Differences In Molybdenum Uptake, Utilization And Its Regulation Mechanism Between Two Winter Wheat Cultivars

Molybdenum (Mo) is an essential trace element for higher plants. Plants can have genotypic differences in Mo uptake and use. There were significient differences in dry matter (DM), grain yield, Mo concentration, Mo accumulation, Mo enzyme activities and their regulating processes between Mo-efficient winter wheat cultivar97003and Mo-inefficient winter wheat cultivar97014. Based on the results of earlier studies, soil and solution culture experiments were conducted to investigate the differences in Mo uptake, utilization and its regulation mechanisms between the two winter wheat cultivars. The main results were as follows.1. Rhizobox and hydroponic culture experiments were conducted to investigate the difference in rhizosphere precesses affecting availability of Mo between the two cultivars.The results showed that the available Mo concentration in the rhizosphere soil of the two cultivars was significantly inceased relative to the control without plants, and Mo concentration in the rhizosphere soil of the two cultivars was significantly higher than that in the non-rhizosphere soil in the-Mo treatment. In the+Mo treatment, the available Mo concentration in the rhizosphere soil of the two cultivars was significantly lower relative to the control without plants, and that in the rhizosphere soil of the two cultivars was significantly lower than that in the non-rhizosphere soil. This suggests that winter wheat can adapt to Mo deficiency stress through root activation of soil Mo. Under Mo-deficient conditions, the pH in the rhizosphere soil of the two cultivars was significantly inceased relative to the control without plants, and in the rhizosphere soil of the two cultivars was significantly higher than that in the non-rhizosphere soil. This indcates that rhizosphere processes of winter wheat can increase pH of the rhizosphere soil and thus increase the availability of Mo in the rhizosphere soil. The two cultivars absorbtion of NO3-and excretion of NH4+induced an influx of proton (H+), suggesting that H+influx into the roots of winter wheat might be the reason for increased pH of the rhizosphere soil. The two cultivars had differential abilities for activating soil Mo via rhizosphere processes. The available Mo in the rhizosphere soil of97003was higher than that in the rhizosphere soil of97014both in the-Mo and+Mo treatments. This suggests that97003has a greater ability for activating soil Mo. The pH of the rhizosphere soil of97003was lower than that of97014, which might be due to a lower net and mean H+influx into97003under Mo-deficient conditions.97003absorbed less NO3-and excreted less NH4+than97014, and thus induced a lower H+influx. However, the differences in the pH of the rhizosphere soil between the two cultivars might be nothing to do with the difference in the available Mo in the rhizosphere soils.2. A hydroponic culture experiment was conducted to investigate the difference in the expression of gene related to Mo uptake, transport and assimilation between the two cultivars.The results indicated a close relationship between Mo uptake and TaSultr5.1, TaSultr5.2and TaCnxl expression, according to a stepwise regression analysis of the time course of Mo uptake in the two cultivars. Meanwhile, expression of TaSultr5.2in roots also showed a positive relationship with Mo uptake rates.97003had stronger Mo uptake than97014at low Mo-application rates (less than1μmol Mo L-1) due to the higher expression of TaSultr5.2, TaSultr5.1and TaCnxl in roots. In contrast, Mo uptake of97003was weaker than97014at high Mo application rates (ranging from5to20μmol Mo L-1), which was related to significant down-regulation of TaSultr5.2and TaCnx1genes in roots of97003compared to97014. This indicates that the differential-expression intensities of TaSultr5.2, TaSultr5.1and TaCnxl could be the cause of the difference in Mo uptake between the two cultivars at low and high Mo application levels.3. A hydroponic culture experiment was conducted to investigate the influence of competitive ions on the expression of gene related to Mo uptake and transport in the two cultivars. The results showed that S and P deficiency increased Mo concentrations in shoots and roots, Mo translocation coefficients, and accumulation per plant, respectively. This shows that S and P deficiency can enhance Mo uptake and translocation in winter wheat, respectively. Meanwhile, S and P deficiency decreased the expression of TaSultr1.1and TaPht1.1but increased expression of TaSullr5.1and TaSultr5.2in roots of the two cultivars. This shows that S and P deficiency can enhance Mo uptake and translocation, which might be related to the increased TaSultr5.1and TaSultr5.2expression. There were different responses to S and P deficiency in Mo uptake, Mo translocation and expression of gene relted to Mo uptake and translocation between the two cultivars.4. A hydroponic culture experiment was conducted to investigate the difference in the expression of gene related to Moco biosynthesis between the two cultivars. The results showed that the activities of nitrate reductase (NR) and sulfite oxidase (SO) in leaves or roots of97003were higher than those of97014, but the activities of aldehyde oxidase (AO) and xanthine dehydrogenase (XDH) in leaves of97003were lower than those of97014under Mo-deficient conditions. Under Mo-sufficient conditions,97003had higher SO activity in leaves, AO and XDH activities in roots but lower SO activity in roots, and AO and XDH activities in leaves than97014. The expression of NR in leaves of97003was lower than that of97014, but the expression of AO and XDH in leaves and roots of97003were lower that those of97014in the-Mo treatment.97003showed lower expression of AO and XDH in leaves and roots than97014in the+Mo treatment. This suggests that the difference in Mo enzyme activities and their gene expression between the two cultivars was different under Mo-deficient and Mo-suficient conditions respectively.97003had lower expression of TaCnx2and TaCnx5in leaves and roots than97014, which might result in the lower accumulation of cyclic pyranopterin monophosphate (cPMP) and molybdopterin (MPT) in97003. The expression of TaCnxl in leaves and roots of97003was significantly higher than that of97014, indicating more Moco formation in97003.97014had higher expression of TaAba3in roots than97003, which might be related to the increase of TaAba3expression incuded by the Moco shortage.Possible mechanisms of differences in the Mo uptake and utilization between two cultivars are summarized. Under Mo-deficient conditions, the available Mo concentration in the rhizosphere soil of97003was higher than that in the rhizosphere soil of97014, which suggested that97003had a stronger ability of activating soil Mo via rhizosphere processes; The expression of TaSultr5.2and TaSultr5.1in roots of97003were higher than those of97014, which suggested that97003had a stronger ability of Mo uptake and transport; The expression of TaCnxl in leaves and roots of97003were higher than those of97014, which suggested that97003had a stronger ability of Moco biosynthesis. Therefore, under Mo-deficient conditions, Mo-efficient cultivar using soil Mo efficiently was due to the stronger ability of activating soil Mo, Mo uptake and transport, and Moco biosynthesis. The difference in the ability of Mo uptake and transport might be the most important reason for the difference in the Mo uptake and utilization between the two winter wheat cultivars.