Abstract
Root tropisms are important responses of plants, allowing them to adapt their growth direction. Research on plant tropisms is indispensable for future space programs that envisage plant-based life support systems for long-term missions and planet colonization. Root tropisms encompass responses toward or away from different environmental stimuli, with an underexplored level of mechanistic divergence. Research into signaling events that coordinate tropistic responses is complicated by the consistent coincidence of various environmental stimuli, often interacting via shared signaling mechanisms. On Earth the major determinant of root growth direction is the gravitational vector, acting through gravitropism and overruling most other tropistic responses to environmental stimuli. Critical advancements in the understanding of root tropisms have been achieved nullifying the gravitropic dominance with experiments performed in the microgravity environment. In this review, we summarize current knowledge on root tropisms to different environmental stimuli. We highlight that the term tropism must be used with care, because it can be easily confused with a change in root growth direction due to asymmetrical damage to the root, as can occur in apparent chemotropism, electrotropism, and magnetotropism. Clearly, the use of Arabidopsis thaliana as a model for tropism research contributed much to our understanding of the underlying regulatory processes and signaling events. However, pronounced differences in tropisms exist among species, and we argue that these should be further investigated to get a more comprehensive view of the signaling pathways and sensors. Finally, we point out that the Cholodny-Went theory of asymmetric auxin distribution remains to be the central and unifying tropistic mechanism after 100 years. Nevertheless, it becomes increasingly clear that the theory is not applicable to all root tropistic responses, and we propose further research to unravel commonalities and differences in the molecular and physiological processes orchestrating root tropisms.
Highlights
Plants are sessile organisms, their organs including roots are not motionless
Oxytropism, and phonotropism will have to determine if these growth responses are bona fide tropisms and what the underlying networks are that control these responses
Crucial to future empirical investigation into tropisms and their and regulation will be the development of experimental tools that enable the study of a single tropism stimulus in isolation, without confounding effects of other environmental gradients of any kind that may elicit a tropistic response
Summary
Plants are sessile organisms, their organs including roots are not motionless. Evidence of a distinct cell population in the part of the EZ more distal from the base of the root has been presented in the last three decades This region was dubbed distal elongation zone (DEZ) initially, and later transition zone (TZ), due to its unique characteristics (Ishikawa and Evans, 1993; Verbelen et al, 2006; Baluška et al, 2010). During the final decades of the twentieth century, the focus moved to studies on the molecular mechanisms of root tropisms, enabled by new techniques in molecular genetics and supported in the FIGURE 1 | Schematic representation of a longitudinal cross section of an Arabidopsis root apex, indicating the four distinct developmental zones: the meristematic zone (MZ; pink), the transition zone (TZ; purple), known as distal elongation zone (DEZ), the elongation zone (EZ; blue), and the differentiation zone (DZ; green). BL, blue light; RL, red light. *Specific localization in the cortex of the EZ. **Suspected localizations
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