Rumor Has It…: Relay Communication of Stress Cues in Plants

Recent evidence demonstrates that plants are able not only to perceive and adaptively respond to external information but also to anticipate forthcoming hazards and stresses. Here, we tested the hypothesis that unstressed plants are able to respond to stress cues emitted from their abiotically-stressed neighbors and in turn induce stress responses in additional unstressed plants located further away from the stressed plants. Pisum sativum plants were subjected to drought while neighboring rows of five unstressed plants on both sides, with which they could exchange different cue combinations. On one side, the stressed plant and its unstressed neighbors did not share their rooting volumes (UNSHARED) and thus were limited to shoot communication. On its other side, the stressed plant shared one of its rooting volumes with its nearest unstressed neighbor and all plants shared their rooting volumes with their immediate neighbors (SHARED), allowing both root and shoot communication. Fifteen minutes following drought induction, significant stomatal closure was observed in both the stressed plants and their nearest unstressed SHARED neighbors, and within one hour, all SHARED neighbors closed their stomata. Stomatal closure was not observed in the UNSHARED neighbors. The results demonstrate that unstressed plants are able to perceive and respond to stress cues emitted by the roots of their drought-stressed neighbors and, via ‘relay cuing’, elicit stress responses in further unstressed plants. Further work is underway to study the underlying mechanisms of this new mode of plant communication and its possible adaptive implications for the anticipation of forthcoming abiotic stresses by plants

Is ABA the exogenous vector of interplant drought cuing?

We have recently demonstrated that root cuing from drought-stressed plants increased the survival time of neighboring plants under drought, which came at performance costs under benign conditions. The involvement of abscisic acid (ABA) was implicated from additional experiments in which interplant drought cuing was greatly diminished in ABA-deficient plants. Here, we tested the hypothesis that ABA is the exogenous vector of interplant drought cuing. Pisum sativum plants were grown in rows of three split-root plants. One of the roots of the first plant was subjected to either drought of benign conditions in one rooting vial, while its other root shared its rooting vial with one of the roots of an unstressed neighbor, which in turn shared its other rooting vial with an additional unstressed neighbor. One hour after subjecting one of the roots of the first plant to drought, ABA concentrations were 106% and 145% higher around its other root and the roots of its unstressed neighbor, compared to their respective unstressed controls; however, the absolute concentrations of ABA found in the rooting media were substantially low. The results may indicate that despite its involvement in interplant drought and the commonly observed exchange of ABA between drought-stressed plants and their rhizospheres, ABA is not directly involved in exogenous interplant drought cuing. However, previous studies have shown that even minute concentrations of ABA in the rhizosphere can prevent ABA leakage from roots and thus to significantly increase endogenous ABA levels. In addition, under drought conditions, plants tend to accumulate ABA, which could markedly increase internal ABA concentrations over time and ABA concentrations in close proximity to the root surface might be significantly greater than estimated from entire rooting volumes. Finally, phaseic acid, an ABA degradation product, is known to activate various ABA receptors, which could enhance plant drought tolerance. It is thus feasible that while the role of ABA is limited, its more stable degradation products could play a significant role in interplant drought cuing. Our preliminary findings call for an extensive investigation into the identity and modes of operation of the exogenous vectors of interplant drought cuing

Ecophysiology of root systems-environment interaction

There is a scarcity of detailed information regarding the ecophysiology of root systems and the way root system functioning is affected by both internal and external factors. Furthermore, global climate change is expected to increase the intensity of climate extremes, such as severe drought, heat waves and periods of heavy rainfall; in addition other stresses such as salinization of soils are increasing world-wide. Recently an increasing awareness has developed that understanding plant traits will play a major role in breeding of future crop plants. For example, there is increasing evidence that the traits of root systems are defined by the properties of individual roots. However, further knowledge on the functional importance of root segments and the molecular/physiological mechanisms underlying root system functioning and persistence is needed, and would specifically allow modifying (crop) root system functionality and efficiency in the future. Another major gap in knowledge is localized at the root-soil interface and in regard to the potential adaptive plasticity of root-rhizosphere interactions under abiotic stress and/or competition. It is currently unknown whether adaptations in microbe communities occur, for example due to modified exudation rates, and what are the subsequent influences on nutrient mobilization and uptake. Furthermore, uncovering the mechanisms by which roots perceive neighboring roots may not only contribute to our understanding of plant developmental strategies, but also has important implications on the study of competitive interactions in natural communities, and in optimizing plant performance and resource use in agricultural and silvicultural systems. In this Research Topic, we aimed to provide an on-line, open-access snapshot of the current state of the art of the field of root ecology and physiology, with special focus on the translation of root structure to function, and how root systems are influenced by interplay with internal and external factors such as abiotic stress, microbes and plant-plant interaction. We welcomed original research papers, but reviews of specific topics, articles formulating opinions or describing cutting-edge methods were also gladly accepted

Testing the validity of GC-MS quantification of mannitol.

<p>Stomatal width of plants, the roots of which have been subjected to 0 (water controls) or 3.02×10⁻⁷ M mannitol, showing that even 100-fold the minimal mannitol concentration detectable by GC-MS analyses did not evoke stomatal closure in the experimental plants.</p

Stomatal responses to stress and communicated stress cues.

<p>Stomatal width of induced plants (pointed at by black arrows) and their SHARED (T1–T5) and UNSHARED (C1–C5) neighbours immediately before (0 min; A), 15 (B) and 60 (C) minutes after one of the roots of the IND plant, was injected with either water (blue) or mannitol (red). Data represent means ±1 s.e.m.; N = 6. ***: p<0.001; **: p<0.01; *: p<0.05; +: 0.05</p

Testing for stress cuing - the experimental setup.

<p>Circles represent rooting receptacles and connector lines represent split-root plants. Plants neighbouring the externally-induced plant (IND) either shared (SHARED; T1–T5) or did not share (UNSHARED; C1–C5) their rooting volumes with their immediate neighbours. External induction was carried out by injecting either water (control) or mannitol (osmotic stress) to the red rooting receptacle. Stomatal width was destructively measured in different experimental sets immediately before (0 min), and 15 and 60 minutes after the red receptacle was injected with either water or mannitol.</p