Yoshikuni Masaoka

Dissolution of ferric phosphate by alfalfa (Medicago sativa L.) root exudates

Alfalfa (Medicago sativa L.) was grown in hydroponic culture to investigate adaptation to Fe-deficiency. Root exudates released into the nutrient solution from Fe-deficient plants were trapped and condensed on an amberlite XAD-4 resin column. The diethyl ether fraction of these exudates dissolved ferric phosphate remarkably. The dissolving capability was about 62 times higher than that of root exudates obtained from Fe-sufficient plants in complete nutrient solution. The Fe-dissolving compound was separated and identified. It was a new natural compound with molecular formula C14H10O5 and was identified as 2-(3’,5’-dihydroxyphenyl)-5,6-dihydroxybenzofuran by means of mass spectrometry and 1 H-nuclear magnetic resonance. This new compound worked as a phytoalexin and inhibited completely the fungal growth of Fusarium oxysporum f. sp. phaseoli.

The responses of red clover (Trifolium pratense L.) to iron deficiency: a root Fe(III) chelate reductase

Abstract Red clover ( Trifolium pratens e L. cv. Kenland) was found to be an Iron (Fe)-efficient plant while it was sensitive to deficiencies of Cu, Zn and Mn or toxicities of Cu and Mn. Under Fe deficiency, the activity of Fe(III) chelate reductase in roots displayed two distinct peaks during 24 days of treatment. The root Fe(III) chelate reductase was induced within 1 day, during which no proton extrusion was observed. Both roots and shoots are involved in the reductase induction. The roots alone were able to rapidly induce reductase activity, while the shoots were necessary for the later enhancement of reductase activity. It is proposed that the rapidly-induced root reductase plays an important role in overcoming Fe deficiency in calcareous soils. The relationships between reductase induction, proton extrusion, and the role of shoots and roots are also discussed.

Ability of salt glands in Rhodes grass (Chloris gayana Kunth) to secrete Na+ and K+

Abstract Some plants are able to tolerate salinity using salt glands on the leaf surface, which secrete excess salts transported into the leaves. Rhodes grass (Chloris gayana Kunth) reportedly possesses such salt glands, but the features of secretion remain unclear. In the present study, we compared the ability of Rhodes grass salt glands to secrete sodium and potassium with the aim of clarifying the preference of cations for secretion. In both whole plant experiments and detached leaf experiments, NaCl treatment increased Na+ secretion and KCl treatment increased K+ secretion. When the ratios of the amount of secreted ions to their concentration in the leaves were compared between Na+ and K+, the ratio was greater with Na+ than K+. The addition of NaCl to the KCl solution significantly decreased K+ secretion, but the addition of KCl to the NaCl solution did not have a significant effect on Na+ secretion. These results indicate that the salt glands of Rhodes grass can secrete both Na+ and K+, but the ability to secrete Na+ is greater than that of K+ secretion. To examine the secretion mechanisms, various ion transport inhibitors were applied to detached leaves. All five inhibitors examined (orthovanadate, barium [Ba2+], ouabain, tetraethylammonium [TEA] and verapamil) significantly inhibited Na+ secretion, but only three inhibitors (ouabain, TEA and verapamil) inhibited K+ secretion. These results indicate that the secretion mechanisms of Na+ and K+ differ in Rhodes grass, at least partially, possibly resulting in the different secretion abilities of Na+ and K+.

A benzofuran derivative released by Fe-deficient Medicago sativa

Abstract A novel benzofuran derivative was isolated from exudates released by roots of iron-deficient Medicago sativa , together with coumestrol, formononetin and 7,4′-dihydroxyflavone. The structure of the novel compound was shown to be 2-(4-hydroxy-2-methoxyphenyl)-6-methoxy-3-benzofuran-carboxylic acid methyl ester by chemical and spectroscopic methods.

Tolerance of Grasses to Calcium Chloride, Magnesium Chloride and Sodium Chloride

Abstract The tolerance of six cool-season grasses and six warm-season grasses to three kinds of salt was examined in solution culture. Among the cool-season grasses, tall fescue (Festuca arundinacea Schreb.) was the most tolerant to all three salts. Among the warm-season grasses, bermudagrass (Cynodon dactylon (L.) Pers.) was the most tolerant to excess calcium chloride and sodium chloride, while bahiagrass (Paspalum notatum Flugge) was the most tolerant to excess magnesium chloride. A positive and significant correlation was found between estimates of the concentration at which plant growth decreases by 50% (C50) in the presence of excess CaCl2 and those in the presence of excess NaCl. The C50 estimates in excess MgCl2, however, were not correlated with those in the other two salts. The results suggest that common physiological mechanism confers tolerance to both excess CaCl2 and excess NaCl, but a different mechanism to excess MgCl2.

EFFECT OF IRON DEFICIENCY ON ROOT FERRIC CHELATE REDUCTASE, PROTON EXTRUSION, BIOMASS PRODUCTION AND MINERAL ABSORPTION OF CITRUS ROOT STOCK ORANGE JASMINE (MURRAYA EXOTICA L.)

