in the sixteenth century. However, it is only since 1943 that Freidrich Laibach reported for the first time the potential of this plant as a model organism for genetic studies (Sommerville and Koornneef, 2002). He was particularly interested in natural variation and the effects of light quality and quantity on flowering time and seed dormancy. One of the features of Arabidopsis that attracted him was the large variation in physiological traits among accessions, which he started to collect systematically in 1937. In 1943, he outlined the suitability of Arabidopsis as a model for genetic and developmental biological research because it produced large numbers of progeny and developed rapidly, was easy to cultivate in limited space, had abundant natural variation, produced fertile hybrids and had a relatively low chromosome number (Laibach, 1943).
          Arabidopsis is a member of brassicaceae family, which includes important crops such as rape, cabbage and radish. It has no agronomic significance, but offers important advantages for basic research in genetics and molecular biology. It possesses a rapid life cycle (about 8 weeks from germination to mature seed) and one of smallest genome (125 Mb) amongst plant species. Its genome has been almost integrally sequenced in the year 2000 (SequenceViewer, A.G.I.), which permitted to build up a reference data base for plant genomics. Furthermore, because of the syntenic relationships existing between plant genomes, the information gained with Arabidopsis are surmised to be of paramount importance for the characterization of those genes that govern agronomic traits in crops. Most importantly, Arabidopsis can be very easily and efficiently transformed by simply spraying flowers with bacteria (Agrobacterium tumefaciens) that contain a gene of interest in a plasmid (Bechtold et al., 1993). In particular, this allowed the creation of large collections of insertion mutants based upon a random integration of T-DNA inserts into the plant nuclear genome. Array). Large collections of characterized mutations and transgenic plants are also available, in which most aspects of plant growth and development have been disrupted (see The Arabidopsis Information Resource (TAIR):; MIPS: ; Stock Centers).

        Seed development consists of a conversion of the integument of the ovule into a resistant seed coat, the development of the endosperm and of the embryo. All these events take place within the original ovary. Later during development the seed, containing a full size embryo, undergoes maturation during which food reserves accumulate and dormancy and desiccation tolerance develop (Raz et al., 2001).

        Seed dormancy refers in seeds to the failure of a viable seed to germinate even when given favorable environmental conditions. In nature, dormancy mechanisms ensure that seeds will germinate at the proper time. Modern crops have been bred to exhibit reduced dormancy so that seeds can germinate immediately after sowing.

        Maturation drying is the normal terminal event in the vast majority of seeds, after which they pass into a metabolically quiescent state. Remarkably, seeds may remain in this state for many years, from decades to millennia, and still retain their viability (Hoekstra et al., 2001). Upon hydration under suitable conditions, the seed, if not dormant, reactivates its metabolism and commences germination, giving rise to a new plant (Bewley and Black, 1994; Bewley, 1997). Seed germination can be divided into three phases, imbibition, increased metabolic activity, and initiation of growth, which loosely parallel the triphasic water uptake of dry mature seeds (Figure 1). Morphologically, initiation of growth corresponds to radicle emergence; subsequent growth is generally defined as seedling growth. By definition, germination sensu stricto incorporates those events that start with the uptake of water by the quiescent dry seed and terminate with the protrusion of the radicle and the elongation of the embryonic axis (Bewley, 1997).

        Besides affording the appropriate physico-chemical conditions for reducing metabolic activity, the massive water loss occurring during late maturation is surmised to play a role in the switch in cellular activities from an exclusively developmental program to an exclusively germination/growth-oriented program (Kermode, 1990, 1995). Indeed, during seed development, metabolism is largely anabolic, being characterized by the synthesis and deposition of reserves. In contrast, during germination and initial plant growth, mobilization of the stored reserves occurs so as to provide an energy source for the growing seedling (Bewley and Black, 1994; Job et al., 1997; Eastmond and Graham, 2001; Gallardo et al., 2001). Despite numerous studies, the nature of the mechanisms involved in fine regulation of metabolic activity during maturation-drying and germination is still unclear.

        We are interested in determining the biochemical and genetic mechanisms that regulate the transition from quiescence to highly active metabolism during germination and seedling establishment. To this end, we develop a proteome analysis of the model plant Arabidopsis thaliana (Gallardo et al., 2001, 2002a,b). This global proteomics approach would allow characterizing the accumulation pattern of key metabolic enzymes in germinating seeds. mRNA profiling expression studies during Arabidopsis seed development (Girke et al., 2000) and barley seed germination (Potokina et al., 2002) have recently been reported.

        The data presented in this website are from our initial studies (Gallardo et al., 2001, 2002a,b). Newly identified proteins will be included progressively.

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