Analysis of the function and the regulation of autophagy in Arabidopsis thaliana
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Plants have developed sophisticated mechanisms to survive under adverse growth conditions. Autophagy is activated in response to multiple abiotic stresses, pathogen infection and senescence in plants. Upon induction of autophagy, portions of cytoplasm are engulfed by double membrane structures termed autophagosomes and are delivered to vacuole for degradation. Over the past decade, the identification of many autophagy-related genes in plants has greatly enhanced our molecular and physiological understanding of plant autophagy. However, the upstream regulatory components in the autophagy pathway and the function of autophagy under various stresses are still unclear. This thesis summarizes my efforts in studying the function and the regulatory pathways of autophagy in Arabidopsis thaliana.
Previous research indicates that autophagy is required for plant tolerance of nutrient and oxidative stress. To further explore the function of autophagy in other abiotic stresses, the induction of autophagy was investigated in response to high salinity, osmotic and ER stresses. Our results showed that autophagy is induced by high salinity and osmotic stresses in Arabidopsis. Autophagy-defective plants are more sensitive to salt and drought stresses than wild-type plants, indicating the participation of autophagy in the response to these stresses. The ER stress-inducers tunicamycin and dithiothreitol (DTT) both caused rapid activation of autophagy in Arabidopsis. Microscopy analysis suggests a role for autophagy in the transport of ER fragments to the vacuole for degradation during ER stress in plants. In addition, one of the ER stress sensors, IRE1b, was found to be required for ER stress-induced autophagy. Interestingly, autophagy was found to be regulated by different pathways during different abiotic stresses. In nutrient starvation and salt stress, autophagy is regulated by an NADPH oxidase-dependent pathway, whereas in osmotic and ER stresses, autophagy is regulated by an NADPH oxidase-independent pathway.
Target of rapamycin (TOR) kinase is the central controller of many cellular functions in eukaryotic cells in response to stress and nutrient deprivation. In yeast and animals, TOR is shown to negatively control the autophagy pathway, whereas little is known about its role in autophagy in plants. To obtain more knowledge on the molecular function of TOR in autophagy, RNA interference (RNAi)-TOR transgenic plants with decreased expression of TOR were generated. These plants showed constitutive autophagy even under favorable growth conditions, indicating that TOR is also a negative regulator of autophagy in Arabidopsis.
Together our results revealed that autophagy is activated in response to salt, osmotic and ER stresses, which suggests that autophagy is a general defense mechanism to protect plants during unfavorable environments. We also provide evidence that TOR negatively controls autophagy in plants, increasing our understanding of the regulatory mechanism of autophagy in plants.