We compared the plasma membrane, tonoplast and mitochondrial Mg-ATPase
activities of microsomes isolated
from mycorrhizal and non-mycorrhizal sunflower (Helianthus annuus
L.) and onion (Allium cepa L.) roots during
a time-course experiment (21–60 d). No significant difference
was observed between the protein levels of root
microsomes from mycorrhizal and non-mycorrhizal sunflower. However,
the protein content of microsomes
obtained from mycorrhizal onion roots increased when the symbiosis was
well established (more than 50% of root
length colonized by the arbuscular mycorrhizal fungus). Kinetic studies
revealed that vanadate-sensitive (plasma
membrane-associated) ATPase activity of sunflower root microsomes
increased with mycorrhizal colonization.
However, the vanadate-sensitive ATPase activity of microsomes from
mycorrhizal onion roots decreased
compared with those from non-mycorrhizal controls. Increased activity
of nitrate (tonoplast-associated) and azide
(mitochondrial membrane-related)-sensitive ATPase activities was
detected in extracts obtained from 45- and 60-d-old mycorrhizal onion
plants. A slight increase in azide-sensitive ATPase activity was detected
in microsomes
of mycorrhizal sunflower. The existence of different physiological
and biochemical strategies in sunflower and
onion in relation to the establishment of this mutualistic symbiosis is
discussed.