During Arabidopsis seed development large quantities of mucilage, composed of pectins, are deposited into the apoplast underneath the outer wall of the seed coat. Upon imbibition of mature seeds, the stored mucilage expands through hydration and breaks the outer cell wall that encapsulates the whole seed. Mutant seeds carrying loss-of-function alleles of AtSBT1.7 that encodes one of 56 Arabidopsis thaliana subtilisin-like serine proteases (subtilases) do not release mucilage upon hydration. Microscopic analysis of the mutant seed coat revealed no visible structural differences compared with wild-type seeds. Weakening of the outer primary wall using cation chelators triggered mucilage release from the seed coats of mutants. However, in contrast to mature wild-type seeds, the mutant’s outer cell walls did not rupture at the radial walls of the seed coat epidermal cells, but instead opened at the chalazal end of the seed, and were released in one piece. In atsbt1.7, the total rhamnose and galacturonic acid contents, representing the backbone of mucilage, remained unchanged compared with wild-type seeds. Thus, extrusion and solubility, but not the initial deposition of mucilage, are affected in atsbt1.7 mutants. AtSBT1.7 is localized in the developing seed coat, indicating a role in testa development or maturation. The altered mode of rupture of the outer seed coat wall and mucilage release indicate that AtSBT1.7 triggers the accumulation, and/or activation, of cell wall modifying enzymes necessary either for the loosening of the outer primary cell wall, or to facilitate swelling of the mucilage, as indicated by elevated pectin methylesterase activity in developing atsbt1.7 mutant seeds.
At the time of fertilization a typical angiosperm ovule consists of one or two protective coats – the integuments – and a central tissue – the nucellus. Often the integuments and the nucellus are clearly differentiated only in the region of the micropyle – the minute pore in the integuments through which, in many species, the pollen tube enters the nucellus. The ovule is attached to the wall of the ovary by a stalk – the funicle.
Pollination and Fertilization
Seed development is initiated by fertilization, the union of a haploid male nucleus from the pollen grain with a haploid female nucleus within the ovule to form a new diploid organism. Fertilization must be preceded by pollination, the arrival of a pollen grain on the stigma of the female flower in angiosperms or close to the micropyle of the gymnosperm ovule. It is important to distinguish the two separate processes of pollination and fertilization (Fritsch & Salisbury 1947). In most angiosperms the elongation of the pollen tube is rapid and the interval between pollination and fertilization is only a few days or even hours. In a few angiosperms (e.g. Liquidambar, some species of Quercus) and many gymnosperms (e.g. Pseudotsuga, Larix, Picea) the interval is several weeks or months, while in other species of Quercus and in many Pinus it is a year to 14 months (Krugman et al. 1974, Kozlowski 1971).
Angiosperm Seed Development
From the forester’s point of view dormancy has some disadvantages. Delayed and irregular germination in the nursery is a serious constraint on efficient nursery management (Bonner et al. 1974). Much research has therefore gone into devising effective artificial treatments to remove dormancy, in order to ensure that the seeds germinate quickly and evenly in the nursery beds. These treatments are described in Chapter 8.