Towards a grammar of plant stress: modular signalling conveys meaning

Abstract Stress resilience is central for plant survival. The appropriate adaptive response not only depends on the type of stress, but also on the context with other stresses, the developmental state of the plant, and the history of preceding stress experiences. The response to stress combinations cannot be a mere addition of the responses to the individual factors. For instance, heat stress requires stomatal opening to cool the leaf by increased transpiration, while drought stress needs stomatal closure to reduce water loss by transpiration. However, heat and drought are often coming in concert, such that the plant needs to reach a prioritised decision. Thus, the response to stress combinations constitutes a new quality transcending the addition of individual stress components. In other words: to survive under combined stress, plants need to render real decisions. We propose a model, where different stress inputs share one or more transducing elements, that can be recruited for different downstream pathways. Competition for these shared elements allows for such qualitative decisions, depending on the relative activities in upstream signalling of the individual stress components. Using different types of osmotic stress as paradigm I demonstrates, how signal modularity and differences in temporal sequence can generate qualitatively different outputs. Thus, plant-stress signalling makes use of a limited set of molecular players to generate, by specific rules for their combination and sequence, different “meanings”. This can be compared to human language, where information-bearing elements (words) are combined according to grammatical rules to generate a semantic space. (249 words)