Im. Juszczuk et al., Regulation of alternative oxidase activity during phosphate deficiency in bean roots (Phaseolus vulgaris), PHYSL PLANT, 113(2), 2001, pp. 185-192
Cyanide-resistant respiration was studied in mitochondria isolated from the
roots of bean plants (Phaseolus vulgaris L. cv. Zlota Saxa) grown hydropon
ically up to 16 days on a phosphate-sufficient (+ P, control) or phosphate-
deficient (- P) medium. Western blotting indicated that the alternative oxi
dase (AOX) was present only in its reduced (active) form, both in phosphate
-sufficient and phosphate-deficient roots, but in the latter, the amount of
AOX protein was greater. Addition of pyruvate to the isolation, washing an
d reaction media made mitochondria from + P roots cyanide-insensitive, simi
lar to mitochondria from - P roots. The doubled activity of NAD-malic. enzy
me (NAD-ME) in - P compared with + P root mitochondria may suggest increase
d pyruvate production in - P mitochondria. Lower cytochrome c oxidase (COX)
activity and no uncoupler effect on respiration indicated limited cytochro
me chain activity in - P mitochondria. In - P mitochondria, the oxygen upta
ke decreased and the level of Q reduction increased from 60 to 80%. With no
pyruvate present (AOX not fully activated), inhibition of the cytochrome p
athway resulted in an increased level of the ratio of reduced ubiquinone (Q
r) to total ubiquinone (Qt) (Qr/Qt) in + P mitochondria, but did not change
Qr/Qt in - P mitochondria. When pyruvate was present, the kinetics for AOX
were similar in mitochondria from - P and + P roots. It is suggested that
AOX participation in - P respiration may provide an acclimation to phosphat
e deficiency. Stabilization of the ubiquinone reduction level by AOX might
prevent the harmful effect of an increased formation of reactive oxygen spe
cies.