Rapid reduction of arsenate in the medium mediated by plant roots

Microbes detoxify arsenate by reduction and efflux of arsenite. Plants have a high capacity to reduce arsenate, but arsenic efflux has not been reported. Tomato (Lycopersicon esculentum) and rice (Oryza sativa) were grown hydroponically and supplied with 10 microm arsenate or arsenite, with or without phosphate, for 1-3 d. The chemical species of As in nutrient solutions, roots and xylem sap were monitored, roles of microbes and root exudates in As transformation were investigated and efflux of As species from tomato roots was determined. Arsenite remained stable in the nutrient solution, whereas arsenate was rapidly reduced to arsenite. Microbes and root exudates contributed little to the reduction of external arsenate. Arsenite was the predominant species in roots and xylem sap. Phosphate inhibited arsenate uptake and the appearance of arsenite in the nutrient solution, but the reduction was near complete in 24 h in both -P- and +P-treated tomato. Phosphate had a greater effect in rice than tomato. Efflux of both arsenite and arsenate was observed; the former was inhibited and the latter enhanced by the metabolic inhibitor carbonylcyanide m-chlorophenylhydrazone. Tomato and rice roots rapidly reduce arsenate to arsenite, some of which is actively effluxed to the medium. The study reveals a new aspect of As metabolism in plants.     

Direct evidence showing the effect of root surface iron plaque on arsenite and arsenate uptake into rice (Oryza sativa) roots

The present study aimed to investigate the effects of root surface iron plaque on the uptake kinetics of arsenite and arsenate by excised roots of rice (Oryza sativa) seedlings. The results demonstrated that the presence of iron plaque enhanced arsenite and decreased arsenate uptake. Arsenite and arsenate uptake kinetics were adequately fitted by the Michaelis-Menten function in the absence of plaque, but produced poor fits to this function in the presence of plaque. Phosphate in the uptake solution did not have a significant effect on arsenite uptake irrespective of the presence of iron plaque; however phosphate had a significant effect on arsenate uptake. Without iron plaque, phosphate inhibited arsenate uptake. The presence of iron plaque diminished the effect of phosphate on arsenate uptake, possibly through a combined effect of arsenate desorption from iron plaque.     

Purification and characterization of an alkaline invertase from shoots of etiolated rice seedlings

One alkaline invertase and two acid invertase activities were detected in the shoots of etiolated rice ( Oryza sativa ) seedlings. The alkaline invertase (AIT) was purified to homogeneity through steps of ammonium sulphate fractionation, concanavalin A-Sepharose affinity chromatography (non-retained), DEAE-Sephacel chromatography and preparative electrophoresis. The pH optimum of AIT was 7.0 and the molecular mass, determined by gel filtration, was 240 kDa. It is apparently a homotetrameric enzyme (subunit molecular mass 60 kDa). The isoelectric point was 4.4 by isoelectric focusing. The best substrate of the enzyme was sucrose, with a K _m of 2.53 mM. The enzyme also hydrolysed raffinose, but not maltose or lactose, so it is a β-D-fructofuranosidase. It gave negative glycoprotein staining. Of the hydrolysis products, fructose was a competitive inhibitor and glucose was a non-competitive inhibitor. Treatment with an alkaline phosphatase could activate AIT, whereas other proteins such as BSA, concanavalin A and urease had no effect on the enzyme activity. The enzyme activity was inhibited by Tris, thiol reagents and heavy metal ions.     

A simple and rapid procedure to establish embryogenic cell suspensions as a source of protoplasts for efficient plant regeneration from two Chinese commercial rice cultivars

An efficient and rapid procedure has been developed to establish embryogenic cell suspension cultures of two Japonica Chinese commercial rice cultivars, Zhonghua 8 and Eryi 105. Embryogenic cell suspensions of both varieties were established from 0.5–1.0 g fresh weight of embryogenic callus in AA medium within 2.5 months of the initiation of callus from sterilised seeds. The previously reported subculture of callus on semi-solid medium for 4–8 weeks prior to transfer into liquid medium was unnecessary and caused delay in the establishment of embryogenic cell suspensions. Protoplasts were isolated reproducibly from cell suspensions up to 18 months after their initiation, with protoplast plating efficiencies attaining 0.15–0.37%. Reproducible plant regeneration from 14–26% of the protoplast-derived tissues was achieved without the requirement for nurse cells. This revised version was published online in June 2006 with corrections to the Cover Date.