Increase in NAD(P)H-dependent generation of active oxygen species and changes in lipid composition of microsomes isolated from roots of zinc-deficient bean plants

The role of zinc (Zn) in maintaining the structural and functional integrity of plant membranes was investigated in the present work. The relationship between the activity of NAD(P)H oxidases generating active oxygen species and changes in lipid composition and peroxidation was evaluated in microsomal membrane vesicles isolated from roots of Zn-deficient bean (Phaseolus vulgaris L., cv. Bobis) plants. Zinc content of bean root microsomal membranes was decreased by about 30% by Zn deficiency. Microsomes isolated from roots of Zn-deficient plants showed higher rates of NAD(P)H oxidation and NAD(P)H-dependent O2- generation than Zn-sufficient roots. Microsomal O2- consumption, measured in the presence of pyridine nucleotides, was also considerably enhanced by Zn deficiency. This latter activity was greatly stimulated by Fe(III)EDTA, while inhibited by superoxide dismutase (SOD) and catalase, indicating that active oxygen species were produced during the oxygen consuming enzyme reaction. Zinc deficiency caused a decline in microsomal phospholipid (PL) content. In addition, saturated fatty acids were present at a higher proportion than unsaturated fatty acids in microsomes from Zn-deficient roots. Sterol content of microsomal vesicles was also modified by Zn deficiency, which led to an increase in the planar sterol campesterol and a concomitant decrease in stigmasterol and sitosterol content. NADPH-dependent lipid peroxidation, directly measured in microsomal vesicles as malondialdehyde (MDA) production, was slightly enhanced by Zn deficiency. These results support the idea that Zn deficiency determines an enhanced generation of harmful oxygen species by membrane associated enzymes and show that this activity can be more pronounced in the presence of iron (Fe), which accumulates in Zn-deficient tissues. The relationship between the occurrence of this phenomenon and the changes in membrane lipid profile is discussed.