Root cap-mediated evaluation of soil resistance towards graviresponding roots of maize (Zea mays L.) and the relevance of ethylene.

Background and Aims: Besides biological and chemical impacts, mechanical resistance represents an important obstacle that growing roots face. Graviresponding roots must assess the mechanical resistance of the substrate and take decisions on whether they change growth direction and grow around obstacles or tolerate growth conditions impaired to varying degrees. To test the significance of the root cap, we measured pressure and growth behaviour of single intact, as well as decapped, roots encountering diverse mechanical obstacles. We examined ethylene emission in intact roots as well as roots without a root cap, thereby lacking the capacity to deviate. Methods: Roots of fixed seedlings were grown vertically onto diverse mechanical obstacles. Developing pressure profiles of vertically growing roots encountering horizontal mechanical obstacles were measured employing electronic milligram scales, with and without root caps in given local environmental conditions. The evolution of root-borne ethylene was measured in intact roots and roots without the root cap. Key Results: In contrast to decapped roots, intact roots develop a tentative, short-lasting pressure profile, the resolution of which is characterized by a definite change of growth direction. Similarly, pressure profiles and strengths of roots facing gradually differing surface resistances differ significantly between the two. This correlates in the short term with root cap-dependent ethylene emission which is lacking in roots without caps. Conclusions: The way gravistimulated and graviresponding roots cope with exogenous stimuli depends on whether and how they adapt to these impacts. With respect to mechanical hindrances, roots without caps do not seem to be able to evaluate soil strengths in order to respond adequately. On encountering resistance, roots with intact caps emit ethylene, which is not observed in decapped roots. It therefore appears that it is the root cap which specifically orchestrates the resistance needed to overcome mechanical resistance by specifically inducing ethylene.