NBR1-mediated selective autophagy targets insoluble ubiquitinated protein aggregates in plant stress responses

Abstract

Plant autophagy plays an important role in delaying senescence, nutrient recycling, and stress responses. Functional analysis of plant autophagy has almost exclusively focused on the proteins required for the core process of autophagosome assembly, but little is known about the proteins involved in other important processes of autophagy, including autophagy cargo recognition and sequestration. In this study, we report functional genetic analysis of Arabidopsis NBR1, a homolog of mammalian autophagy cargo adaptors P62 and NBR1. We isolated two nbr1 knockout mutants and discovered that they displayed some but not all of the phenotypes of autophagy-deficient atg5 and atg7 mutants. Like ATG5 and ATG7, NBR1 is important for plant tolerance to heat, oxidative, salt, and drought stresses. The role of NBR1 in plant tolerance to these abiotic stresses is dependent on its interaction with ATG8. Unlike ATG5 and ATG7, however, NBR1 is dispensable in age- and darkness-induced senescence and in resistance to a necrotrophic pathogen. A selective role of NBR1 in plant responses to specific abiotic stresses suggest that plant autophagy in diverse biological processes operates through multiple cargo recognition and delivery systems. The compromised heat tolerance of atg5, atg7, and nbr1 mutants was associated with increased accumulation of insoluble, detergent-resistant proteins that were highly ubiquitinated under heat stress. NBR1, which contains an ubiquitin-binding domain, also accumulated to high levels with an increasing enrichment in the insoluble protein fraction in the autophagy-deficient mutants under heat stress. These results suggest that NBR1-mediated autophagy targets ubiquitinated protein aggregates most likely derived from denatured or otherwise damaged nonnative proteins generated under stress conditions.

Figure 1. BiFC analysis of NBR1 interaction with ATG8a in planta.

Fluorescence was observed in the transformed N. benthamiana leaf epidermal cells, which results from complementation of the N-terminal part of the YFP fused with ATG8a (ATG8a-N-YFP) by the C-terminal part of the YFP fused with NBR1 (NBR1-C-YFP). No fluorescence was observed when ATG8a-N-YFP was coexpressed with unfused C-YFP or with mNBR1-C-YFP or when unfused N-YFP was coexpressed with NBR1-C-YFP. YFP epifluorescence images, bright-field images and overlay images of the same cells are shown.

Figure 2. Induction of autophagy genes by heat stress.

Five weeks-old Arabidopsis wild-type Col-0 plants were placed in the 22°C and 45°C growth chambers and total RNA was isolated from leaf samples collected at indicated times. Transcript levels were determined using real-time qRT-PCR. Error bars indicate SE (n = 3).

Figure 3. Determination of accumulation of autophagosomes using GFP-ATG8a.

(A) Four-weeks old transgenic wild-type Col-0 (WT) and atg7-2 mutant plants expressing GFP-ATG8a were treated with (45°C) or without (22°C) heat shock for 3 h and then placed at room temperature for 0.5 h. The leaves were visualized by fluorescence confocal microscopy of GFP signal. (B) Numbers of punctate GFP-ATG8a spots representing autophagosomes per 10,000 µm² section. Means and SE were calculated from three experiments. According to Duncan's multiple range test (P = 0.05), means do not differ significantly if they are indicated with the same letter. Bar = 20 µm.

Figure 4. Increased formation of punctate structures containing GFP-NBR1 under heat stress.

(Top panel) Four-weeks old transgenic wild-type Col-0 (WT), atg7-2 and nbr1-1 mutant plants expressing GFP-NBR1 were treated with (45°C) or without (22°C) heat shock for 3 h and then placed at room temperature for 0.5 h. The leaves were visualized by fluorescence confocal microscopy of GFP signal. (Bottom panel) Numbers of punctate GFP-NBR1 spots per 10,000 µm² section. Means and SE were calculated from three experiments. According to Duncan's multiple range test (P = 0.05), means do not differ significantly if they are indicated with the same letter. Bar = 20 µm.

Figure 5. Enhanced sensitivity of atg5, atg7 and nbr1 mutants to heat and oxidative stresses.

(A) Five weeks-old Arabidopsis Col-0 wild type (WT), atg5, atg7 and nbr1 mutant plants were placed in 22°C and 45°C growth for 10 hours and then moved to room temperature for 3-day recovery or (B) sprayed with 20 µM methyl viologen (MV) and kept under light for two days before the picture of representative plants was taken. The experiments were repeated three times with similar results.

Figure 6. Enhanced sensitivity of atg5, atg7, and nbr1 mutants to heat stress.

