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The myrosinase-glucosinolate system is considered to be a major component of the preformed defence system of Brassicaceae species. This hypothesis has influenced the belief that the components of the myrosinase-glucosinolate system are present at fixed levels which are independent of environmental factors. In the present study we show that external availability of nutrients can modulate the expression levels of myrosinase enzymes (EC 3.2.3.1). Nutrients such as sulphate, iron, copper, zinc and manganese were tested for their modulation effect on myrosinase expression levels and activity in roots, stems, cotyledons and buds of Sinapis alba seedlings at four different developmental stages. The most sensitive organ was the bud where iron deficiency approximately doubled the myrosinase activity. Removal of sulphate and all four micronutrients reduced the myrosinase activity to approximately half of the activity in plants supplemented with all these nutrients. The myrosinase polypeptides can be divided into classes based on molecular mass after reduction. The nutritional status influenced mainly the 68-kDa class of myrosinases as revealed by western blot and laser scan densitometry of the immunolabelled blots. [ABSTRACT FROM AUTHOR]

15N-Nitrate and 35S-sulphate labelling experiments were performed with spring wheat (Triticum aestivum L. cv. Timmo) 44, 64, 79, 95 and 115 days after sowing (growth stages arbitrarily denoted I to V). Label was fed to the plants via a fraction of the root system, termed “donor root”, whereas the rest of the root (“receiver root”) was fed non-labelled nutrient solution. Net uptake rates for both nitrate and sulphate per unit root weight changed little from growth stage I to IV, but were considerably lower at stage V. On a whole-plant weight basis, uptake declined from stage I to IV, because root contribution to total plant weight declined. Between 80 and 95% of absorbed label was translocated to the shoot at all growth stages. At stage V, up to 30% of absorbed label was recovered in the ears. Labelling of the receiver root indicated that, at all growth stages, 10 to 17% of N and 12 to 32% of S translocated to the shoot was retranslocated to the root. This corresponds to between 35 and 85% of the label actually recovered in the roots. Analysis of 15N-labelling of xylem sap collected from receiver roots at growth stages I to IV indicated that about half of the reduced N in the sap is derived from cycling through roots of recently assimilated N. Evidence of cycling was also obtained at stage V. Labelled sulphate was the only form of S cycled in the plant, but it accounted for only 1 to 7% of the sulphate in the xylem sap. [ABSTRACT FROM AUTHOR]

The accumulation of labelled ions was measured in seedlings of Arabidopsis thaliana (L.) Heynh. grown in submerged cultures. Genotypes used were wild type and the nitrate-uptake mutant B1, which is altered in the uptake of nitrate and chlorate from concentrations above 1 mM when grown under normal conditions. At low and high concentrations of chlorate, chloride, and K+, significantly less radioactivity was accumulated in seedlings of B1 than in seedlings of wild type. The influx of sulphate did not decrease in B1. The results indicate that the effect of the mutation in B1 is restricted to the uptake of monovalent ions. [ABSTRACT FROM AUTHOR]