Gating of water channels (aquaporins) in cortical cells of young corn roots by mechanical stimuli (pressure pulses): effects of ABA and of HgCl2

Abstract

Hydraulic properties (half-time of water exchange, T1/2, and hydraulic conductivity, Lp; T1/2 approximately 1/Lp) of individual cells in the cortex of young corn roots were measured using a cell pressure probe for up to 6 h to avoid variations between cells. When pulses of turgor pressure of different size were imposed, T1/2 (Lp) responded differently depending on the size. Pulses of smaller than 0.1 MPa, which induced a small proportional water flow, caused no changes in T1/2 (Lp). Medium-sized pulses of between 0.1 and 0.2 MPa caused an increase in T1/2 (decrease in Lp) by a factor of 4 to 23. The effects caused by medium-sized pulses were reversible within 5-20 min. When larger pulses of more than 0.2 MPa were employed, changes were not reversible within 1-3 h, but could be reversed within 30 min in the presence of 500 nM of the stress hormone ABA. Cells with a short T1/2 responded to the aquaporin blocker mercuric chloride (HgCl2). The treatment had no effect on cells which exhibited long T1/2 following a mechanical inhibition by the large-pulse treatment. Step decreases in pressure resulted in the same inhibition as step increases. Hence, the treatment did not cause a stretch-inhibition of water channels and was independent of the directions of both pressure changes and water flows induced by them. It is concluded that inhibition is caused by the absolute value of intensities of water flow within the channels, which increased in proportion to the size of step changes in pressure. Probable mechanisms by which the mechanical stimuli are perceived are (i) the input of kinetic energy to the channel constriction (NPA motif of aquaporin) which may cause a conformational change of the channel protein (energy-input model) or (ii) the creation of tensions at the constriction analogous to Bernoulli's principle for macroscopic pores (cohesion-tension model). Estimated rates of water flow within the pores were a few hundred micro m s-1, which is too small to create sufficient tension. They were much smaller than those proposed for AQP1. Based on literature data of single-channel permeability of AQP1, a per channel energy input of 200 kBxT (kB=Boltzmann constant) was estimated for the energy-input model. This should be sufficient to initiate changes of protein conformation and an inactivation of channels. The data indicate different closed states which differ in the amount of distortion and the rates at which they relax back to the open state.

Polarity of Water Transport across Epidermal Cell Membranes in Tradescantia virginiana

“…Data are means 6 SD of n = 6 microcapillaries tested. (Wan et al, 2004). For the latter hydraulic disturbance, we have similarly noted increased T 1/2 in T. virginiana epidermal cells when applying P steps of greater than 0.15 MPa (data not shown).…”

“…Consistent with other studies in higher plant cells, Moore and Cosgrove (1991) Cosgrove and Steudle (1981) reported that a substantial (6-fold) and rapid (within 20 s) reduction in L p could occur in the same cell, and in hindsight, this presumably reflected the influence of aquaporins. Cosgrove and Steudle (1981) did not consider the lower L p as indicative of the L p in situ, and Wan et al (2004) reported that a reduction in L p was associated with perturbations to P cell on the order of 0.1 MPa. Hence, if measured membrane L p itself can exhibit substantial changes over relatively short periods of time in the same cell, then further study of systematic differences between L p OUT and L p IN will require a robust hydrostatic methodology (PC or PR) that can reversibly and reproducibly apply small perturbations in pressure (P) to individual cells over short periods of time.…”

Rapid changes in root hydraulic conductivity and aquaporin expression in rice (Oryza sativaL.) in response to shoot removal – xylem tension as a possible signal

“…5). Wan et al (2004) proposed a gating mechanism for AQPs which involves the water flow rate through AQPs. This gating mechanism will affect cellular water status and, through this, could provide the link between xylem tension, cellular water flow rate and nuclear AQP transcription response.…”

Aquaporin structure–function relationships: Water flow through plant living cells

“…There is no direct evidence for mechanosensitivity of aquaporin proteins, although some studies illustrate the potential for MIPS to be involved in volume or turgor homeostasis in plants [3,73,74], and a different mechanism of mechanical inhibition was recently reported in young maize roots [75]. The dependence of mercury-induced closure of water channels on tissue turgor indicates that changes in turgor can induce changes in the conformation of the plasma membrane, or even in the aquaporins themselves [76].…”

“…The responsiveness of each aquaporin to ABA were different, implying that the regulation of aquaporin expression involves both ABA-dependent and ABA-independent signaling pathways. Both energy-input and tension-gating mechanisms might be used by the plant to sense changes in turgor pressure and surrounding water availability, and to adapt the membrane water permeability in an ABAdependent manner [75]. In many species including sunflower, barley, sorghum and maize [96][97][98], exogenous ABA enhanced root L p .…”