Basic instinct – protons pulled back as air supply fails
Higher plants are essentially aerobic organisms. A complete lack
of oxygen (anoxia) leads to a failure in mitochondrial electron transport. The
subsequent impact on cytoplasmic processes is quite well documented. However,
it is the contention of Hubert Felle (Geissen, Germany,
pp. 1085–1093) that studies of this topic have ignored the apoplast. This
is a very important compartment, having multiple functions that include
transport, storage, defence and a limited range of enzyme catalysis. So, what
happens to the apoplast under low oxygen tension? Firstly, using non-invasive
micro-probe techniques, the author showed that treatment of barley seedlings
with fusicoccin, which stimulates the plasma membrane H+ pump,
results, as expected, in hyper-polarization of the plasma membrane and an
acidification of the apoplast. Cyanide, which inhibits mitochondrial electron
transport, by contrast reduced H+ pumping, leading to a
depolarization of the plasma membrane and alkalinization of the apoplast.
However, the loss of H+ ions from the apoplast was not enough to
explain the decrease in cytoplasmic pH, suggesting that H+ also
entered the cytoplasm from another source (e.g. vacuole). In leaves exposed to anoxic or hypoxic conditions,
responses were very similar to those seen with cyanide, but under hypoxia both
membrane potential and apoplastic pH started to recover after a few minutes. A key question now
arises: are these changes local or are they systemic? Plant shoots were
maintained in air while the roots were kept anoxic under nitrogen. Very
interestingly, after a delay, the apoplastic pH increased, as it did if roots
were treated with cyanide. Signalling from root to shoot caused the leaf cell
apoplasts to respond as if leaf cells were anoxic. However, this did not happen
within the 2 h if the roots
were simply flooded, possibly indicating that the oxygen tension in the roots
did not, in the time of the experiment, drop low enough to trigger export of an
‘anoxia signal’.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
