Ethylene assists auxin in the regulation of root cap renewal in Arabidopsis thaliana L.

Motivation and Aim: In plants, a root cap (RC) is an extraordinary organ, which continuously grows while maintaining a constant size. The tissue size homeostasis is due to a synchronization of columella stem cell proliferation and cell detachment at the periphery of RC, and it is essential for the root functioning. Auxin gradient in the RC is a key regulator, which couples the two processes. It is assumed that cell division occurs at an auxin maximum, and detachment -at a minimum However, the mechanisms of how auxin coordinates the two processes are unknown. Here we accomplish in vivo and in silico analysis of root cap self-renewal in Arabidopsis thaliana to get a new insight into the underlying mechanisms. Methods and Algorithms: For in silico analysis, we developed a hybrid 1D model of auxin-regulated cell dynamics in columella, which implements "division at a maximum -detachment at a minimum" concept Confocal and light microscopy techniques were used to estimate RC cell dynamics in vivo. Results: To formally describe RC cell dynamics observed both in planta and in a computer simulation, we used easily detectable structural parameters: the number of attached/detached/total RC cell layers. Comparing the results of in vivo experiments and mathematical modeling, we found that the model properly describes RC cell dynamics only under normal conditions but not under auxin excess, implying that "division at a maximum -detachment at a minimum" concept needs a refinement. A plant hormone ethylene is an essential regulator of root development, wherein the two hormones are known to reciprocally regulate each other's activity Using 1-aminocyclopropane-1carboxylic acid (ACC) treatment and a mutant assay, we demonstrated that alterations of both ethylene production and signaling affect root cap dynamics in planta. Next, utilizing mathematical modeling, we predicted that simultaneous fine-tuning of auxin effect on cell growth and division could provide for ethylene-related RC phenotypes. Finally, the combined results of in vivo and in silico analyses of auxin response in mutants deficient in ethylene signaling supported the role of ethylene in auxin-mediated regulation of RC cell dynamics. Conclusion: Taken together, our results demonstrate that ethylene assists auxin in the regulation of root cap renewal in Arabidopsis thaliana.