Naoko Iwata

Leaf senescence in rice due to magnesium deficiency mediated defect in transpiration rate before sugar accumulation and chlorosis.

Magnesium (Mg) is an essential macronutrient supporting various functions, including photosynthesis. However, the specific physiological responses to Mg deficiency remain elusive. In this study, 2-week-old rice seedlings (Oryza sativa. cv. Nipponbare) with three expanded leaves (L2-L4) were transferred to Mg-free nutrient solution for 8 days. In the absence of Mg, on day 8, L5 and L6 were completely developed, while L7 just emerged. We also studied several mineral deficiencies to identify specific responses to Mg deficiency. Each leaf was analyzed in terms of chlorophyll, starch, anthocyanin and carbohydrate metabolites, and only absence of Mg was found to cause irreversible senescence of L5. Resupply of Mg at various time points confirmed that the borderline of L5 death was between days 6 and 7 of Mg deficiency treatment. Decrease in chlorophyll concentration and starch accumulation occurred simultaneously in L5 and L6 blades on day 8. However, nutrient transport drastically decreased in L5 as early as day 6. These data suggest that the predominant response to Mg deficiency is a defect in transpiration flow. Furthermore, changes in myo-inositol and citrate concentrations were detected only in L5 when transpiration decreased, suggesting that they may constitute new biological markers of Mg deficiency.

The analysis of magnesium transport system from external solution to xylem in rice root

We report the characterization of magnesium (Mg) transport from external solution to xylem by using xylem fluid analysis in rice (Oryza sativa L. Nipponbare). It was indicated that Mg uptake and/or xylem loading was mediated by two transport systems, a saturable and a linear component. The K m value of the saturable component was approximately 20 µM, and the saturated concentration was calculated to be 1.5 mM. Using a multi-compartment transport box with excised roots, the transfer from external solution to xylem was shown to be inhibited by hexaamminecobalt(III) {[Co(III)(NH3)6], Co-Hex}, a selective inhibitor of CorA-mediated influx of Mg2+, suggesting that the MRS2/MGT genes that belong to the superfamily of CorA-type membrane proteins are involved in the Mg transport system.

Expression and Functional Analysis of the CorA-MRS2-ALR- Type Magnesium Transporter Family in Rice

Maintenance of an appropriate magnesium ion (Mg(2+)) concentration is essential for plant growth. In Arabidopsis thaliana, the CorA-MRS2-ALR-type proteins, named MRS2/MGT family proteins, are reportedly localized in various membranes and they function in Mg transport. However, knowledge of this family in other plant species is extremely limited. Furthermore, differential diversification among dicot and monocot plants suggested by phylogenetic analysis indicates that the role of the Arabidopsis MRS2/MGT family proteins is not the same in monocot plants. For a further understanding of this family in higher plants, functional analysis and gene expression profiling of rice MRS2/MGT family members were performed. A phylogenetic tree based on the isolated mRNA sequences of nine members of the OsMRS2 family confirmed that the MRS2/MGT family consists of five clades (A-E). A complementation assay in the yeast CM66 strain showed that four of the nine members possessed the Mg(2+) transport ability. Transient green fluorescent protein (GFP) expression in the isolated rice protoplast indicated that OsMRS2-5 and OsMRS2-6, belonging to clades D and A, respectively, localized in the chloroplast. Expression levels of these genes were low in the unexpanded yellow-green leaf, but increased considerably with leaf maturation. In addition, diurnal oscillation of expression was observed, particularly in OsMRS2-6 expression in the expanded leaf blade. We conclude that OsMRS2 family members function as Mg transporters and suggest that the genes belonging to clade A encode the chloroplast-localized Mg(2+) transporter in plants.

Different magnesium uptake and transport activity along the rice root axis revealed by 28Mg tracer experiments

We characterized magnesium (Mg) uptake and transport along the primary root of six-day-old rice seedlings (Oryza sativa L. cv. Nipponbare) using 28Mg as a radioactive tracer. We used a multi-compartment transport box, which separated the root region every 1 cm in order to apply 28Mg to a particular root region. Then, the root region was defined as R-A, R-B, and R-C in order from the root tip region to the lateral root-developing area. The amount of 28Mg taken up in the R-C region in 15 min, 1 h, and 3 h was nearly three times as much as that of R-A and R-B. Thirty percent to 50% of 28Mg absorbed from R-C was transported toward the lower part of the root. Furthermore, when 28Mg was absorbed from R-C, it was detected only in the root part lower than R-C after 5 min. In contrast, less than 5% of 28Mg absorbed from R-B was transported downward. This characteristic uptake and transport behavior was not observed with 32P(phosphorus)-phosphate or 45Ca (calcium). Our results demonstrate that Mg uptake and transport activity differ between root regions and further suggest that phloem loading of Mg occurs at R-C within minutes after uptake from the culture solution. Then, Mg is transported toward the root tip directly without going up the shoot.

Analysis of the mineral composition of taro for determination of geographic origin.

The mineral composition of taro ( Colocasia esculenta (L.) Schott) was analyzed to develop a method to distinguish taro produced in Japan and China. The concentrations of 15 elements (Al, Ca, Cl, Mg, Mn, Br, Co, Cr, Cs, Fe, K, Na, Rb, Sc, Zn) were assayed using instrumental neutron activation analysis. The concentrations of NO(3)(-), SO(4)(2-), H(2)PO(4)(-), Cl(-), malate, and oxalate were measured by ion chromatography. The mean concentrations of H(2)PO(4)(-), Co, Cr, and Na significantly differed (P < 0.01) between taro grown in Japan and that grown in China. Discriminant analysis was performed to identify the most efficient combination of elements and compounds to discriminate the taro geographic origin. The highest percentage of correct classification was achieved with a two-variable model including H(2)PO(4)(-) and Co (100% for Japanese, 93.75% for Chinese). Principal component analysis and cluster analysis using all of the assayed elements and compounds were also conducted to determine which elements significantly accounted for the variation of the taro mineral composition. We report on the potential of H(2)PO(4)(-) and Co concentrations to differentiate taro grown in China and Japan and discuss the sources of variability in the taro mineral composition of our samples.

Mineral composition of frozen taro for determination of geographic origin

The mineral composition of frozen food of taro [Colocasia esculenta (L.) Schott] was analyzed to categorize the geographical production place of taro. The concentrations of Co and H2PO4− were found to be useful to separate the producing place between Japan and China. The analysis was performed by instrumental neutron activation analysis (INAA) and ion chromatography (IC). In the case of INAA, the samples were dried and sealed in a vinyl bag and irradiated with thermal neutrons from JRR3M, installed at Japan Atomic Energy Agency (JAEA). The activated samples were cooled down for a few weeks and the elements (Co, Cr, Fe, Rb, Zn) were determined. Cobalt concentration of frozen taro from China was higher than that from Japan. The tendency was the same in the fresh sample of taro. When concentration of H2PO4− of frozen sample was measured, taro from Japanese product was higher than that of Chinese one, contrary to fresh sample. This result might be caused by the leakage of H2PO4− during freezing process, indicating that we should be careful to apply the discrimination indicators. In addition to Co, there was a significant difference of Rb and Fe concentrations between frozen taro from Japan and China.