Analysis of the Mechanisms Underlying the Specificity of the Variation Potential Induced by Different Stimuli

Abstract

Plants are able to perceive diverse environmental factors and form an appropriate systemic functional response. Systemic responses are induced by stimulus-specific long-distance signals that carry information about the stimulus. Variation potential is proposed as a candidate for the role of such a signal. Here, we focus on the mechanisms that determine the specificity of the variation potential under the action of different local stimuli. Local stimuli such as heating, burning and wounding cause variation potential, the parameters of which differ depending on the type of stimulus. It was found that the stimulus-specific features of the hydraulic signal monitored by changes in leaf thickness and variation potential, such as a greater amplitude upon heating and burning and a significant amplitude decrement upon burning and wounding, were similar. The main features of these signals are the greater amplitude upon heating and burning, and a significant amplitude decrement upon burning and wounding. Together with the temporal correspondence of signal propagation, this evidence indicates a role for the hydraulic signal in the induction of stimulus-specific variation potential. Experiments using mechanosensitive channel inhibitors have demonstrated that the hydraulic signal contributes more to the induction of the variation potential in the case of rapidly growing stimuli, such as burning and wounding, than in the case of gradual heating. For thermal stimuli (gradual heating and burning), a greater contribution, compared to wounding, of the chemical signal related to reactive oxygen species to the induction of the variation potential was demonstrated. Thus, the specificity of the parameters of the variation potential is determined by the different contributions of hydraulic and chemical signals.

Keywords: abiotic stress; electrical signal; hydraulic signal; signal transduction; variation potential.

Figure 1

Variation potentials (VPs) induced by local heating, burning or wounding in wheat plants. (A) Averaged VP traces. The arrow indicates the moment of mechanical wounding, burning or the beginning of gradual heating of the leaf tip. (B) Dependence of the VP amplitude on the distance to the area of local stimulation. Data are means ± SEM. Different uppercase letters indicate statistically significant differences between stimuli; different lowercase letters indicate statistically significant differences between distances within a single stimulus (p < 0.05). (C) Schematic representation of the experimental design for monitoring surface potentials in wheat plants. E1, E2, E3: surface electrodes.

Figure 2

Systemic changes in leaf thickness induced by local heating, burning or wounding in wheat plants. (A) Simultaneous representative recordings of leaf thickness changes (Δh, solid lines) and variation potentials (ΔU, dashed lines) at a distance of 4.5 cm and 3 cm, respectively, from the stimulation area. The arrow indicates the moment of mechanical wounding, burning or the beginning of gradual heating of the leaf tip. (B) Averaged recordings of leaf thickness changes (Δh); shaded regions (envelopes) represent SEM. The arrow indicates the moment of mechanical wounding, burning or the beginning of gradual heating of the leaf tip. (C–E) Dependences of the amplitude (C), duration (D) and rate (E) of leaf thickening on the distance to the area of local stimulation. Data are means ± SEM. Different uppercase letters indicate statistically significant differences between stimuli; different lowercase letters indicate statistically significant differences between distances within a single stimulus (p < 0.05). (F) Schematic representation of the experimental design for simultaneous monitoring of surface potentials and changes in wheat leaf thickness. E1, E2, E3: surface electrodes. Yellow lines are light bands between the sender units and the receiver units of the optical micrometers (gray rectangles).

Figure 3

The dynamics of stomatal conductance (g_S) induced by local heating, burning or wounding in the unstimulated part of the wheat plant. Lines are means; shaded regions (envelopes) represent SEM. The dashed line indicates the moment of generation of the variation potential.

Figure 4

Variation potentials (VPs) induced by local heating, burning or wounding in the detached wheat leaf. (A) Averaged VP traces. The arrow indicates the moment of mechanical wounding, burning or the beginning of gradual heating of the leaf tip. (B) Comparison of the dependences of the VP amplitude on the distance to the area of local stimulation in detached leaves (solid lines) and whole wheat plants (dashed lines). Data are means ± SEM. For detached leaves only, different uppercase letters indicate statistically significant differences between stimuli; different lowercase letters indicate statistically significant differences between distances within a single stimulus (p < 0.05). (C) Schematic representation of the experimental design for monitoring surface potentials in wheat plants. E1, E2, E3: surface electrodes.

Figure 5

Systemic changes in leaf thickness (Δh) induced by local heating, burning or wounding in the detached wheat leaf. Lines are means; shaded regions (envelopes) represent SEM. The arrow indicates the moment of mechanical wounding, burning or the beginning of gradual heating of the leaf tip.

Figure 6

Effects of the reactive oxygen species (ROS) scavenger N,N′-dimethylthiourea (DMTU) and ROS-producing enzyme inhibitors diphenyleneiodonium chloride (DPI) and salicylhydroxamic acid (SHAM) on the amplitudes of the variation potentials (VPs) induced by local heating, burning or wounding in the detached wheat leaf. VP amplitude is represented as the percentage of control, which is the VP amplitude in untreated leaves (without scavengers and inhibitors). Data are means ± SEM. * indicates data significantly different from untreated leaves (p < 0.05).

Figure 7

Effects of the H⁺-ATPase inhibitor Na₃VO₄, the Са²⁺-permeable channel blocker LaCl₃ and the mechanosensitive channel inhibitor GdCl₃ on the variation potentials (VPs) induced by local heating, burning or wounding in the detached wheat leaf. (A) Averaged VP traces observed at a distance of 6 cm from the stimulation area in untreated leaves (dashed lines) or those treated with the inhibitor/blocker (solid lines). (B–D) VP amplitude at a distance of 6 cm from the area of local stimulation upon treatment with Na₃VO₄ (B), LaCl₃ (C) or GdCl₃ (D). VP amplitude is represented as the percentage of control, which is the VP amplitude in untreated leaves (without inhibitor). Data are means ± SEM. Statistically significant differences between untreated and treated leaves were found for all stimuli. * indicates significant differences between the stimuli (p < 0.05).