Dicot-specific ATG8-interacting ATI3 proteins interact with conserved UBAC2 proteins and play critical roles in plant stress responses

Selective macroautophagy/autophagy targets specific cargo by autophagy receptors through interaction with ATG8 (autophagy-related protein 8)/MAP1LC3 (microtubule associated protein 1 light chain 3) for degradation in the vacuole. Here, we report the identification and characterization of 3 related ATG8-interacting proteins (AT1G17780/ATI3A, AT2G16575/ATI3B and AT1G73130/ATI3C) from Arabidopsis. ATI3 proteins contain a WxxL LC3-interacting region (LIR) motif at the C terminus required for interaction with ATG8. ATI3 homologs are found only in dicots but not in other organisms including monocots. Disruption of ATI3A does not alter plant growth or development but compromises both plant heat tolerance and resistance to the necrotrophic fungal pathogen Botrytis cinerea. The critical role of ATI3A in plant stress tolerance and disease resistance is dependent on its interaction with ATG8. Disruption of ATI3B and ATI3C also significantly compromises plant heat tolerance. ATI3A interacts with AT3G56740/UBAC2A and AT2G41160/UBAC2B (Ubiquitin-associated [UBA] protein 2a/b), 2 conserved proteins implicated in endoplasmic reticulum (ER)-associated degradation. Disruption of UBAC2A and UBAC2B also compromised heat tolerance and resistance to B. cinerea. Overexpression of UBAC2 induces formation of ATG8- and ATI3-labeled punctate structures under normal conditions, likely reflecting increased formation of phagophores or autophagosomes. The ati3 and ubac2 mutants are significantly compromised in sensitivity to tunicamycin, an ER stress-inducing agent, but are fully competent in autophagy-dependent ER degradation under conditions of ER stress when using an ER lumenal marker for detection. We propose that ATI3 and UBAC2 play an important role in plant stress responses by mediating selective autophagy of specific unknown ER components.