Hybrid Effects on the Release of Phytosiderophores in Winter Wheat (Triticum aestivum)

Fe (iron) deficiency is an important nutritional problem particularly in crop plants grown on calcareous soils. Phytosiderophore (PS) release has been suggested to be linked to the ability of graminaceous species and genotypes to overcome Fe_deficiency chlorosis. Thus, enhancing PS release is a critical step to improve Fe nutrition of plants grown on Fe stressed soils. The heterosis of PS release rate in common wheat was studied by analyzing PS release from roots of three hybrids and their four parents grown in Fe_deficiency nutrient solution under controlled environmental conditions. PS release rates were determined at two or three day intervals after onset of Fe_deficiency symptoms by the measurement of Fe mobilizing capacity of root exudates from freshly precipitated FeⅢ hydroxide. High amounts of phytosiderophores were released from the roots of all wheat genotypes under Fe_deficiency, and the amount progressively increased with the development of Fe_deficiency chlorosis. The results revealed that the hybrids had more sensitive feedback systems which secreted more phytosiderophores under Fe_deficiency than their parents. By analyzing the relationship between each hybrid and its parents, it was also found that the parents should be selected on the basis of the rate of PS release and the combining ability by using the heterosis to improve Fe utilizability of crop plants.

Effect of Root Surface Iron Plaque on Se Translocation and Uptake by Fe-Deficient Rice

Under anaerobic conditions, ferric hydroxide deposits on the surface of rice roots and affects uptake and translocation of certain nutrients. In the present study, rice plants were cultured in Fe-deficient or sufficient solutions and placed in a medium containing selenium （Se） for 2 h. Then, FeSO4 was added at the various concentrations of 0, 10, 40, or 70 mg L-1 to induce varying levels of iron plaque on the root surfaces and subsequent uptake of Se was monitored. The uptake of Se was inhibited by the iron plaque, with the effect proportional to the amount of plaque induced. The activity of cysteine synthase was decreased with increasing amounts of iron plaque on the roots. This may be the important reason for iron plaque inhibition of Se translocation. At each level of iron plaque, Fe-deficient rice had more Se than Fe-sufficient rice. Furthermore, with plaque induced by 20 mg Fe L-1, plants from Fe-deficient media accumulated more Se than those from Fe-sufficient media, as the Se concentration was increased from 10 to 30 or 50 mg L^-1. We found that phytosiderophores, highly effective iron chelating agents, could desorb selenite from ferrihydrite. Root exudates of the Fe-deficient rice, especially phytosiderophores in the exudates, could enhance Se uptake by rice plants with iron plaque.

Responses of Aerobic Rice (Oryza sativa L.) to Iron Deficiency

Aerobic rice has the advantage of saving water. Most published work has focused on improving its yield, while few reported on its micronutrient status. In fact, Fe deficiency is a common nutritional problem in the production of aerobic rice. Short- term hydroponic culture experiments were conducted to study the response of aerobic rice to Fe deficiency and the effect of root exudates from Fe-deficient wheat on its Fe uptake ability. The results indicate that the amount of phytosiderophores （PS） released from aerobic rice did not increase under Fe deficient conditions. The Fe（III） reducing capacity of Fe-deficient aerobic rice did not increase and the solution pH did not decrease significantly. What＇s more, no obvious swelling was observed in the root tips. Aerobic rice did not show special responses to improve their Fe nutrition under Fe deficiency as both strategy I and II plants though they were very sensitive to Fe deficiency. This may be a reason which causes Fe deficiency problem in aerobic rice. However, root exudates from Fe-deficient wheat （PSw） could improve its Fe nutrition in the presence of insoluble Fe（OH）3. This suggests that aerobic rice could utilize Fe activated by PSw.

Secretion of Phy tosiderophore and Its Effects on Soil Fe Availability

Large amounts of phytosiderophore are detected from both the solution and the rhizosphere soil when cereal crops are under Fe deficiency stress. The extension of phytosiderophore in the rhizosphere soil is found only within 1 mm apart from the root surface. The rate of phytosiderophore secretion is negatively related to chlorophyll content in young leaves and positively related to the Fe-solubilizing capacity. Results from in vitro experiments show 10 μmoles mugineic acid can dissolve 501 μg Fe from iron hydroxide and 146 ug from strengite. Thus, phytosiderophore can considerably enhance the soil iron availability by increasing the solubility of amorphous iron hydroxide and iron phosphate, and active Fe is consequently accumulated in the plant rhizosphere , 43% higher than in the bulk soils. There is evidence to support that mugineic acid chelates with Fe3+at a rate of 1:1 in the acid condition. In addition ,we observe mugineic acid has certain effects on mobilization of P as well as Fe by dissolving the insoluble iron phosphate. And phytosiderophore seems to be an effective remedy for the chlorosis of dicotyledonous plants.

Selection of Inbred Lines for Breeding of Maize with High Efficiency in Iron Utilization

Crops are often subjected to iron(Fe)-deficiency due to the limited solubility of this essential element in most neutral or basic soils. Developing cultivars with high efficiency in Fe utilization via breeding programs can provide solutions to this problem as a long term strategy. In the present study, to select inbred lines for breeding of maize with high efficiency in Fe utilization, we screened 123 inbred lines at the seedling stage by analyzing secretion pattern of phytosiderophores, a class of non-protein amino acids released by graminaceous species for Fe utilization, using high-performance liquid chromatography. One hundred and twenty three inbred lines were clustered into nine groups. The low PS secretion rate under Fe-sufficient condition and high PS secretion rate increment after Fe-deficiency treatment type were the ideal inbred lines for breeding of maize with high efficiency in Fe utilization.

Transporters Contributing to Iron Trafficking in Plants

This review will discuss recent progress in understanding the many roles of transporters in the whole-plant physiological processes that maintain iron （Fe） homeostasis. These processes include uptake from the soil via roots, control of transport from roots to above-ground parts of the plant, unloading of Fe from the xylem in above-ground parts, loading of Fe into mitochondria and plastids, transport of Fe to reproductive parts of the plant, and Fe mobilization during seed germination. In addition, we will discuss the mechanisms that plants use to cope with an apparently unintended conse- quence of Fe acquisition： the uptake of toxic heavy metals via Fe transporters. Rapid progress has been made in under- standing the transport processes involved in each of these areas in the last 5 years and this review will focus on this recent progress. We will also highlight the key questions regarding transport steps that remain to be elucidated.