| Iron (Fe) is an essential micronutrient for plant growth. Plants develop a set of mechanism of Fe absorption and mobilization to adapt Fe deficiency and Fe toxicity. Understanding of the regulation mechanism of plant Fe uptake and homeostasis is important for the crop improvement for both Fe fortification and tolerance to Fe deficiency or overdose stress.Plants are divided into two categories based on the Fe acquisition mechanisms: Strategy I plants, which include dicotyledons and non-graminaceous monocotyledons, could reduce Fe (Ⅲ) into Fe (Ⅱ) by ferric reducatse oxidase (FRO) and take Fe (Ⅱ) by Fe (Ⅱ) transporter. Strategy Ⅱ plants, which include most graminaceous plants, secrete phytosiderophores (PS) into soil to bind Fe (Ⅲ) and then the specific transporters will uptake the PS-Fe (Ⅲ) complex. Adpated to the submerged padding soil where the major form of Fe is the reduced form—Fe (Ⅱ), rice could take both Fe (Ⅲ) and Fe (Ⅱ). Rice is considered as a mixed model plant for Fe uptake.Ethylene, an important hormone in plant growth and stress adaption, participates in regulating of Fe deficiency response of strategy I plant and rice. The study focused on strategy I plant Arabidopsis and the mixed strategy plant rice to analyze the mechanism of regulation of Fe deficiency induced ethylene production in these two plants. Result showed that the ethylene production was induced quickly but shortly under Fe deficiency. The transcript levels of several genes encoding the rate-limiting enzyme of ethylene biosynthesis, ACSs, were induced in both leaves and roots under Fe deficiency. In Arabidopsis, AtACS2, AtACS6, AtACS7 and AtACS11 were induced in both leaves and roots. AtACS8 was induced in leaves and AtACS9 was induced in roots. In rice, only OsACS2 was induced in both leaves and roots. The loss of function of OsACS2 decreased the basal level of ethylene production of rice in Fe-sufficient solution, but it had little influence on the induction of ethylene production under Fe deficiency. However, the mutation of its Arabidopsis homologous gene, AtACS2, led to the reduced ethylene production under Fe deficiency. The absence of AtACS2 and OsACS2 resulted in the down-regulation of Fe-related genes in plant.Furthermore, we investigated the role of MPK3 and MPK6 in ethylene production under conditions of Fe deficiency. MPK3 and MPK6 were induced under Fe deficiency at transcription level as well as their enzyme activity. The enzyme activity of MPK6 lasted longer than the MPK3’s. They could influence the ethylene production through the regulation of expression of AtACS2. The mutation of either MPK3 or MPK6 decreased the ethylene production. Three Fe-deficiency response genes, including the FIT, FR02 and IRT1, were down regulated in the mutants. Meanwhile, the ferric-chelate reducatse (FCR) activity in roots of Arabidopsis was also down regulated in them. Fe concentration analysis on mpk3/6 mutants suggested that the chlorotic phenotype was caused by the lower soluble Fe but not the total Fe.Ferric reductase oxidase (FRO), is important in Fe absorption and homeostasis in different organs and cellular internal environment of strategy I plant. Rice has two genes encoding these enzymes but their function is not clear to date. In this study, the function of the rice ferric reductase oxidase OsFRO1 was investigated. OsFRO1 is located at vacuolar membrane. The spatial expression pattern of OsFRO1 suggested that it is mainly function in shoots. Both OsFRO1 promoter-GUS transgenic plant and qRT-PCR result indicated that the expression of OsFRO1 was down regulated under high concentration of Fe. Transgenic plants, including the overexpress lines (OE) and RNAi lines of OsFRO1 were developed for the function analysis. While OE seedlings showed a sensitive phenotype to high concentration of Fe, the RNAi seedlings showed the opposite performance to high Fe stress. There was no significant difference between wild type and transgenic plant in Fe concentration under both Fe sufficient and deficient treatment.In summary, this study investigated the regulatory mechanism of Fe deficiency induced ethylene production. Results suggest that the MPK3/MPK6 and ACS2 are part of the Fe deficiency-induced ethylene production signaling pathway. Besides, the study found that OsFRO1 is involved in Fe homeostasis between vacuoles and cytoplasm. |
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