STORED RESERVES IN SEED

Transcription

1 What do seeds store? STORED RESERVES IN SEED Carbohydrates Proteins Lipids Other protective or metabolically important compounds Phytate or phytic acid Tannins Cork, mucilage, alkaloids Hormones, vitamins Environmental effects Soil fertility seed size and weight. Water availability during flowering and seed fill decrease seed size. Early plant development - seed number Maternal growth environment and seed reserves ss-leaves-farmers-ranchers-high-and-dry/ Environmental effects Temperature high temperatures during seed fill tend to reduce seed size and seed germination (Gibson and Mullen, 1996; Egli et al., 2005). Light in general, reduced light results in smaller seeds Position in the plant related to sink- source effect and competition for limiting photosynthesis Phases of Seed Development and Maturation Seed depends on plant + environment Seed moisture & quality depends on environment Pollination & fertilization 1

2 Substrates needed by the seed A source of carbon for metabolic energy and synthesis of sugars and starch A source of nitrogen for synthesis of amino acids Other elements (potassium, calcium, phosphorus, etc.) CARBOHYDRATES Cereal grain: only % from carbohydrates stored in vegetative parts of the plant (pre-anthesis) Where does the required C for seed filling comes from? 85 to 90% of C originates in current photosynthesis Mobilization of stored carbohydrates in the plant Seed major sink for photosynthesis products during grain development Longevity of vegetative and reproductive biomass are critical Carbohydrates Starch is the main polysaccharide in the plants Starch granules consist primarily of amylose (~30%), amylopectin (~70%), and in some cases phytoglycogen 2

3 Floury endosperm in sorghum at physiological maturity showing starch granules and proteins Floury endosperm in sorghum after defoliation Goggi et al., 1993 Goggi et al., 1993 Carbohydrates - Storage polysaccharides other than starch Hemicellulose (major carbohydrate in nonendospermic seeds) are deposited in the cell walls Mucilage usually a seed dispersal mechanism recovers seed coat in buckhorn plantain and flax, also in endosperm of legumes Pectic compounds cell wall Fig. 2.Ruthenium red staining of wild-type and mutant seeds. A, Wild-type seed placed directly into stain without agitation. Two layers of mucilage are present, an outer, cloudy layer, and an inner, intensely staining layer. B, Wildtype seed first shaken in water, then stained; the outer layer of mucilage is not stained. Note sharp outline to the columellae. C, mum2-1 seed stained after shaking in water. No capsule of mucilage is apparent and the columellae are less defined than in B. D, mum5-1 seed stained after shaking in water. Columellae are sharply outlined and a thin layer of palely staining mucilage is apparent directly around the seed. E, mum5-1 seed placed directly in stain without shaking. Both inner and outer layers of mucilage are present, as in A. F, Wild-type seed stained after first shaking in presence of EDTA. Only a thin layer of palely staining mucilage is visible, as in D. Scale bars = 200 μm. Western et al., 2001 Other carbohydrates stored in seeds 5 to 15 % of the dry matter in pea and bean seed are stored oligosaccharides of the raffinose family Free sugars (rarely) sugar maple up to 11% of dry wt. of mature seed PROTEINS 3

4 The biological role of storage proteins is to provide carbon, nitrogen and sulphur for seedling growth 70 % of human demand for protein is met by direct or indirect (animal) consumption of seed Cereals: 10 15% of DW Legumes: up to 40 50% of DW T.B. Osborne (1924) Classification of proteins according to solubility Albumins, soluble in H 2 O Globulins, soluble in diluted salt solutions Glutelins, soluble in diluted alkali and acids solutions Prolamins, soluble in aqueous ethanol Figure (A,B) Changes in the endosperm protein fractions during kernel development of normal Bomi barley (A) and the high-iysine barley mutant Risø 1508 (B)., Albumins (plus free amino acids); *, globulins;, hordeins (prolamins);, glutelins. (C) The accumulation of vicilin ( ), legumin ( ), and albumins ( ) in developing cotyledons of broad bean (Vicia faba). (D) Accumulation of, ', and subunits of the 7 S storage protein -conglycinin in developing soybean seeds. A and B, after Brandt (1976); C, after Manteuffel et al. (1976); and D, after Gayler and Sykes (1981). Storage Proteins Usually have no enzymatic activities Hygroscopic nature aggregate into small particles Deposited in the cell within protein bodies surrounded by a single membrane Transmission electron micrographs of protein bodies from (A) normal sorghum. Henley et al., In: Advances in Food and Nutrition Research, Volume 60 (Chapter 2). LIPIDS 4

