Root hydraulic conductance: diurnal aquaporin expression and the effects of nutrient stress

It has been shown that N-, P- and S-deficiencies result in major reductions of root hydraulic conductivity (Lp(r)) which may lead to lowered stomatal conductance. but the relationship between the two conductance changes is no t understood. In a variety of species, Lp(r) decreases in the early stages of NO3-, H2PO42- and SO42- deprivation. These effects can be reversed in 4- 24 h after the deficient nutrient is re-supplied. Diurnal fluctuations of r oot Lp(r) have also been found in some species, and an example of this is g iven for Lotus japonicus. In nutrient-sufficient wheat plants, root Lp(r) i s extremely sensitive to brief treatments with HgCl2; these effects are com pletely reversible when Hg is removed. The low values of Lp, in N- or P-dep rived roots of wheat are not affected by Hg treatments. The properties of p lasma membrane (PM) vesicles from wheat roots are also affected by NO3--dep rivation of the intact plants. The osmotic permeability of vesicles from N- deprived roots is much lower than that of roots adequately supplied with NO 3- and is insensitive to Hg treatment. In roots of Lp(r) japonicus, gene tr anscripts are found which have a strong homology to those encoding the PIP1 and PIP2 aquaporins of Arabidopsis. There is a very marked diurnal cycle i n the abundance of mRNAs of aquaporin gene homologues in roots of L. japoni cus. The maxima and minima appear to anticipate the diurnal fluctuations in Lp(r) by 2-4 h. The temporal similarity between the cycles of the abundanc e of the mRNAs and root Lp(r) is most striking. The aquaporin encoded by At PIP1 is known to have its water permeation blocked by Hg binding. The lack of Hg-sensitivity in roots and PMs from N-deprived roots provides circumsta ntial evidence that lowered root Lp(r) may be due to a decrease in either t he activity of water channels or their density in the PM. It is concluded t hat roots are capable, by means completely unknown, of monitoring the nutri ent content of the solution in the root apoplasm and of initiating response s that anticipate by hours or days any metabolic disturbances caused by nut rient deficiencies. It is the incoming nutrient supply that is registered a s deficient, not the plant's nutrient status. At some point, close to the i nitiation of these responses, changes in water channel activity may be invo lved, but the manner in which monitoring of nutrient stress is transduced i nto an hydraulic response is also unknown.