M.S. Naik

Nitrate assimilation-its regulation and relationship to reduced nitrogen in higher plants

The regulation of nitrate assimilation in higher plants is reviewed in relation to the availability and accumulation of reduced nitrogen. The effects of light on these processes are also considered.

Changes in the nucleic acid and protein fractions in Opaque-2 maize kernels during development

Abstract Different RNA fractions, as well as DNA disappeared from the endosperm of Opaque-2 maize 31 days after pollination. At the same time, these nucleic acids were present in the embryo. As compared with normal maize, Opaque-2 endosperm showed high RNase activity, which increased about ten-fold 31 days after pollination. Most of the activity was due to RNase A during the early stages. The disappearance of nucleic acids from the endosperm at a critical stage might be responsible for depressed zein synthesis in Opaque-2, since little of this protein is synthesised in the earlier stages after pollination.

Effect of photosynthesis on dark mitochondrial respiration in green cells

Green plant cells can generate ATP in both chloroplasts and mitochondria. Hence the effect of photosynthesis on dark mitochondrial respiration can be considered at a variety of levels. Turnover of ceitric acid cycle dehydrogenases, which is essential for supply of carbon skeletons for amino acid synthesis, seems to be largely unaffected during photosynthesis. The source of carbon for the anaplerotic function of the citric acid cycle in light is however, not known with certainty. NADH generated in these reactions is probably not oxidised via the mitochondrial electron transfer chain coupled to ATP synthesis. However, it may be oxidised by the alternative cyanide‐insensitive pathway, exported to the cytosol via the oxaloacetate‐malate dicarboxylate shuttle or directly utilised for cytosolic nitrate reduction. Oxidation of succinate via cytochrome oxidase may also be similarly inhibited in light. Whether increase in the cytosolic ATP/ADP ratio in light is responsible for the inhibition of mitochondrial electron transfer to O2 is not clearly established, because the ATP/ADP ratio is reported to be already quite high in the dark. Effective collaboration between photophosphorylation and oxidative phosphorylation in order to maintain the cytosolic energy charge at a present high level is discussed.

Characterization of polysomes and incorporation in vitro of leucine and lysine in normal and opaque-2 zea mays endosperm during development

Abstract Polysome preparations obtained from opaque-2 and normal maize endosperms during development did not show any significant difference in sedimentation coefficient or nucleotide composition. The pattern of incorporation in vitro of lysine and leucine, however, differed quite distinctly in these two preparations. During early stages of maturity the polysomes from opaque-2 incorporated substantially more lysine and less leucine as compared with those from normal maize. Although the trend was reversed at 25 days post-pollination, this did not result in any significant zein accumulation since very little total protein was synthesized after that stage in opaque-2 maize endosperm. It is, therefore, suggested that the opaque-2 gene exerts a regulatory control on m RNA synthesis, required for zein formation at early stages of maturation.

Inactivation of nitrate reductase from wheat and rice leaves

Abstract In fresh leaves, the inactivation of nitrate reductase was rapid at high temperatures as compared to low temperatures. In leaves subjected to freeze-thaw treatment, the loss of enzyme activity was extremely rapid particularly at high temperatures. Pre-incubation with NADH not only protected the enzyme against inactivation, but also stimulated its activity. In dialysed extracts of rice leaves, NADH alone offered some protection while nitrate alone did not protect the enzyme from inactivation. Addition of both NADH and nitrate during pre-incubation enhanced the enzyme activity considerably. It is suggested that stimulation of nitrate reduction by NADH and nitrate may be of physiological significance to the plant, in the sense that in the presence of sufficient supplies of reluctant and nitrate, the process of nitrate assimilation would be accelerated.

Changes in protein fractions and leucine-[14C] incorporation during Sorghum grain development

Abstract Incorporation of leucine and changes in different protein fractions have been studied during Sorghum grain development. Most of the label from the injected leucine-[ 14 C] was found in glutelin and residue fraction towards later stages of maturity. The label in albumin, globulin and prolamin decreased with a concomitant increase in label in glutelin and residue proteins. The concentration of lysine, aspartic acid and glycine decreased while that of leucine, proline, alanine, tyrosine, phenylalanine, and cystine increased during grain development. Increase in serine, methionine, valine and isoleucine was only marginal. The proportion of glutamic acid was high at all stages of grain development. Glutelin fraction resolved into two peaks on gel chromatography, only one of which with higher MW was labelled, while in albumin both the peaks were found to be labelled. Tannin content also increased during grain development.

Reevaluation of in vivo assay of nitrate reductase activity in wheat leaves

Abstract During in vivo assay of nitrate reductase in wheat leaf discs, the amount of nitrate released in the medium was greatly influenced by the pH of the medium, although it had no effect on total nitrite formed in the leaf tissue. In an acidic medium nitrite excreted out was negligible, while it increased considerably at alkaline pH-values. Extraction of leaf discs in boiling water resulted in significant loss of nitrite at acidic pH. In order to study the effect of malonate, succinate and L-malate, these acids were infiltrated into leaf discs at acidic pH while subsequent incubation and extraction was done at pH 7.5. It was observed that malonate and succinate significantly inhibited in vivo nitrate reduction while L-malate was without any effect.

Protein synthesis and changes in nucleic acids during grain development of Sorghum

Abstract Changes in DNA, RNA, nitrogen, nucleotide composition and in vitro incorporation of leucine/lysine by polysomes have been studied during sorghum grain development. Both DNA, RNA and protein content increased substantially during grain development. Although RNase activity increased, it did not affect RNA accumulation. Minor changes in the nucleotide composition of r RNA and s RNA were observed during grain development. In vitro incorporation of leucine and lysine by polysomes indicate qualitative change in the m RNA during later stages of grain development and the substantial accumulation of proteins during this period ultimately results in accumulation of proteins rich in leucine and poor in lysine.

Regulation of nitrate assimilation in plants in light and dark

Abstract Assimilation of nitrate and nitrite accumulated in leaves was much more rapid in the light than in the dark. Under light aerobic and dark anaerobic conditions the extent of nitrate reduction was almost equal. Dark assimilation of nitrite was drastically inhibited by 2,4-dinitrophenol (2,4-DNP). Although the uncoupler did not show any significant effect on oxidation of NADH and succinate in isolated leaf mitochondria, it inhibited 14CO2 evolved from endogenously labelled sugars, probably by blocking the formation of glucose 6-phosphate (G 6-P). It is suggested that for dark assimilation of nitrite, NADPH generated in the oxidative pentose phosphate pathway is the source of reductant.

RNA POLYMERASE FROM OPAQUE-2 AND NORMAL ZEA MAYS ENDOSPERM

Abstract RNA polymerase from Opaque-2 and normal maize showed qualitative differences during endosperm development. DEAE-Sephadex column chromatography indicated the presence of one and three RNA polymerases respectively at 15 and 25 days post-pollination. The polymerases from Opaque-2 and normal endosperms at 15 days post-pollination showed considerable differences in Mn 2+ optimum. The optimum Mn 2+ for normal polymerase was ten times higher than for Opaque-2 polymerase. The polymerase activity from endosperms at 15 days post-pollination was due to nucleoplasmic RNA polymerase II.