AtBAG4-mediated stress tolerance requires autophagy and interaction with the selective autophagy substrate receptor NBR1 and chaperone Hsp70

N. THANTHRIGE1, S.D BHOWMIK1, B. J. FERGUSON2, M. KABBAGE3, S. MUNDREE1, B. WILLIAMS1

1 Centre for Agriculture and the Bioeconomy, Queensland University of Technology, Queensland, Australia, 2 School of Agriculture and Food Sciences, University of Queensland, Queensland, Australia,3 Department of Plant Pathology, University of Wisconsin-Madison, Wisconsin, USA.

The Bcl-2 associated athanogene (BAG) family is an evolutionarily conserved, multifunctional group of co-chaperones that participate in stress responses, proliferation, migration, and cell death. Arabidopsis encodes seven BAG members (AtBAG1-7), the functions of many remain largely unelucidated. AtBAG4 is a proven candidate for improving plant stress tolerance. Although crops expressing AtBAG4 are more stress tolerant, the molecular mechanisms driving this tolerance are unknown. Using protein microarrays and yeast two hybrid analyses we show that AtBAG4 interacts with the autophagy substrate Neighbour of BRCA1 (NBR1). Furthermore, we used genetic and mutational studies to unequivocally demonstrate that AtBAG4-mediated stress tolerance requires autophagy. Arabidopsis plants overexpressing AtBAG4 in autophagy defective genetic backgrounds (atg4, atg8) are hypersensitive to salinity and nutritional deprivation. Conversely, autophagy does not require AtBAG4, and using electron microscopy we observed similar numbers of autophagosomes in atbag4 mutants and wild type plants. Together, we showed that autophagy plays a central role in AtBAG4-mediated stress tolerance. The linkage between AtBAG4, stress tolerance and autophagy pathways provide more evidence for the central role of the plant BAG family and autophagy in plant stress response regulation and the execution of programmed cell death pathways.