Three-week iron (Fe) deficiency stress experiments were conducted using two citrus root stocks, Fe-deficiency tolerant Orange Jasmine (OJ, Murraya exotica L.) and the sensitive Flying Dragon [FD, Poncirus trifoliata var. monstrosa (T. Ito) Swingle]. Root ferric chelate reductase activity and proton extrusion increased in OJ between 12 and 18 d of stress, whereas there was no change in FD. Dry weight of OJ roots increased in contrast to FD which decreased. The Mn content in OJ remained the same even under Fe stress. Zn content in OJ roots doubled while that of FD increased 4-folds. The shoot/root Fe accumulation ratio increased in OJ while it decreased in FD. OJ apparently has mechanisms for increasing root biomass, controlling Fe reutilization and regulating manganese (Mn) and zinc (Zn) absorption in response to Fe deficiency. These mechanisms could help maintain homeostasis under heavy metal stress, which would be useful for improved growth of economically important citrus species.

Advantages of a diluted nutrient broth medium for isolating N2-producing denitrifying bacteria of α-Proteobacteria in surface and subsurface upland soils

Abstract This research aimed to determine whether a diluted nutrient broth (DNB) medium was different from a conventional nutrient broth (NB) medium when counting and isolating denitrifying bacteria in surface and subsurface upland soils. To this end, we investigated populations of denitrifying bacteria isolated from the surface to a depth of 4 m of subsurface upland soil that had received slurry. The DNB medium gave higher viable counts of denitrifying bacteria than the NB medium and a higher isolation ratio of denitrifying bacterial isolates. In total, 74 isolates from the DNB medium (D-isolates) and 26 isolates from the NB medium (N-isolates) were collected. We characterized their denitrifying activity and analyzed the diversity of 16S rDNA and denitrifying-related genes. Seventy-three percent of the D-isolates were oligotrophic denitrifying bacteria. The N2-producing, oligotrophic denitrifying bacteria, largely of α-Proteobacteria, increased in the D-isolates. The D-isolates and the N-isolates had some taxonomic overlapping on a phylogenetic tree based on 16S rDNA. It was not possible to identify the denitrification phenotype (N2-producing or N2O-producing) on the phylogenetic tree. Phylogenetic groups of isolates corresponded to nirK groups, except in some isolates in which horizontal gene transfer might have occurred. The terminal gas emission of the isolates was consistent with the existence of the nosZ gene. The DNB medium may be very useful in isolating N2-producing denitrifying α-Proteobacteria. Its use highlights the ecological significance of oligotrophic isolates and the different viable counts resulting from the selectivity of conventional and diluted media.

Interactions between plant nutrition and symptom expression in mandarin trees infected with the disease huanglongbing

A survey of Siem mandarin trees (Citrus reticulata Blanco) grown in different soil types and given different fertiliser regimes showed that applications of a foliar fertiliser reduced the symptom expression of trees infected with huanglongbing by ∼40%. In contrast, infected trees growing in a sandy soil, or in a clay-loam, did not differ in their level of symptom expression, despite differences in the fertility of the two soils. These data suggest that infection restricts either nutrient uptake or transport and, therefore, that foliar applied minerals may prolong tree life and reduce yield losses.

Effects of Excess Magnesium on the Growth and Mineral Content of Rice and Echinochloa

Abstract The tolerance of three cultivars of rice (Oryza sativa L.) and three species of the genus Echinochloa to excess magnesium was examined in solution culture. In Echinochloa species, excess MgCl2 or MgSO4 in the culture solution (30 mM) reduced the growth to 33-42% of that in the control plants and caused symptoms resembling those of calcium deficiency. In rice cultivars, however, excess Mg in the culture solution reduced the growth only to 54-67% of that in the control and did not cause the symptoms like those of Ca deficiency. The effect of excess Mg on the mineral contents of plants differed between rice (Nipponbare) and Echinochloa oryzicola. The Mg content of the whole plants in rice increased in proportion to MgCl2 concentration in the culture solution up to 30 mM, while that in E. oryzicola leveled off when MgCl2 concentration exceeded 10 mM. The excess MgCl2 treatment greatly reduced the calcium content of the whole plants in E. oryzicola and slightly in rice. In rice, the excess Mg treatment increased the Mg content of shoots and roots, and the potassium and chloride contents of roots, but slightly decreased the Ca and K contents of shoots. In E. oryzicola, the excess Mg treatment increased the K and Cl contents of shoots and the Mg and K contents of roots, and slightly increased the Mg content of shoots, but greatly decreased the Ca content of shoots. These results indicate that rice is more tolerant than Echinochloa to excess Mg and that the tolerance is related to Ca deficiency.

A copper-deficiency-induced root reductase is different from the iron-deficiency-induced one in red clover (Trifolium pratense L.)

There is increasing evidence that Cu deficiency can induce root reductase activity, but the ecological and physiological significance of this is unknown. This study compared the characteristics of root reductase activity induced by Cu deficiency with those induced by Fe deficiency in red clover (Trifolium pratenseL. cv. Kenland), a Fe-efficient plant. Effects of other nutritional stresses were also investigated for comparison. Compared with the effect of Fe deficiency, Cu deficiency induced only a moderate level of root reductase activity, while other nutrient stresses had no effect, or even inhibited the root reductases activity, especially in the case of Zn deficiency. Compared with Fe deficiency-induced Fe(III)-chelate reductase, Cu deficiency-induced reductase displayed a different pattern of induction. The activity of the Cu deficiency-induced reductase in intact plants increased with time; in decapitated plants it showed a distinct peak at a later stage of the treatment. The Fe concentration in the roots was significantly increased under Cu deficiency. Furthermore, the reductase activity was presented in the entire root system, contrary to what was observed for the Fe-deficiency-induced reductase activity, which was confined to the root apex. Cu deficiency did not increase proton extrusion from the roots, even when growth was significantly affected. The present results suggest that in red clover Cu deficiency induces a root reductase that is different from the reductase induced by Fe deficiency.