A functional link between ethylene biosynthesis and autophagy

<p>Ethylene is a gaseous plant hormone that regulates many plant growth and developmental processes,including  fruit  ripening,  leaf  senescence,  germination,  and  response  to  various  biotic and abiotic stimuli. To respond promptlyand adapt to developmental transitions and harsh stress conditions, the strict regulation of ethylene biosynthesis under given growth conditions is essential for the survival of plants. Accumulating studies suggested that 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS), a rate-limiting enzyme in ethylene biosynthesis, acts as a crosstalk point that transduces various internal and external stimuli to the cells via the interaction with other cellular  signaling  pathways.  Autophagy,  an  evolutionarily  conserved  catabolic  process  for cytoplasmic contents, is one such signaling pathway that ethylene likely interacts with to respond to many stimuli. However, despite the fact that both pathways are well investigated, the functional link between ethylene and autophagy is largely unknown. A recent study from our laboratory has demonstrated  that  ACS  plays  a  role  in  response  to  carbonstarvation  stress  by  modulating  the protein  abundance  of  regulatory  proteins  involved  in  autophagy. This  study revealed  aninsight into the connection between ethylene biosynthesis and autophagy, but the detailed mechanism that underlies the crosstalk isstill largely missing.</p> <p>In this study, we have investigated several aspects of functional links between ethylene and autophagy to fully understand the crosstalkbetween ethylene and autophagy. The two main areas of studies include autophagy-mediated regulation of ACS protein stability and investigation of the signaling role of ACC, a direct precursor of ethylene, in autophagy. The autophagy-mediated ACS stability regulation works showed that a subset of ACS isozymes are localized to autophagosomes,and the stability of ACS5 is regulated by autophagy. The second aspect of the works revealed that ACC  positively  regulates  autophagy  as  a  signaling  molecule,  whereas  ethylene  negatively influences  autophagy.  Together,  our  studies  have  revealed  that  ethylene  biosynthesis  and autophagy  are  tightly  associated  at  the  levels  of  post-translational  regulation  of  ACS  isozymes. Moreover,  we  revealed  the  novel  aspects  of  ACC  that  it  controls  autophagy  as  an  independent signaling molecule.</p>