Discovering the Plants’ Sense of Smell: The Role of Stomata in the Ability to Detect Olfactory Signals

Throughout the course of evolution, plants have developed different mechanisms to perceive environmental cues. Since plants cannot move from one place to another as other organisms can, understanding what is happening around them is of great importance. To the surprise of many, these perception systems are more sophisticated than people thought. For example, recent studies have demonstrated their ability to emit sounds that might inform other organisms about the physiological state of the plant, as well as their capability to “observe” and mimic plants in the surroundings, possibly through simple eye-like light receptors known as ocelli.

The ability of plants to detect ecologically relevant olfactory signals has been one of the sensory systems that have captivated researchers. When plant tissues are damaged by stress, they emit volatile organic compounds (VOCs) that distal plant tissues or neighbouring plants can detect as a warning to prepare for future attacks. Once detected, plants that perceive it can prepare to defend themselves by either volatile-mediated defence induction or defence priming. Despite the several studies that speak about this type of communication, major questions in volatile-mediated plant communication still remain, including the underlying mechanism of how plants perceive such signals and the location where this perception occurs. Current hypotheses suggest that volatile compounds are sensed within plant tissues and that stomata –the tiny pores found in plant epidermis– function as a port of entry for many of these compounds.

Another key factor for plant communication that has been little explored so far is the role of environmental context. Plants respond to stimuli caused by environmental changes with distinct changes related to their development and physiology. In fact, stomata predictably respond to environmental conditions opening in response to light cues and closing them in the absence of light or during drought. In this sense, and since stomata are potential sites for the uptake of volatile compounds, changes in stomata aperture caused by the environment should impact plant communication through VOCs.

To contribute to answering these questions, Natalie M. Aguirre –a PhD Student at Texas A&M University– and her colleagues conducted a fascinating study to examine how the environmental context modulates plant volatile communication and defence priming  in addition to evaluating the role of stomata in the perception of olfactory signals. To do so, the authors exposed maize plants to a variety of volatile organic compounds produced in response to herbivory (known as HIPVs) and simulated herbivore attack under different environmental conditions (light, dark, drought, and Abscisic acid application) that affect stomatal opening or defence signalling and then quantified the phytohormones synthesised by plants and the volatile compounds emitted.

The results show that the perception of volatile compounds and subsequent defence priming was impaired under environmental conditions that reduce stomatal conductance or alter defence signalling. In non-stressed conditions and when exposed to light, the plants kept their stomata open, suggesting they could perceive the signals from the HIPVs. In fact, these plants showed higher levels of substances known to play a role in inducing herbivore defences, such as jasmonic acid (JA) and abscisic acid (ABA). Contrastingly, plants under darkness or drought conditions had their stomata closed and produced less of such substances. This indicates that volatile compounds uptake and subsequent plant communication may be impaired in environmental conditions that reduce stomatal openness.

Although there are still unanswered questions, such as what is the effect of other environmental conditions that could alter plant volatile emissions, uptake, perception and defence priming, this exciting work provides a better understanding of how environmental contexts affect physiological responses and defence signalling and offers new insights into plant volatile communication.

READ THE ARTICLE
Aguirre, N. M., Grunseich, J. M., Lima, A. F., Davis, S. D., & Helms, A. M. (2023). Plant communication across different environmental contexts suggests a role for stomata in volatile perception. Plant, Cell & Environment. Available at: https://doi.org/10.1111/pce.14601

About the author.

Andrés Reyes is a biotechnology engineer working in plant molecular biology at the National Institute of Agricultural Research (INIAP) in Ecuador. Passionate about science communication, he believes that connecting science with the community is fundamental to generating real and lasting changes. You might follow him on Twitter at @f_andresreyes.