(A) Electrolyte leakage. (B) Fv/Fm images. (C) Average values for the Fv/Fm images. Five weeks-old Arabidopsis Col-0 wild type (WT), atg5, atg7 and nbr1 mutant plants were placed in 22°C and 45°C growth chambers. Fully expanded leaves were sampled after 10 hours in the indicated temperatures and immediately measured for electrolyte leakage and Fv/Fm. Error bars in (A) and (C) indicate SE (n = 5). According to Duncan's multiple range test (P = 0.05), means of EL or Fv/Fm do not differ significantly if they are indicated with the same letter. The experiments were repeated twice with similar results.

Figure 7. Enhanced sensitivity of atg5, atg7, and nbr1 mutants to drought and salt stresses.

(A) Five weeks-old Arabidopsis Col-0 wild type (WT), atg5, atg7 and nbr1 mutant plants were placed into a growth chamber with approximately 50% humidity. The photograph of representative plants was taken 10 days after withholding watering. The experiment was repeated twice with similar results. (B) Seven days-old seedlings Col-0 WT, atg5, atg7 and nbr1 grown on solid MS medium were transferred to the same medium (control) or the same medium containing 0.16 M NaCl and photographed 5 days later. The survived seedlings were scored 5 days after the transfer and the average values and SE were calculated from three experiments. According to Duncan's multiple range test (P = 0.05), means do not differ significantly if they are indicated with the same letter.

Figure 8. Normal phenotypes of the nbr1 mutant plants in age- and darkness-induced senescence.

(A) Eight weeks old wild type (WT), atg5, atg7 and nbr1 plants grown under normal growth conditions in a growth chamber. (B) Four weeks old WT, atg5, atg7 and nbr1 plants after being kept in the dark for 6 days. (C) Percentage of plants that survived the various lengths in dark as determined by resumption of growth. Each point represents the average of 10 plants. The experiments were repeated twice with similar results.

Figure 9. Normal phenotypes of the nbr1 mutant plants in resistance to Botrytis.

(A) Responses of Col-0 wild type (WT), atg5, atg7 and nbr1 plants to Botrytis. Wild-type and mutant plants were inoculated by spraying with spore suspension at a density of 2.5×10⁵ spores ml⁻¹, and kept at high humidity. Pictures of representative plants were taken at 4 and 5 days post inoculation (dpi). (B) Quantitative real-time PCR analysis of the B. cinerea ActA (BcActA) transcript levels in infected Arabidopsis plants at 5 dpi. The experiments were repeated twice with similar results.

Figure 10. Requirement of the LIR motif for the critical role of NBR1 in heat tolerance.

(A) Diagrams of wild-type and mutant NBR1 proteins. The conserved W and I residues in the LIR motif between the two UBA domains were changed to A residues in the NBR1W661A/I664A (mNBR1) mutant protein. (B) Five weeks-old Arabidopsis Col-0 wild type (WT), nbr1 and transgenic nbr1 plants expressing the wild-type NBR1 (lines 3 and 6; see Figure S2) or the NBR1W661A/I664A (mNBR1) (lines 2 and 6; see Figure S2) mutant gene were placed in a 45°C growth chambers for 10 hours and then moved to room temperature for 3-day recovery before the picture as taken. The experiment was repeated three times with similar results.

Figure 11. Accumulation of NBR1 and its increased enrichment in the insoluble protein fraction in the atg7 mutant under heat stress.

Leaf tissues from wild type (WT) and atg7 expressing NBR1-TAP were collected at indicated hours (h) under 45°C and prepared for soluble and insoluble proteins as described in Materials and Methods. Proteins from the first supernatants (S) and last pellets (P) were subjected to SDS-PAGES and probed with a peroxidase-conjugated anti-peroxidase antibody for detection of NBR1-TAP. Protein samples prepared from non-transgenic Col-0 wild-type plants were included as controls.

Figure 12. Increased accumulation of insoluble protein aggregates in the atg7 and nbr1 mutants under heat stress.

(A) Accumulation of insoluble proteins. Leaf tissues from wild-type (WT), atg7 and nbr1 mutants collected at indicated hours (h) under 45°C for preparation of total, soluble and insoluble proteins as described in Materials and Methods. Total proteins in the starting homogenates and insoluble proteins in the last pellets were determined the percentages of insoluble proteins to total proteins were calculated. (B) Profiles of soluble and insoluble proteins. Proteins from the first supernatants (S) and last pellets (P) were subjected to SDS-PAGES and stained with Coomassie brilliant blue. Major proteins accumulated in the pellets from heat-stressed atg7 mutant plants were indicated by arrows.

Figure 13. Ubiquitination of insoluble protein aggregates in the atg7 and nbr1 mutants under heat stress.

Leaf tissues from wild type (WT), atg7 and nbr1 mutants were collected at indicated hours (h) under 45°C and prepared for soluble and insoluble proteins as described in Materials and Methods. Proteins from the first supernatants (S) and last pellets (P) were subjected to SDS-PAGES and probed with anti-ubiquitin monoclonal antibody. The experiment was repeated three times with similar results.