Abstract
Cell wall is the major component of root apoplast which is the main reservoir for iron in roots, while nitric oxide (NO) is involved in regulating the synthesis of cell wall. However, whether such regulation could influence the reutilization of iron stored in root apoplast remains unclear. In this study, we observed that iron deficiency elevated NO level in tomato (Solanum lycopersicum) roots. However, application of S-nitrosoglutathione, a NO donor, significantly enhanced iron retention in root apoplast of iron-deficient plants, accompanied with a decrease of iron level in xylem sap. Consequently, S-nitrosoglutathione treatment increased iron concentration in roots, but decreased it in shoots. The opposite was true for the NO scavenging treatment with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO). Interestingly, S-nitrosoglutathione treatment increased pectin methylesterase activity and decreased degree of pectin methylation in root cell wall of both iron-deficient and iron-sufficient plants, which led to an increased iron retention in pectin fraction, thus increasing the binding capacity of iron to the extracted cell wall. Altogether, these results suggested that iron-deficiency-induced elevation of NO increases iron immobilization in root apoplast by decreasing pectin methylation in cell wall.
Highlights
Cell wall is the major component of root apoplast which is the main reservoir for iron in roots, while nitric oxide (NO) is involved in regulating the synthesis of cell wall
Cell wall is the major component of root apoplast, consisting of pectin, cellulose and hemicellulose, and containing highly negatively charged sites that may serve as a sink for most cationic nutrients[6]
Application of the NO specific scavenger cPTIO substantially diminished the Fe-deficiency-induced NO-associated green fluorescence (Fig. 1a). These results suggested that Fe deficiency promoted NO production in roots of tomato plants
Summary
Cell wall is the major component of root apoplast which is the main reservoir for iron in roots, while nitric oxide (NO) is involved in regulating the synthesis of cell wall. S-nitrosoglutathione treatment increased pectin methylesterase activity and decreased degree of pectin methylation in root cell wall of both iron-deficient and iron-sufficient plants, which led to an increased iron retention in pectin fraction, increasing the binding capacity of iron to the extracted cell wall. Since cell wall are full of negative charges, metal ions can be fixed into cell wall component, the alteration of cell walls components could be expected to affect the binding capacity of metal ions with cell wall[6] In this sense and taking account of the important role of root apolast in Fe homeostasis in plant, we assumed that elevation of NO production may affect the Fe retention in root apoplast and its subsequent reutilization by plant through a mechanism of modulating the composition of cell wall. We found that Fe deficiency-induced NO production increased the Fe immobilization in the Fe-deficient root apoplastic space through lowering the degree of methyl esterification of cell wall, preventing Fe translocation from roots to shoots
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