No abstract

Silicon (Si) is generally considered a beneficial element for the growth of higher plants, especially under stress conditions, but the mechanisms remain unclear. Here, we tested the hypothesis that Si improves salt tolerance through mediating important metabolism processes rather than acting as a mere mechanical barrier. Seedlings of sorghum (Sorghum bicolor L.) growing in hydroponic culture were treated with NaCl (100 mm) combined with or without Si (0.83 mm). The result showed that supplemental Si enhanced sorghum salt tolerance by decreasing Na(+) accumulation. Simultaneously, polyamine (PA) levels were increased and ethylene precursor (1-aminocyclopropane-1-carboxylic acid: ACC) concentrations were decreased. Several key PA synthesis genes were up-regulated by Si under salt stress. To further confirm the role of PA in Si-mediated salt tolerance, seedlings were exposed to spermidine (Spd) or a PA synthesis inhibitor (dicyclohexylammonium sulphate, DCHA) combined with salt and Si. Exogenous Spd showed similar effects as Si under salt stress whereas exogenous DCHA eliminated Si-enhanced salt tolerance and the beneficial effect of Si in decreasing Na(+) accumulation. These results indicate that PAs and ACC are involved in Si-induced salt tolerance in sorghum and provide evidence that Si plays an active role in mediating salt tolerance.

Non-symbiotic hemoglobins (ns-Hbs) are found in all plants, although their physiological function remains to be determined. The present study was undertaken to explore the mode of induction of ns-Hb genes by metabolites of nitrate assimilation using cultured rice (Oryza sativa L.) cells. Two class-1 ns-Hb genes, ORYsa GLB1a and ORYsa GLB1b, were strongly induced by nitrate, nitrite and nitric oxide (NO) donors, S-nitroso-N-acetylpenicillamine and sodium nitroprusside. The rapid and transient accumulation of ORYsa GLB1a and ORYsa GLB1b transcripts in response to nitrate, nitrite and NO donors was similar to that of nia1, which encodes NADH-nitrate reductase (NR), although repression by glutamine and asparagines was significant only for nia1. In the mutants defective in NR mRNA expression, nitrate, nitrite and NO donors failed to induce not only nia1 but also ORYsa GLB1a and ORYsa GLB1b transcripts, indicating that the induction of ns-Hb genes is closely associated with that of the NR gene. Although the kinetics of induction by nitrate, nitrite and NO donors are similar for the two ns-Hb genes, an inhibitor study demonstrated that de novo synthesis of the protein in cytoplasm is essential for inducing ORYsa GLB1b. In contrast, ORYsa GLB1a, like nia1, can be induced in the primary response to these signals without de novo protein synthesis. The role of nitrate, nitrite and NO in the induction of ns-Hb gene expression in rice cells and the possible cellar function of ns-Hbs were discussed in relation to nitrate reduction pathways.

Some studies on the effects of xanthble oxiase inhibitor a_opurIo 14-hydroxypyrazolo(34-)pyri.idel on alato met of soybean plants (Glychae max cv. T _ ) are reported.Soyben 'as asepticaly germinated for 96 hours on agar containing I _ilhlmlar allopurinol, contained only slight amounts of aUantoin, allantoic acid, and urea as compared with controls. Analysis of puries and pyrimidines of the allopurinol-treated seedlings sbowed marked accumulation of xanthine both in the cotyedons and seedlin axes. No hypoxanthine accumulation was found. Xanthine accumulation due to allopurinol treatment was relatively low after the cotyedons had fallen. For nodulated plants, allopurinol caused a sigificant drop in allantoin (+allantoic acid) in the stems and nodules, accompanied by a strking accumulatfon of xanthine in the nodules. The xanthine concentration in the nodules far exceeded that in the germinated slings. Allopurl at a concentration of 50 micromolar strongly inbibited xasthine oxidase prepared from soybean nodules.The results suggested that the main pathway of alantoin formation in soybean plants was through purine decomposition, via xanthine-uric acid. It was specially noted that a very active purine-decomposing system existed in soybean nodules.Allantoin is well known as one of the principal end products of nitrogen metabolism in animals and is produced by a biosynthetic sequence involving the formation of inosinic acid and its subsequent stepwise breakdown. Generally a variety of microorganisms, e.g. bacteria (2, 7), yeasts (30), fungi (3), and algae (1) have the ability to decompose oxypurines and consequently allantoin.Some higher plants such as maple, comfrey, and leguminous plants also produce and accumulate large amounts of allantoin, up to half the amount of their total N (24). In contrast to the animal systems which excrete allantoin as a waste material, these plants accumulate it within cells, transport it to various organs, and further reutilize its nitrogen (24,29). Several authors (5,10,17), by experiments using 'C-labeled purines or glycine, have demonstrated that allantoin or allantoic acid is formed through oxidative purine decomposition by a well defined reaction sequence in animals and microorganisms. Mothes (24) and Reinbothe (29) have, however, discussed in their reviews another plausible mode of allantoin synthesis, i.e. direct formation from allantoic acid via condensation of urea and glyoxylic acid. This simple reverse reaction may be supported by the finding that when ['4Clurea was fed to banana leaves allantoic acid was heavily labeled (12), and by a report of Brunel (9) suggesting an enzymic synthesis of allantoic acid from urea and glyoxylic acid in higher fungi. While it is likely that in higher plants most allantoin and allantoic acid are formed by the same pathway as that demonstrated in microorganisms and in animal tissues, the possibility that some might be made by the condensation of urea with glyoxylic acid has not been eliminated.Allopurinol [4-hydroxypyrazolo(3,4-d)pyrimidi...

We examined the nitrogenase reductase (nifH) genes of endophytic diazotrophic bacteria expressed in field-grown sweet potatoes (Ipomoea batatas L.) by reverse transcription (RT)-PCR. Gene fragments corresponding to nifH were amplified from mRNA obtained from the stems and storage roots of field-grown sweet potatoes several months after planting. Sequence analysis revealed that these clones were homologous to the nifH sequences of Bradyrhizobium, Pelomonas, and Bacillus sp. in the DNA database. Investigation of the nifH genes amplified from the genomic DNA extracted from these sweet potatoes also showed high similarity to various α-proteobacteria including Bradyrhizobium, β-proteobacteria, and cyanobacteria. These results suggest that bradyrhizobia colonize and express nifH genes not only in the root nodules of leguminous plants but also in sweet potatoes as diazotrophic endophytes.