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.

Selective autophagy limits cauliflower mosaic virus infection by NBR1-mediated targeting of viral capsid protein and particles

“…(24). Recent reports suggested that plant NBR1 binds to polyubiquitinated substrates similar to the binding in mammalian counterparts, although the identity of individual plant proteins targeted by NBR1 for autophagic destruction has yet to be determined (22,24). We found direct ubiquitin-independent interaction of NBR1 with CaMV P4.…”

“…We next monitored the expression of the autophagy core genes ATG8a and ATG8e and the autophagy receptor NBR1, which are all elevated as part of an induced autophagy response during plant heat stress (22). Both CaMV isolates stimulated the accumulation of the autophagic markers' transcripts and proteins relative to the noninfected control ( Fig.…”

“…WT plants were A. thaliana ecotype Columbia (Col-0). Mutants atg5-1, atg7-2, nbr1-2, npr1-1, atg5-1 npr1-1, and the GFP-ATG8a transgenic line have been described previously (19,22,38,39). For infection experiments, Arabidopsis plants were grown in soil in a growth cabinet under short-day conditions (8/16-h light/dark cycles), and for transient expression assays N. benthamiana plants were cultivated in a growth room under long-day conditions (16/8-h light/dark cycles) at 150 μE·m −2 ·s −1 , 21°C, and 70% relative humidity.…”

Enhanced Oxidative Stress Resistance through Activation of a Zinc Deficiency Transcription Factor inBrachypodium distachyon

“…5). Consistent with the role of ATG8 during abiotic stress (Xiong et al, 2007) as well as its transcriptional induction under heat stress (Zhou et al, 2013), paraquat treatment in BdbZIP10o/e plants significantly increased BdATG8 levels above that of wild-type treated plants (Fig. 5).…”

Disrupting Autophagy Restores Peroxisome Function to anArabidopsis lon2Mutant and Reveals a Role for the LON2 Protease in Peroxisomal Matrix Protein Degradation

“…NBR1 functions as a pexophagy receptor in mammals (Deosaran et al, 2013). It will be interesting to learn whether Arabidopsis NBR1 (Svenning et al, 2011;ZientaraRytter et al, 2011), which functions in autophagy of insoluble ubiquitinated aggregates (Zhou et al, 2013), similarly targets peroxisomes for pexophagy in Arabidopsis and whether any ubiquitinated proteins in the peroxisome membrane are required for pexophagy. It is noteworthy that several peroxisomal proteins are found in a recent proteomic analysis of ubiquitinated Arabidopsis proteins (Kim et al, 2013).…”