SUMMARY Extensive use of chemical insecticides adversely affects both environment and human health. One of the most popular biological pest control alternatives is bioinsecticides based on Bacillus thuringiensis. This entomopathogenic bacterium produces different protein types which are toxic to several insect, mite, and nematode species. Currently, insecticidal proteins belonging to the Cry and Vip3 groups are widely used to control insect pests both in formulated sprays and in transgenic crops. However, the benefits of B. thuringiensis-based products are threatened by insect resistance evolution. Numerous studies have highlighted that mutations in genes coding for surrogate receptors are responsible for conferring resistance to B. thuringiensis. Nevertheless, other mechanisms may also contribute to the reduction of the effectiveness of B. thuringiensis-based products for managing insect pests and even to the acquisition of resistance. Here, we review the relevant literature reporting how invertebrates (mainly insects and Caenorhabditis elegans) respond to exposure to B. thuringiensis as either whole bacteria, spores, and/or its pesticidal proteins.
Fatty acids are synthesized in the stroma of plant and algal chloroplasts by the fatty acid synthase complex. Newly synthesized fatty acids are then used to generate plastidial lipids that are essential for chloroplast structure and function. Here, we show that inhibition of fatty acid synthesis in the model alga activates autophagy, a highly conserved catabolic process by which cells degrade intracellular material under adverse conditions to maintain cell homeostasis. Treatment of cells with cerulenin, a specific fatty acid synthase inhibitor, stimulated lipidation of the autophagosome protein ATG8 and enhanced autophagic flux. We found that inhibition of fatty acid synthesis decreased monogalactosyldiacylglycerol abundance, increased lutein content, down-regulated photosynthesis, and increased the production of reactive oxygen species. Electron microscopy revealed a high degree of thylakoid membrane stacking in cerulenin-treated cells. Moreover, global transcriptomic analysis of these cells showed an up-regulation of genes encoding chloroplast proteins involved in protein folding and oxidative stress and the induction of major catabolic processes, including autophagy and proteasome pathways. Thus, our results uncovered a link between lipid metabolism, chloroplast integrity, and autophagy through a mechanism that involves the activation of a chloroplast quality control system.
Autophagy is an intracellular self-degradation pathway by which eukaryotic cells recycle their own material in response to specific stress conditions. Exposure to high concentrations of metals causes cell damage, although the effect of metal stress on autophagy has not been explored in photosynthetic organisms. In this study, we investigated the effect of metal excess on autophagy in the model unicellular green alga Chlamydomonas reinhardtii. We show in cells treated with nickel an upregulation of ATG8 that is independent of CRR1, a global regulator of copper signaling in Chlamydomonas. A similar effect on ATG8 was observed with copper and cobalt but not with cadmium or mercury ions. Transcriptome sequencing data revealed an increase in the abundance of the protein degradation machinery, including that responsible for autophagy, and a substantial overlap of that increased abundance with the hydrogen peroxide response in cells treated with nickel ions. Thus, our results indicate that metal stress triggers autophagy in Chlamydomonas and suggest that excess nickel may cause oxidative damage, which in turn activates degradative pathways, including autophagy, to clear impaired components and recover cellular homeostasis. Eukaryotic cells are able to degrade and recycle their own material when they are exposed to nutrient starvation or other adverse conditions through a catabolic pathway known as macroautophagy or autophagy. This process is characterized by the formation of double-membrane vesicles termed autophagosomes that engulf and deliver cytosolic components to the vacuole/lysosome for degradation (1-4). The primary function of autophagy is to recycle cytoplasmic material as well as to clear damaged organelles or toxic cellular components generated during stress in order to maintain cellular homeostasis. In higher eukaryotes, autophagy has also been implicated in cell differentiation, development and cell death, and several human pathologies, such as cancer and neurodegenerative diseases (5, 6).Autophagy is mediated by highly conserved autophagy-related (ATG) genes, which have been described in organisms ranging from yeasts to mammals. Some ATG proteins are required for the formation of the autophagosome and constitute the core autophagy machinery (4,7,8). This group of proteins includes the ATG8 and ATG12 ubiquitin-like systems required for vesicle expansion. The ATG8 protein has been widely used to monitor autophagy in many systems (9) because, unlike other ATG proteins, this protein firmly binds to the autophagosome membrane through a covalent bond to phosphatidylethanolamine (PE). Most of the core ATG proteins are conserved in land plants (10-12) and in evolutionarily distant algae, including freshwater species, such as the model green alga Chlamydomonas reinhardtii (herein referred to as Chlamydomonas) (13) and marine species (14). Our current knowledge about autophagy in algae is still limited compared to our knowledge about autophagy in other eukaryotes, but recent studies, mainly performed in Chlamydomon...
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