The relationship between the methyl-erythritol phosphate pathway leading to emission of volatile isoprenoids and abscisic acid content in leaves

Abstract

It was investigated whether the methyl-erythritol phosphate (MEP) pathway that generates volatile isoprenoids and carotenoids also produces foliar abscisic acid (ABA) and controls stomatal opening. When the MEP pathway was blocked by fosmidomycin and volatile isoprenoid emission was largely suppressed, leaf ABA content decreased to about 50% and leaf stomatal conductance increased significantly. No effect of fosmidomycin was seen in leaves with constitutively high rates of stomatal conductance and in plant species with low foliar ABA concentration. In all other cases, isoprene emission was directly associated with foliar ABA, but ABA reduction upon MEP pathway inhibition was also observed in plant species that do not emit isoprenoids. Stomatal closure causing a midday depression of photosynthesis was also associated with a concurrent increase of isoprene emission and ABA content. It is suggested that the MEP pathway generates a labile pool of ABA that responds rapidly to environmental changes. This pool also regulates stomatal conductance, possibly when coping with frequent changes of water availability. MEP pathway inhibition by leaf darkening, and its down-regulation by exposure to elevated CO2, was also associated with a reduction of foliar ABA content. However, stomatal conductance was reduced, indicating that stomatal aperture is not regulated by the MEP-dependent foliar ABA pool, under these specific cases.

Figure 1.

Stomatal conductance rates in leaves of P. australis (isoprene emitter, circles) and Q. ilex (monoterpene emitter, triangles). The increment of stomatal conductance after inhibiting isoprenoid formation (x axis, values expressed as percent of the stomatal conductance in the same leaves before isoprene inhibition [=100]) is plotted versus the stomatal conductance rates of each leaf, measured before isoprenoid inhibition (y axis). Isoprene was inhibited by feeding 20 μm fosmidomycin through petioles. In some leaves of P. australis (black squares) the increment of stomatal conductance after endogenous isoprene inhibition was again measured after reconstituting the internal pool with exogenous isoprene (3 ppm of gaseous isoprene in the air flowing over the leaf in the gas exchange cuvette) for 1 h (white squares).

Figure 2.

Relationship between isoprene emission and ABA content in P. australis leaves characterized by stomatal conductances variable between 0.15 to 0.35 mol m⁻² s⁻¹. Also shown is the linear best fit generated by the Sigmaplot 2002 software (Systat; r² = 0.83).

Figure 3.

Time course of changes in ABA content (A), isoprene emission (B; black circles), and stomatal conductances (white circles) upon feeding leaves of P. alba with fosmidomycin 20 μm. The inhibitor was fed at time 0. The measurements shown are means ± se (n = 3).

Figure 4.

Time course of changes in ABA content and isoprene emission upon a light-to-dark transition in P. alba leaves. The illumination (1,000 μmol photons m⁻² s⁻¹) was switched off at time 0. The values are means of measurements on three leaves of different plants expressed as relative to isoprene emission and ABA content in illuminated leaves (=100). The se of these measurements were always <10% of the respective means. Also shown as means ± se are the values of stomatal conductance at the beginning and at the end of the experiment.

Figure 5.

Time course of changes in photosynthesis, isoprene emission, ABA content, and stomatal conductance during a day in P. alba leaves. The measurements shown are means ± se (n = 4). Asterisks (***) represent statistical separation between means of each parameter measured at 9:30 am and at 1:00 pm (P ≤ 0.01, Tukey's test).

Figure 6.

Isoprene emission (gray bars), ABA content (black bars), and stomatal conductance (white circles) of P. alba leaves in response to exposure to different concentrations of CO₂. Asterisks represent statistical separation between means of each parameter under the three CO₂ concentrations (n = 3; * = P ≤ 0.10, ** = P ≤ 0.05; Tukey's test).