Beta vulgaris plasma membrane aquaporins do not need protection against dephosfhorilation to remain active.

KARINA ALLEVA; RICARDO DORR; MARIO PARISI; GABRIELA AMODEO

Punta de Este, Uruguay

Congreso; XI Reunión Latinoamericana de Fisología Vegetal. XXIV Reunión Argentina de Fisiología Vegetal; 2002

Sociedad Argentina de Fisiología Vegetal

Water transport across plasma membrane is essential for plant metabolism. During the last decade it was demonstrated that water channels or aquaporins, are responsible for this kind of transport (1). It is known that tonoplast membrane (TP) and plasma membrane (PM) of plant cells abundantly express these specific proteins, suggesting that water transport across these membranes must be efficient for fast responses to osmotic changes and also endeavor the possibility of this response to be finely regulated. Both hypothesis have been tested by us and other researchers by measuring water transport in isolated PM (plasma membrane) and TP (tonoplast membrane) vesicles. Stopped-flow measurements done by Maurel et al (2) and Niemietz & Tyerman (3) revealed low water permeability values, associated with a high energy activation (Ea) for PM vesicles and on the other hand, high permeability values, with low Ea for TP vesicles. At tissue level, plant cell vacuoles do not represent an important resistance to water transport, therefore the significance of higher permeabilities for TP remains unclear. It is also not well understood why when PM aquaporin genes are so abundantly expressed, the observed water permeability is sufficiently low to assume that it is not the main pathway for water transport across this membrane. Zang & Tyerman (4) proposed that some functional properties of the PM water channels could be lost during the isolation of PM vesicles. The mechanisms that lead to aquaporin inactivation are unknown and may be due to dissociation from cellular elements such as the cytoskeleton (5) or to changes in the levels of phosphorylation (6,7). In order to verify if the lack of a high Pf could be a result of dephosphorylation due to the isolation conditions, we tested two protocols for isolating PM vesicles isolated from B. vulgaris storage roots: a standard protocol and a modified one, where the effects of divalent ions were prevented and the protection against phosphatases were enhanced. The water permeability of PM vesicles was then studied by means of stopped-flow spectrophotometry.