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

Abstract

It has been shown that N-, P- and S-deficiencies result in major reductions of root hydraulic conductivity (Lpr) which may lead to lowered stomatal conductance, but the relationship between the two conductance changes is not understood. In a variety of species, Lpr decreases in the early stages of NO3-, H2PO4(2-) and SO4(2-) deprivation. These effects can be reversed in 4-24 h after the deficient nutrient is re-supplied. Diurnal fluctuations of root Lpr have also been found in some species, and an example of this is given for Lotus japonicus. In nutrient-sufficient wheat plants, root Lpr is extremely sensitive to brief treatments with HgCl2; these effects are completely reversible when Hg is removed. The low values of Lpr in N- or P-deprived roots of wheat are not affected by Hg treatments. The properties of plasma membrane (PM) vesicles from wheat roots are also affected by NO3(-)-deprivation of the intact plants. The osmotic permeability of vesicles from N-deprived roots is much lower than that of roots adequately supplied with NO3-, and is insensitive to Hg treatment. In roots of L. japonicus, gene transcripts are found which have a strong homology to those encoding the PIP1 and PIP2 aquaporins of Arabidopsis. There is a very marked diurnal cycle in the abundance of mRNAs of aquaporin gene homologues in roots of L. japonicus. The maxima and minima appear to anticipate the diurnal fluctuations in Lpr by 2-4 h. The temporal similarity between the cycles of the abundance of the mRNAs and root Lpr is most striking. The aquaporin encoded by AtPIP1 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 circumstantial evidence that lowered root Lpr may be due to a decrease in either the activity of water channels or their density in the PM. It is concluded that roots are capable, by means completely unknown, of monitoring the nutrient content of the solution in the root apoplasm and of initiating responses that anticipate by hours or days any metabolic disturbances caused by nutrient deficiencies. It is the incoming nutrient supply that is registered as deficient, not the plant's nutrient status. At some point, close to the initiation of these responses, changes in water channel activity may be involved, but the manner in which monitoring of nutrient stress is transduced into an hydraulic response is also unknown.

A 1H NMR study of water flow in Phaseolus vulgaris L. roots treated with nitrate or ammonium

“…A detailed study at the cellular level revealed that N deprivation reduced the hydraulic conductance of root cortical cells, whereas turgor pressure of these cells increased (Radin and Matthews 1989). Dependence of root hydraulic properties on N supply was later re-investigated by Clarkson et al (2000). These authors reported that root hydraulic conductance of plants well supplied with nitrate was affected both by N deprivation and by exposure to the aquaporin inhibitor mercuric chloride in the same way.…”

Comparison of the S-, N- or P-Deprivations’ Impacts on Stomatal Conductance, Transpiration and Photosynthetic Rate of Young Maize Leaves

“…Four days later, the most uniform of those plants were selected and maintained in a hydroponic batch culture for 3 days in well-aerated distilled H 2 …”

“…Although transported, metabolized and utilized in different ways, all the aforementioned nutrients, under deficiency conditions, produce similar effects on g s and hydraulic conductivity. Clarkson et al (2000) [2] have discussed in depth the question whether stomatal closure and diminished hydraulic conductivity are the primary effects of nutrient-stress's response.…”

Consequences of nitrogen deficiency induced by low external N concentration and by patchy N supply in Picea abies and Thuja occidentalis

“…Our measurements of SWUR indicated greater specific water uptake in side roots of Thuja, which were exposed to the N-rich nutrient solution (Table 2). This effect could be mediated by lower hydraulic resistance of roots in presence of nitrate (Clarkson et al 2000;Gloser et al 2007) presumably due to altered function of aquaporins in membranes of root cells (Carvajal et al 1996).…”