The regulation of carbon (C) and nitrogen (N) supply to the developing maize kernel by the vegetative organs is a subject of great interest because it is responsible for the accumulation of starch, proteins and corresponding increases in dry weight of the kernel. Evidence has indicated that the relative proportion of starch and proteins in the endosperm may be determined by the nutrient supply, the sink demand and the interaction between them (Motto et al., Oxford Survey Plant Mol. Cell Biol. 6:87-114, 1989). The biochemical and physiological background of this relationship is complex and not fully understood.
Previous work in our laboratory, using field grown plants (Reggiani et al., Genet. Agr. 39:221-232, 1985) and endosperm cell cultures (Balconi et al., Plant Sci. 73:1-9, 1991) of the Illinois High Protein (IHP, a low kernel C/N ratio cultivar) and the Illinois Low Protein (ILP, a high kernel C/N ratio cultivar) also indicated that the composition of nutrients translocated from the plant to the developing grains may influence the relative concentration of starch and proteins in the grains. Nutrient availability in the endosperm is also dependent on transfer layer, on the rate and efficiency of two different mechanisms:phloem unloading, and the uptake and utilization of assimilates for seed growth and storage protein formation (Gifford et al., Science 225:801-808, 1984). In maize, nutrients are unloaded from the phloem terminals located in the maternal tissue at the base of the seed, the pedicel. Susequently, the nutrients enter the apoplast of the placento-chalazal region, which is the tissue that is directly in contact with the endosperm. Evidence shows that the processes of protein and starch biosynthesis are extremely dependent upon metabolic events taking place in these regions (Misra and Oaks, Plant Physiol. 77:520-523, 1985; Miller and Chourey, Plant Cell 4:297-305). The objective of this research was to evaluate the possible role of the cob and pedicel-placento-chalazal tissues in the amino acid (glutamine) supply in affecting zein and starch synthesis in the developing kernels.
Ears of the IHP and ILP maize strains were harvested at 9 days after pollination (DAP) and were cut into blocks containing 10 kernels per block as described by Gengenbach (Planta 134:91-93, 1977). The blocks were cultured on agar media containing salts as described in Nitsch and Nitsch (Science 163:85-87, 1969), 75 g/l sucrose and 4 g/l glutamine. After 7 days the blocks were transferred, until 25 DAP, to the same media supplemented with L-(U-14C) glutamine (12.5 uCi/ml; 4.56 uCi/mmol). The 14C content in endosperm, pericarp, pedicel and cob tissues was determined. The data indicated that the 14C content of cob and kernel tissues was lower in the ILP strain than in the IHP strain (Table 1). In Table 2 the concentrations, as percentage of dry matter, of various protein fractions (zeins, albumins plus globulins and glutelins) and carbohydrates (starch and sugars) are reported, in IHP and ILP endosperms harvested at 9 DAP (t0) and grown in vitro until 25 DAP on culture media containing 4 g/l glutamine (t25). At 9 DAP (t0) low zeins and starch content was observed in all tissues of IHP and ILP kernels. At 25 DAP, both IHP and ILP kernels grown in vitro showed a phenotype similar to that observed in kernels grown to maturity in field conditions; in particular the ILP strain showed a reduced accumulation of the zein fraction and a higher starch content in comparison to the IHP strain; on the other hand, the sugar content was higher in IHP kernels than in ILP kernels.
In summary, the results of this research suggest that the cob and pedicel-placento-chalazal tissues may be involved in determining the ILP phenotype, through a less efficient system of amino acid uptake, interconversion, and transport to the endosperm, in comparison with the IHP strain.
Table 1. Distribution of 14C (cpm/20mg) in tissue extracts from IHP
and ILP in vitro cultured caryopses until 25 DAP on culture medium containing
4 g/l glutamine.
Table 2. Protein fractions, starch and soluble sugars content as percent
of the dry matter of the IHP and ILP endosperms at 9 DAP (t0) and grown
in vitro until 25 DAP on culture medium containing 4 g/l glutamine (t25).
|Albumins plus globulins||1.16||1.88||1.00||1.70|
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