Intracellular phosphate recycling systems for survival during phosphate starvation in plants

Phosphorus (P) is an essential nutrient for plant growth and plants use inorganic phosphate (Pi) as their P source, but its bioavailable form, orthophosphate, is often limited in soils. Hence, plants have several mechanisms for adaptation to Pi starvation. One of the most common response strategies is “Pi recycling” in which catabolic enzymes degrade intracellular constituents, such as phosphoesters, nucleic acids and glycerophospholipids to salvage Pi. Recently, several other intracellular degradation systems have been discovered that salvage Pi from organelles. Also, one of sphingolipids has recently been identified as a degradation target for Pi recycling. So, in this mini-review we summarize the current state of knowledge, including research findings, about the targets and degradation processes for Pi recycling under Pi starvation, in order to further our knowledge of the whole mechanism of Pi recycling

Pexophagy suppresses ROS-induced damage in leaf cells under high-intensity light

Although light is essential for photosynthesis, it has the potential to elevate intracellular levels of reactive oxygen species (ROS). Since high ROS levels are cytotoxic, plants must alleviate such damage. However, the cellular mechanism underlying ROS-induced leaf damage alleviation in peroxisomes was not fully explored. Here, we show that autophagy plays a pivotal role in the selective removal of ROS-generating peroxisomes, which protects plants from oxidative damage during photosynthesis. We present evidence that autophagy-deficient mutants show light intensity-dependent leaf damage and excess aggregation of ROS-accumulating peroxisomes. The peroxisome aggregates are specifically engulfed by pre-autophagosomal structures and vacuolar membranes in both leaf cells and isolated vacuoles, but they are not degraded in mutants. ATG18a-GFP and GFP-2×FYVE, which bind to phosphatidylinositol 3-phosphate, preferentially target the peroxisomal membranes and pre-autophagosomal structures near peroxisomes in ROS-accumulating cells under high-intensity light. Our findings provide deeper insights into the plant stress response caused by light irradiation

The role of reticulophagy under early phase phosphate starvation in plant cells

Inorganic phosphate (Pi) is one of the most important nutrients for plant growth. Under Pi starvation, intracellular components are degraded for Pi recycling in plant cells. Macroautophagy/autophagy is a process for vacuolar degradation of cytoplasmic components including organelles, but it is still unclear whether this process is involved in plant growth and Pi recycling during Pi starvation. Recently, we reported that the degradation of endoplasmic reticulum (ER) by selective autophagy, termed reticulophagy, contributes to Pi recycling and is an important stress response in the early phase of Pi starvation. During this phase, oxidized lipids are accumulated in the plant cell in an iron ion-dependent manner, and this accumulation causes ER stress which induces reticulophagy. As a result, the Pi contents are maintained at a sufficient level during early Pi starvation, suppressing any late Pi starvation responses, such as membrane lipid remodeling. Thus, we proposed that ER stress-induced reticulophagy is an important Pi salvage system during the early phase of Pi starvation in plants

Autophagy as a possible mechanism for micronutrient remobilization from leaves to seeds.

Seed formation is an important step of plant development which depends on nutrient allocation. Uptake from soil is an obvious source of nutrients which mainly occurs during vegetative stage. Because seed filling and leaf senescence are synchronized, subsequent mobilization of nutrients from vegetative organs also play an essential role in nutrient use efficiency, providing source-sink relationships. However, nutrient accumulation during the formation of seeds may be limited by their availability in source tissues. While several mechanisms contributing to make leaf macronutrients available were already described, little is known regarding micronutrients such as metals. Autophagy, which is involved in nutrient recycling, was already shown to play a critical role in nitrogen remobilization to seeds during leaf senescence. Because it is a non-specific mechanism, it could also control remobilization of metals. This article reviews actors and processes involved in metal remobilization with emphasis on autophagy and methodology to study metal fluxes inside the plant. A better understanding of metal remobilization is needed to improve metal use efficiency in the context of biofortification

An Arabidopsis Homolog of Yeast ATG6/VPS30 Is Essential for Pollen Germination

Yeast (Saccharomyces cerevisiae) Atg6/Vps30 is required for autophagy and the sorting of vacuolar hydrolases, such as carboxypeptidase Y. In higher eukaryotes, however, roles for ATG6/VPS30 homologs in vesicle sorting have remained obscure. Here, we show that AtATG6, an Arabidopsis (Arabidopsis thaliana) homolog of yeast ATG6/VPS30, restored both autophagy and vacuolar sorting of carboxypeptidase Y in a yeast atg6/vps30 mutant. In Arabidopsis cells, green fluorescent protein-AtAtg6 protein localized to punctate structures and colocalized with AtAtg8, a marker protein of the preautophagosomal structure. Disruption of AtATG6 by T-DNA insertion resulted in male sterility that was confirmed by reciprocal crossing experiments. Microscopic analyses of AtATG6 heterozygous plants (AtATG6/atatg6) crossed with the quartet mutant revealed that AtATG6-deficient pollen developed normally, but did not germinate. Because other atatg mutants are fertile, AtAtg6 likely mediates pollen germination in a manner independent of autophagy. We propose that Arabidopsis Atg6/Vps30 functions not only in autophagy, but also plays a pivotal role in pollen germination

Thaumatin-like proteins and a cysteine protease inhibitor secreted by the pine wood nematode Bursaphelenchus xylophilus induce cell death in Nicotiana benthamiana.

Pine wilt disease (PWD) is an infectious disease of pines that typically kills affected trees. The causal pathogen of PWD is the pine wood nematode (PWN), Bursaphelenchus xylophilus. Understanding of the disease has advanced in recent years through the use of a highly sensitive proteomics procedure and whole genome sequence analysis; in combination, these approaches have enabled identification of proteins secreted by PWNs. However, the roles of these proteins during the onset of parasitism have not yet been elucidated. In this study, we used a leaf-disk assay based on transient overexpression in Nicotiana benthamiana to allow functional screening of 10 candidate pathogenic proteins secreted by PWNs. These proteins were selected based on previous secretome and RNA-seq analyses. We found that five molecules induced significant cell death in tobacco plants relative to a GFP-only control. Three of these proteins (Bx-TH1, Bx-TH2, and Bx-CPI) may have a role in molecular mimicry and likely make important contributions to inducing hypersensitive responses in host plants

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