Chemotropic vs Hydrotropic Stimuli for Root Growth Orientation in Microgravity

Abstract

Understanding how plants respond to spaceflight and extraterrestrial environments is crucial to develop life-support systems intended for long-term human explorations. Gravity is a main factor influencing root development and orientation, typically masking other tropisms. Considering that reduced levels of gravity affect many plant responses in space, the interaction of other tropic stimuli in microgravity represents the frontier to be investigated aiming at life-support systems optimization. In this paper we report on MULTITROP (Multiple-Tropism: interaction of gravity, nutrient and water stimuli for root orientation in microgravity), an experiment performed on the International Space Station during the Expedition 52/53. Scientific aim of the experiment was to disentangle hydrotropism from chemotropism for root orientation in absence of the gravity stimulus. Among several species relevant to space farming, Daucus carota was selected for the experiment because of its suitability with the experimental hardware and setup. At launch site, carrot seeds were placed between two disks of inert substrate (one imbibed with water and the other with a disodium phosphate solution) and integrated into a hardware developed, refurbished and flight-certificated by Kayser Italia. Post-flight, a Ground Reference Experiment was performed. Root development and orientation of seedlings grown in microgravity and at 1g condition were measured through 3D-image analysis procedures after imaging with X-ray microtomography. Radicle protruded preferentially from the ventral side of the seed due to the asymmetric position of the embryo. Such a phenomenon did not prevent the achievement of MULTITROP scientific goal but should be considered for further experiments on radicle growth orientation in microgravity. The experiment conducted in space verified that the primary root of carrot shows a positive chemotropism towards disodium phosphate solution in the absence of the gravity stimulus. On Earth, the positive chemotropism was masked by the dominant effect of gravity and roots developed downward regardless of the presence/absence of nutrients in the substrate. Taking advantage of altered gravity conditions and using other chemical compounds, further studies should be performed to deepen our understanding of root chemotropic response and its interaction with other tropisms.

Root Tropisms: Investigations on Earth and in Space to Unravel Plant Growth Direction

“…However, this study was criticized by Gilroy and Masson (2008) for not constituting a "robust chemotropic directional assay" as there was no repositioning of the stimulus. The positive chemotropic effect of disodium phosphate is supported by a recent experiment, where it was elicited in carrot seedlings (Daucus carota) onboard the ISS (Izzo et al, 2019). When confronted with both a hydrotropic and chemotropic stimulus under microgravity conditions, the roots grew preferentially into the substrate containing disodium phosphate.…”

“…When confronted with both a hydrotropic and chemotropic stimulus under microgravity conditions, the roots grew preferentially into the substrate containing disodium phosphate. On the ground, both stimuli were overruled by gravitropism (Izzo et al, 2019). To the best of our knowledge, there is no information available on the underlying mechanisms, or the possible involvement of signal molecules.…”

Settling for Less: Do Statoliths Modulate Gravity Perception?

“…A wide range of techniques and resources are currently available to researchers, including clinostats, sounding rockets, parabolic flights and even prolonged exposure to micro-g on the International Space Station. Here, valuable insights are already contributing to our understanding; for example, in parsing the hierarchy of tropic responses to different stimuli by comparing growth responses on the ISS with those on the ground (gravitropism trumps chemotropism and hydrotropism) [42]. It will be important now to explore the boundaries of these relationships; for example, as salt concentrations increase, which signaling pathways determine that a halotropic response should take precedence over a gravitropic one [43]?…”

Root Tropisms: New Insights Leading the Growth Direction of the Hidden Half

“…Indeed, the magnitude of each stimulus can determine the final root bending, and the response can be sometimes unpredictable, as in the case of the positive phototropism of Arabidopsis roots to blue light in microgravity [ 14 ]. The knowledge regarding phototropic responses of roots already contributes to the improvement of cultivation systems in controlled environments, such as those in space, but other stimuli have also shown potential for applications, as in the case of root chemotropism [ 15 ]. Still, the question remains whether tropisms other than gravitropism play a substantial role in determining root growth direction in a natural setting on Earth, where roots are subjected to several stimuli simultaneously.…”