Knowledge of the mature seed and its germination is of importance in the study of seed formation Indeed, embryogeny and germination are extensions of each other separated by a period of relative metabolic inactivity called quiescence, and are essentially different phases of the continuing process of embryo growth and development Obviously, the nature of the pre-quiescence embryo bears very heavily on early germination behaviour and its control, and answers to questions relating to germination behaviour are increasingly being sought during the period of seed development

The Seed: Germination

HSP101 belongs to the ClpB protein subfamily whose members promote the renaturation of protein aggregates and are essential for the induction of thermotolerance. We found that maize HSP101 accumulated in mature kernels in the absence of heat stress. At optimal temperatures, HSP101 disappeared within the first 3 days after imbibition, although its levels increased in response to heat shock. In embryonic cells, HSP101 concentrated in the nucleus and in some nucleoli. Hsp101 maps near the umc132 and npi280 markers on chromosome 6. Five maize hsp101-m-::Mu1 alleles were isolated. Mutants were null for HSP101 and defective in both induced and basal thermotolerance. Moreover, during the first 3 days after imbibition, primary roots grew faster in the mutants at optimal temperature. Thus, HSP101 is a nucleus-localized protein that, in addition to its role in thermotolerance, negatively influences the growth rate of the primary root. HSP101 is dispensable for proper embryo and whole plant development in the absence of heat stress.

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Maize HSP101 plays important roles in both induced and basal thermotolerance and primary root growth.

Publisher Summary This chapter discusses the early events in germination of seeds that have lost viability through prolonged storage or artificial aging and those seeds that remain moist when mature, the so-called recalcitrant seeds. Three phases of metabolism of development of seeds can be recognized: imbibition, the lag phase, and germination. The seeds of some species can be induced to germinate only if they have first become air dry and then rehydrated during imbibition. This applies also to immature seeds of certain species. The seeds of soybean can be induced to germinate at unusually early stages of development by first air-drying. Desiccation and rehydration terminate the synthesis of proteins normally accumulated during development and initiate the synthesis of different proteins: a germination pattern. Although the seed shows no obvious and outward sign of change during the lag phase, the effect of temperature on the duration of this phase can be taken as evidence that preparations are being made at the metabolic level for the eventual act of germination. The data on seed leakage and on maturation of mitochondria in pea suggest that there is a change in membrane architecture at water contents below 20%–30%. Such a change might possibly mediate the observed effects of desiccation and inhibition on seed development.

Early Events in Germination

The cell cycle is the series of molecular events that allows cells to duplicate and segregate their chromosomes to form new cells. The finding that a protein kinase, the product of the yeast cdc2 gene, was fundamental in the regulation of the G2/M and G1/S transitions, associated with unstable proteins named cyclins, opened a very exciting and dynamic research area. The number of gene products that participate in the development and regulation of the cell cycle may be in the hundreds, and there is a high degree of conservation in protein sequences and regulatory pathways among eukaryotes. Although there are clear differences between plants and animals in cell structure, organization, growth, development and differentiation, the same types of proteins and very similar regulatory pathways seem to exist. Seed germination appears to be an excellent model system for studying the cell cycle in plants. Imbibition will reactivate meristematic cells – most initially with a G1 DNA content – into the cell cycle in preparation for seedling establishment. Early events include a thorough survey of DNA status, since the drying process and seed storage conditions reduce chromosomal integrity. The initiation of cell cycle events leading to G1 and S phases, and of the germination process itself, may depend on a G1 checkpoint control. Most, if not all, cell cycle proteins appear to be already present in unimbibed embryos, although there is evidence of protein turnover in the early hours, suggesting the need for de novo protein synthesis. Regulation also may occur at the level of protein modification, because existing G1, S and G2 cell cycle proteins appear to be activated at precise times during germination. Thus, cell cycle control during seed germination may be exerted at multiple levels; however, knowledge of cell cycle events and their importance for germination is still scarce and fragmentary, and different species may have developed unique control mechanisms, more suited to specific germination characteristics and habitat.

The cell cycle and seed germination

Axes of soybean seeds are tolerant to dehydration at 6 hours of imbibition, but susceptible to dehydration injury if dried at 36 hours of imbibition. Smooth microsomal membranes were isolated from axes imbibed for 6 hours (dehydration tolerant state) and 36 hours (dehydration susceptible state) before and after dehydration treatment. The phase properties and the lipid composition of the membrane fraction were investigated. Wide angle x-ray diffraction patterns of microsomal membranes from axes imbibed for 6 or 36 hours indicated a liquid-crystalline to gel phase transition at approximately 7°C. Membranes from axes dehydrated at 6 or 36 hours of imbibition and rehydrated for 2 hours exhibited a phase transition at 7°C and 47°C, respectively. Changes in fatty acid saturation did not account for the changes in phase properties. However, the increased phase transition temperature of the membranes from dehydration injured axes was associated with an increase in free fatty acid:phospholipid molar ratio and a decrease in phospholipid:sterol ratio. These results suggests that dehydration prompted a deesterification of the linkage between glycerol and fatty acid side chains of the phospholipid molecules in the membrane. The resultant increase in free fatty acid content in the membrane is thought to alter the fluidity and phase properties of the membrane and contribute to dehydration injury.

R. Deltour

Papers

Relation between Water Stress and DNA Synthesis during Germination of Zea mays L.

Root ontogenesis during early germination of Zea mays