Responses of Sugar Beet Roots to Iron Deficiency. Changes in Carbon Assimilation and Oxygen Use

Abstract

Different root parts with or without increased iron-reducing activities have been studied in iron-deficient and iron-sufficient control sugar beet (Beta vulgaris L. Monohil hybrid). The distal root parts of iron-deficient plants, 0 to 5 mm from the root apex, were capable to reduce Fe(III)-chelates and contained concentrations of flavins near 700 m, two characteristics absent in the 5 to 10 mm sections of iron-deficient plants and the whole root of iron-sufficient plants. Flavin-containing root tips had large pools of carboxylic acids and high activities of enzymes involved in organic acid metabolism. In iron-deficient yellow root tips there was a large increase in carbon fixation associated to an increase in phosphoenolpyruvate carboxylase activity. Part of this carbon was used, through an increase in mitochondrial activity, to increase the capacity to produce reducing power, whereas another part was exported via xylem. Root respiration was increased by iron deficiency. In sugar beet iron-deficient roots flavins would provide a suitable link between the increased capacity to produce reduced nucleotides and the plasma membrane associated ferric chelate reductase enzyme(s). Iron-deficient roots had a large oxygen consumption rate in the presence of cyanide and hydroxisalycilic acid, suggesting that the ferric chelate reductase enzyme is able to reduce oxygen in the absence of Fe(III)-chelates.

Increased de novo riboflavin synthesis and hydrolysis of FMN are involved in riboflavin secretion from Hyoscyamus albus hairy roots under iron deficiency

“…4). Similar enhancements of respiration in Fe-deficient roots have been described in cucumber [22,23] and sugar beet [24] monitoring oxygen consumption, indicating an increase in aerobic respiration. In the case of H. albus, a change in the activity of the mtETC must be involved in this increase, as discussed below.…”

Changes in cell size and number and in rhizodermal development contribute to root tip swelling of Hyoscyamus albus roots subjected to iron deficiency

“…An increase in root tip diameter, reported as root tip swelling, has been observed previously in various plant species [7][8][9]11]. Our results obtained with H. albus roots cultured under Fe deficiency indicate that this swelling began in the sub-meristematic zone and gradually extended to the elongation zone.…”

“…Using scanning electron microscopy, the formation of short root hairs has been observed on the surface of sugar beet root tips subjected for 10 d to Fe deficiency [9].…”

“…To meet the increasing demand for food supplies, the breeding of Fe deficiency-tolerant plants is a promising approach. So far, the responses of plants to insufficient levels of Fe have been studied actively in a range of crop plants and vegetables, and these studies have demonstrated the induction of the morphological and metabolic changes that are required to withstand the resultant stress and to maintain Fe homeostasis [2][3][4][5][6][7][8][9].…”

Iron deficiency induces changes in riboflavin secretion and the mitochondrial electron transport chain in hairy roots of Hyoscyamus albus

“…An increase in oxygen consumption under Fe starvation has indeed been reported in roots of cucumber (Espen et al, 2000;) and sugar beet (Lόpez-Millán et al, 2000), although not in tomato roots (Lόpez-Millán et al, 2009); and enhancements of glycolytic enzyme activities, cytosolic dehydrogenase activity, phosphoenolpyruvate carboxylase (PEPCase) activity and organic acid synthesis have all been observed in roots in response to Fe deficiency (Rabotti, 1995;Andaluz et al, 2002: Lόpez-Millán et al, 2009.…”