Abstract
Autophagy is an intracellular process in which a portion of cytoplasm is transported into vacuoles for recycling. Physiological roles of autophagy in plants include recycling nutrients during senescence, sustaining life during starvation, and the formation of central digestive vacuoles. The regulation of autophagy and the formation of autophagosomes, spherical double membrane structures containing cytoplasm moving toward vacuoles, are poorly understood. HVA22 is a gene originally cloned from barley (Hordeum vulgare), which is highly induced by abscisic acid and environmental stress. Homologs of HVA22 include Yop1 in yeast, TB2/DP1 in human, and AtHVA22a to -e in Arabidopsis (Arabidopsis thaliana). Reverse genetics followed by a cell biology approach were employed to study the function of HVA22 homologs. The AtHVA22d RNA interference (RNAi) Arabidopsis plants produced small siliques with reduced seed yield. This phenotype cosegregated with the RNAi transgene. Causes of the reduced seed yield include short filaments, defective carpels, and dysfunctional pollen grains. Enhanced autophagy was observed in the filament cells. The number of autophagosomes in root tips of RNAi plants was also increased dramatically. The yop1 deletion mutant of Saccharomyces cerevisiae was used to verify our hypothesis that HVA22 homologs are suppressors of autophagy. Autophagy activity of this mutant during nitrogen starvation increased in 5 min and reached a plateau after 2 h, with about 80% of cells showing autophagy, while the wild-type cells exhibited low levels of autophagy following 8 h of nitrogen starvation. We conclude that HVA22 homologs function as suppressors of autophagy in both plants and yeast. Potential mechanisms of this suppression and the roles of abscisic acid-induced HVA22 expression in vegetative and reproductive tissues are discussed.
Highlights
Autophagy is an intracellular process in which a portion of cytoplasm is transported into vacuoles for recycling
AtHVA22d cDNA, including parts of its 5# and 3# untranslated regions, was used to make the RNA interference (RNAi) construct for Arabidopsis transformation (Fig. 1A)
It is possible that HVA22 homologs prevent preautophagosomal structure formation by interacting with an unknown protein(s) that contributes to negative regulation of autophagy
Summary
Autophagy is an intracellular process in which a portion of cytoplasm is transported into vacuoles for recycling. We conclude that HVA22 homologs function as suppressors of autophagy in both plants and yeast. Potential mechanisms of this suppression and the roles of abscisic acid-induced HVA22 expression in vegetative and reproductive tissues are discussed. More than 30 S. cerevisiae autophagy genes have been identified and are involved in different aspects of autophagy, such as regulation, autophagosome formation, and degradation of autophagic vesicles in vacuoles (Bassham et al, 2006). One gene involved in the regulation, encoding a Ser/Thr protein kinase called Tor, functions as an autophagy suppressor in S. cerevisiae and mammalian cells (Noda and Ohsumi, 1998; De Virgilio and Loewith, 2006; Inoki and Guan, 2006). The origin of the autophagosome membrane in S. cerevisiae is still under debate (Noda et al, 2002; Yorimitsu and Klionsky, 2005)
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