Abstract
Roots from iron-deficient sugar beet grown in the presence of calcium carbonate exhibit a yellow color and autofluorescence typical of flavin-like compounds, whereas roots of control, iron-sufficient plants exhibited no yellow color and extremely low autofluorescence. The two major flavins whose accumulation is induced by iron deficiency have been shown to be different from riboflavin, FMN, and FAD by reversed-phase high performance liquid chromatography. These flavins, accounting for 82 and 15% of the total flavin concentration in deficient roots, have been shown unequivocally to be riboflavin 3'-sulfate and riboflavin 5'-sulfate, respectively, by electrospray-mass spectrometry, inductively coupled plasma emission spectroscopy, infrared spectrometry, and 1H nuclear magnetic resonance. These flavin sulfates have not been found previously in biological systems. The localization of riboflavin sulfates in deficient roots is similar, but not identical, to that of high iron reductase activity. The concentration of riboflavin sulfates has been estimated from root extracts to be at least 1 mM. We hypothesize, based on the similar localization of flavin and that of iron reduction, that the accumulation of riboflavin sulfates induced by iron deficiency may be an integral part of the turbo iron-reducing system in sugar beet roots.
Highlights
From the $Departmentof Plant Nutrition, Estacion Experimental de Aula DeCi,onsejo Superior de Inuestigaciones Cientificas, Apdo. 202,50080 Zaragoza, Spain, the llDepartments of Neurochemistry and )IBiological Chemistry, Centro de Investigaciony
We show unequivocally by several analytical techniques thatthe flavins induced by iron deficiency in sugar beet are Rbfl 3‘sulfate and Rbfl 5’-sulfate, two flavins not previously found in biological systems
Iron was added in the chelated commercial form Sequestrene 138 from Ciba-Geigy.The pHof the nutrient solution was raised with 1mM NaOH and 1g/liter CaCOa to simulate conditions usually found in the field that lead to iron deficiency; this treatment led to a constant pH of7.5 throughout the 2-week growth period
Summary
Q 1993 by The American Society for Biochemistryand Molecular Biology, Inc. Vol 268, No 28, Issue of October 5,pp. 2095&20965,1993 Printed in U.S.A. We hypothesize, based on the similar localization of flavin and that of iron reduction, that the accumulation of riboflavin sulfates induced by iron deficiency may be an integral part of the turbo iron-reducing system in sugar beet roots. Similar observations were made later for sugar beet [9], sunflower (10, l l ) , pumpkin [12], andpepper [13] It has been generally assumed in the literature that the flavin compound causing the yellow color of the roots of iron-deficient plants was Rbfl since this was the flavin compound thought to be actively excreted to the nutrient solution [7]. May be an integral part of the iron-reducing system in sugar beet roots
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