A proposed role for endomembrane trafficking processes in regulating tonoplast content and vacuole dynamics under ammonium stress conditions in Arabidopsis root cells

Ammonium (NH4+) stress has multiple effects on plant physiology, therefore, plant responses are complex, and multiple mechanisms are involved in NH4+ sensitivity and tolerance in plants. Root growth inhibition is an important quantitative readout of the effects of NH4+ stress on plant physiology, and cell elongation appear as the principal growth inhibition target. We recently proposed autophagy as a relevant physiological mechanisms underlying NH4+ sensitivity response in Arabidopsis. In a brief overview, the impaired macro-autophagic flux observed under NH4+ stress conditions has a detrimental impact on the cellular energetic balance, and therefore on the energy-demanding plant growth. In contrast to its inhibitory effect on the autophagosomes flux to vacuole, NH4+ toxicity induced a micro-autophagy-like process. Consistent with the reduced membrane flux to the vacuole related to macro-autophagy inhibition and the increased tonoplast degradation due to enhanced micro-autophagy, the vacuoles of the root cells of the NH4+-stressed plants showed lower tonoplast content and a decreased perimeter/area ratio. As the endosome-to-vacuole trafficking is another important process that contributes to membrane flux toward the vacuole, we evaluated the effects of NH4+ stress on this process. This allows us to propose that autophagy could contribute to vacuole development as well as possible avenues to follow for future studies.Instituto de Fisiología y Recursos Genéticos VegetalesFil: Robert, German. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Fisiología y Recursos Genéticos Vegetales; ArgentinaFil: Robert, German. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Estudios Agropecuarios (UDEA); ArgentinaFil: Robert, German. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Cátedra de Fisiología Vegetal; ArgentinaFil: Yagyu, Mako. Meiji University. School of Agriculture. Department of Life Sciences; JapónFil: Lascano, Hernán Ramiro. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Cátedra de Fisiología Vegetal; ArgentinaFil: Lascano, Hernán Ramiro. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Estudios Agropecuarios (UDEA); ArgentinaFil: Lascano, Hernán Ramiro. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Fisiología y Recursos Genéticos Vegetales. ArgentinaFil: Masclaux-Daubresse, Céline. Université Paris-Saclay. AgroParisTech. Institut Jean-Pierre Bourgin. FranciaFil: Yoshimoto, Kohki. Meiji University. School of Agriculture. Department of Life Sciences; Japó

Ammonium stress increases microautophagic activity while impairing macroautophagic flux in Arabidopsis roots

Plant responses to NH4 + stress are complex, and multiple mechanisms underlying NH4 + sensitivity and tolerance in plants may be involved. Here, we demonstrate that macro‐ and microautophagic activities are oppositely affected in plants grown under NH4 + toxicity conditions. When grown under NH4 + stress conditions, macroautophagic activity was impaired in roots. Root cells accumulated autophagosomes in the cytoplasm, but showed less autophagic flux, indicating that late steps of the macroautophagy process are affected under NH4 + stress conditions. Under this scenario, we also found that the CCZ1‐MON1 complex, a critical factor for vacuole delivery pathways, functions in the late step of the macroautophagic pathway in Arabidopsis. In contrast, an accumulation of tonoplast‐derived vesicles was observed in vacuolar lumens of root cells of NH4 +‐stressed plants, suggesting the induction of a microautophagy‐like process. In this sense, some SYP22‐, but mainly VAMP711‐positive vesicles were observed inside vacuole in roots of NH4 +‐stressed plants. Consistent with the increased tonoplast degradation and the reduced membrane flow to the vacuole due to the impaired macroautophagic flux, the vacuoles of root cells of NH4 +‐stressed plants showed a simplified structure and lower tonoplast content. Taken together, this study presents evidence that postulates late steps of the macroautophagic process as a relevant physiological mechanism underlying the NH4 + sensitivity response in Arabidopsis, and additionally provides insights into the molecular tools for studying microautophagy in plants.Instituto de Fisiología y Recursos Genéticos VegetalesFil: Robert, German. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Fisiología y Recursos Genéticos Vegetales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). Unidad de Estudios Agropecuarios (UDEA); Argentina. Université Paris‐Saclay. Institut Jean‐Pierre Bourgin, INRAE, AgroParisTech; FranceFil: Yagyu, Mako. Meiji University. School of Agriculture. Department of Life Sciences; JapónFil: Koizumi, Takaya. Meiji University. School of Agriculture. Department of Life Sciences; JapónFil: Naya, Loreto. Université Paris‐Saclay. Institut Jean‐Pierre Bourgin, INRAE, AgroParisTech; FranciaFil: Masclaux‐Daubresse, Céline. Université Paris‐Saclay. Institut Jean‐Pierre Bourgin, INRAE, AgroParisTech; FranciaFil: Yoshimoto, Kohki. Université Paris‐Saclay. Institut Jean‐Pierre Bourgin, INRAE, AgroParisTech; Francia. Meiji University. School of Agriculture. Department of Life Sciences; Japó