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Arabidopsis cargo receptor NBR1 contributes to protein quality control by promoting the formation of protein aggregates and mediating their clearance via selective autophagy., Aggrephagy, a type of selective autophagy that sequesters protein aggregates for degradation in the vacuole, is an important protein quality control mechanism, particularly during cell stress. In mammalian cells, aggrephagy and several other forms of selective autophagy are mediated by dedicated cargo receptors such as NEIGHBOR OF BRCA1 (NBR1). Although plant NBR1 homologs have been linked to selective autophagy during biotic stress, it remains unclear how they impact selective autophagy under non-stressed and abiotic stress conditions. Through microscopic and biochemical analysis of nbr1 mutants expressing autophagy markers and an aggregation-prone reporter, we tested the connection between NBR1 and aggrephagy in Arabidopsis. Although NBR1 is not essential for general autophagy, or for the selective clearance of peroxisomes, mitochondria, or the ER, we found that NBR1 is required for the heat-induced formation of autophagic vesicles. Moreover, cytoplasmic puncta containing aggregation-prone proteins, which were rarely observed in wild-type plants, were found to accumulate in nbr1 mutants under both control and heat stress conditions. Given that NBR1 co-localizes with these cytoplasmic puncta, we propose that Arabidopsis NBR1 is a plant aggrephagy receptor essential for maintaining proteostasis under both heat stress and non-stress conditions.

Many plants require prolonged exposure to cold to acquire the competence to flower. The process by which cold exposure results in competence is known as vernalization. In Arabidopsis thaliana, vernalization leads to the stable repression of the floral repressor FLOWERING LOCUS C via chromatin modification, including an increase of trimethylation on lysine 27 of histone H3 (H3K27me3) by Polycomb Repressive Complex 2 (PRC2). Vernalization in pooids is associated with the stable induction of a floral promoter, VERNALIZATION 1 (VRN1). From a screen for mutants with a reduced vernalization requirement in the model grass Brachypodium distachyon, we identified two recessive alleles of ENHANCER OF ZESTE-LIKE 1 (EZL1). EZL1 is orthologous to A. thaliana CURLY LEAF 1, a gene that encodes the catalytic subunit of PRC2. B. distachyon ezl1 mutants flower rapidly without vernalization in long-day (LD) photoperiods; thus, EZL1 is required for the proper maintenance of the vegetative state prior to vernalization. Transcriptomic studies in ezl1 revealed mis-regulation of thousands of genes, including ectopic expression of several floral homeotic genes in leaves. Loss of EZL1 results in the global reduction of H3K27me3 and H3K27me2, consistent with this gene making a major contribution to PRC2 activity in B. distachyon. Furthermore, in ezl1 mutants, the flowering genes VRN1 and AGAMOUS (AG) are ectopically expressed and have reduced H3K27me3. Artificial microRNA knock-down of either VRN1 or AG in ezl1-1 mutants partially restores wild-type flowering behavior in non-vernalized plants, suggesting that ectopic expression in ezl1 mutants may contribute to the rapid-flowering phenotype.

Leaf senescence is an essential part of the plant lifecycle during which nutrients are re-allocated to other tissues. The regulation of leaf senescence is a complex process. However, the underlying mechanism is poorly understood. Here, we uncovered a novel and the pivotal role of Arabidopsis HDA9 (a RPD3-like histone deacetylase) in promoting the onset of leaf senescence. We found that HDA9 acts in complex with a SANT domain-containing protein POWERDRESS (PWR) and transcription factor WRKY53. Our genome-wide profiling of HDA9 occupancy reveals that HDA9 directly binds to the promoters of key negative regulators of senescence and this association requires PWR. Furthermore, we found that PWR is important for HDA9 nuclear accumulation. This study reveals an uncharacterized epigenetic complex involved in leaf senescence and provides mechanistic insights into how a histone deacetylase along with a chromatin-binding protein contribute to a robust regulatory network to modulate the onset of plant aging. DOI: http://dx.doi.org/10.7554/eLife.17214.001, eLife digest The leaves of many plants turn yellow in the fall as nutrients are recycled to prepare for the winter months. However, if leaves age and yellow too early, it can limit how much energy the plant can harvest from light. Thus, it is crucial for plants to know when they should start the leaf aging process. This is also important for plant biologists because premature leaf yellowing can reduce both the yield and quality of crop plants. Certain aging-related genes tightly control when and how leaves age. Like in many other organisms, plant DNA is packaged around proteins called histones. As such, one of the ways that plants regulate the activity of their genes is by chemically modifying the DNA or histones to alter how tightly the DNA is packaged. For example, to switch particular genes off, enzymes known as histone deacetylases remove an acetyl group from their histones. However, it is not clear how these enzymes know which genes to modify and how this helps to make sure that leaf aging happens at the appropriate time. Chen et al. studied a histone deacetylase called HDA9 in a flowering plant named Arabidopsis. The experiments show that the HDA9 enzyme plays an important role in ensuring the leaves turn yellow at the right time. Without HDA9, the leaf aging process is delayed. HDA9 also needs the help of another protein called PWR that instructs HDA9 to remove acetyl groups from the histones of specific aging-associated genes in order to switch these genes off. The next challenge is to understand how HDA9 and PWR sense developmental and environmental signals to trigger the histone modifications. It will also be important to decipher how this enzyme works with other regulators to trigger leaf aging at the right time. DOI: http://dx.doi.org/10.7554/eLife.17214.002