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Abstract
Autophagy is a mechanism to recycle intracellular constituents such as amino acids and other carbon- and nitrogen (N)-containing compounds. Although autophagy-related (ATG) genes required for autophagy are encoded by many algal genomes, their functional importance in microalgae in nutrient-deficiency has not been appraised using ATG-defective mutants. Recently, by characterization of an insertional mutant of the ATG8 encoding a ubiquitin-like protein indispensable for autophagosome formation in a green alga Chlamydomonas reinhardtii, we have provided evidence that supports the following notions. ATG8 protein is required for the degradation of lipid droplets and triacylglycerol (TAG) triggered by resupply of N to cell culture in N-deficient conditions. ATG8 protein is also necessary for starch accumulation under phosphorus-deficient conditions. Algal autophagy is not necessary for inheritance of chloroplast and mitochondrial genomes. In this review, we discuss the physiological roles of algal autophagy associated with nutrient deficiency revealed by the genetic and biochemical analyses using disruption mutants and reagents that inhibit the fatty acid biosynthesis and vacuolar H+-ATPase.
Highlights
Macroautophagy is a recycling system for degradation of cytoplasmic constituents in the vacuole or lysosomes (Mizushima et al, 2011; Liu and Bassham, 2012; Yang and Bassham, 2015)
Fan et al (2019) reported that basal autophagy is required for TAG biosynthesis by lipid turnover providing fatty acids from organellar membrane lipids in A. thaliana
Studies of plant autophagy have been limited to a few model species for which atg mutants are available, and the role of autophagy in the metabolism of carbon (C)-assimilation products throughout the plant kingdom needs to be further investigated
Summary
Macroautophagy (hereafter autophagy) is a recycling system for degradation of cytoplasmic constituents in the vacuole or lysosomes (Mizushima et al, 2011; Liu and Bassham, 2012; Yang and Bassham, 2015). Studies on atg mutants in rice which showed male sterility concluded that autophagy is necessary for TAG and starch accumulation and lipid droplet formation as well as normal reproductive postmeiotic anther development during pollen maturation (Kurusu et al, 2014). Because in algae the role of autophagy in the accumulation and metabolism of photosynthetic assimilation products remained unclear, studies using autophagy-deficient mutant strains have been considered necessary in addition to those using wild type cells treated with autophagy-inhibiting chemicals. The physiological functions of algal autophagy in response to nutrient deficiency will be discussed, based on recent reports of autophagy-defective mutants in Chlamydomonas reinhardtii (hereafter Chlamydomonas) and the effects of treatment of the wild-type Chlamydomonas cells with cerulenin and concanamycin A for inhibition of fatty acid synthesis and vacuolar lysosomal function, respectively
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