5 Lipids Contained primarily in seed (fruits like olives and avocados) Source of C during seed germination 20 % of human caloric uptake in industrialized countries comes from plant fats 10% of world production is used for industrial purposes Higher content of unsaturated fatty acids than animal fats Lipids For our purpose two kinds of lipids Storage lipids deposited in lipid bodies Gycerol + fatty acid Lipids For our purpose two kinds of lipids Membranes polar with a hydrophilic and an hydrophobic end MEMBRANE STRUCTURE- PHOSPHOLIPID BILAYER Table 1.4. The major fatty acid composition of commercial oils of various plant sources [after Weber (1980) and Miller (1931)]. Palmitic Stearic Oleic Linoleic Linolenic Species (16:0) (18:0) (18:1) (18:2) (18:3) Sunflower Maize Soybeans Canola Cotton Peanut Oil Palm Flax Animal fat

6 Phytin: source of phosphate and mineral ions % of all P in the seed is in the form of phytate Associated with protein bodies of the aleurone layer in cereals Phytin Seeds/Seed-6Cereal.htm Physiological roles of phytin Reserve compound for inositol, phosphate, and K, Mg, Ca, Fe, Mn Control of physiological balance of P in developing seeds and seedlings Important for plant adaptation to the surrounding environment Tannins Tannin Deposited in the cell wall of the seed coat (e.g. cocoa and beans) Defends the seed from attacks by predator by making them less digestible Also protects the seed against light and delays seed decomposition in the soil Restrict germination by limiting gas flow Other storage materials Cork cell walls with suberine between the epidermis and other tissues of the seed Alkaloids morphine (poppy), strychnine (nux vomica), caffeine (coffee, cocoa, tea), nicotine (tabacco) Poppy seed Cork Oak seed 6

7 Hormones in the developing seed ABA Auxin Gibberellins Cytokinins Possible role of seed hormones Seed growth and development Accumulation of storage reserves Storage for later use during germination Tissues close to the developing fruit Vitamins All vitamins and precursors are synthesized in plants, although the role of many is unknown Location of reserves Storage organ Cotyledon Chemical Proteins & Carbs Common name Pea, broad bean, dwarf bean Lipids & Prot. Endosperm Prot. & Carbs Cucumber, squash, lettuce, soybean, cabbages Cereals and grasses Location of reserves (2) Storage organ Chemical Perispem mannans coffee Common name Endosperm Fenugreek, caraway Lipids & Prot. Yucca Stylized diagram of a seed cell to show the major components. CW: cell wall; ER: endoplasmic reticulum and associated ribosomes of rough ER; G: Golgi apparatus; L: lipid bodies; M; mitochondrion; MB: microbodies or glyoxysomes; ML: middle lamella; N: nucleus containing dense nucleoliand disperse DNA chromatin ( c ); P: plastid; PD: plasmodesmata; R: ribosomes; V: Vacuole (some store proteins, phytin, organic acids, phenolics, sugars, amino acids and hormones). From: Black, M., J.D. Bewley and P. Halmer. Ed The encyclopedia of seeds. CABI, Cambridge, MA. 7

8 Figure 2,27, The synthesis of storage proteins and their sequestering within the vacuole/protein body, as occurs typically in the storage parenchyma cells of the cotyledons of legume seeds. ER, endoplasmic reticulum; G, Golgi apparatus; Gv, Golgiderived vesicle; M, mitochondrion; N, nucleus; Nu, nucleolus; PI, plastid; V, vacuole. After Bewley and Greenwood (1990). From: Bewley, J.D. and M. Black Seeds: physiology of development and germination, 2nd ed. pp. 79. Plenum Press, NY. References Bewley, J.D. and M. Black Seeds: physiology of development and germination, 2 nd ed. Plenum Press, NY. Egli, D.B., D. M. TeKrony, J. J. Heitholt, and J. Rupe Air temperature during seed filling and soybean seed germination and vigor. Crop Sci. 45: Heldt, Hans W Plant biochemistry & molecular biology. Oxford University Press. Gibson, L. R. and R. E. Mullen Soybean seed quality reductions by high day and night temperature. Crop Sci. 36: Goggi, A.S., J.C. Delouche, and L.M. Gourley Sorghum [Sorghum bicolor (L.) Moench] seed inernal morphology related to seed specific gravity, weathering, and immaturity. J. of Seed Techn. 17: Seed development and germination Edited by J. Kiegel & G. Galili. Marcel Dekker, Inc. Western, T. L., J. Burn, W. L. Tan, D. J. Skinner, L. Martin-McCaffrey, B.A. Moffatt, and G.W. Haughn Isolation and Characterization of Mutants Defective in Seed Coat Mucilage Secretory Cell Development in Arabidopsis. Plant Phys.127: