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1.  Autophagy

Autophagy was initially defined as a bulk degradation process that causes massive degradation of cellular components; however, in recent years, cumulative evidence has indicated that the recruitment of cargo to autophagosomes is highly selective ^[1][2]. Several selective autophagy receptors have been characterized in plants. These selective autophagy receptors link organelles, protein aggregates or other cargo to the autophagy machinery by binding to both the fated cargo and ATG8 through conserved ATG8-interacting motif (AIM) or ubiquitin-interacting motif (UIM). Several characterized autophagy receptors function in the plant abiotic stress response (Table 1).

The next to BRCA1 gene 1 (NBR1) is a functional hybrid protein of the mammalian autophagy receptor p62 (also known as Sequestosome1/SQSTM1) and a neighbor of BRCA1 (NBR1) that specifically targets stress-induced, ubiquitinated protein aggregates ^[3][4]. Both p62 and NBR1 preferentially target K63-linked polyubiquitylated proteins and mediate their aggregation and autophagic clearance in an LC3-interacting region (LIR)- and UBA-dependent manner. NBR1 and p62 oligomerize but can also function independently ^[5]. Arabidopsis AtNBR1 can bind ATG8 via the AIM motif and ubiquitinate proteins via the ubiquitin-associated domain ^[6]. The expression of AtNBR1 is upregulated under HS in Arabidopsis ^[7]. Moreover, under HS conditions, atnbr1 mutants accumulate more puncta in the cytoplasm compared with WT. GFP-NBR1 puncta accumulate in WT plants but not in atg7 mutants under HS conditions. During the HS recovery phase, more NBR1 puncta accumulate in WT plants, and the NBR1 protein accumulates at substantially higher levels in atg5-1 and atg18a-2 mutants than in WT plants ^[7][8]. These findings demonstrate that NBR1 puncta formation is autophagy dependent and that NBR1 is required not only for the heat-induced formation of autophagosomes but also for the degradation of substrates in an autophagy-dependent pathway throughout the HS stage. Further analysis showed that NBR1 plays a crucial role as a receptor for the selective autophagy-mediated degradation of heat shock protein 90.1 (HSP90.1) and rotamase FKBP1 (ROF1) during recovery from HS to regulate HS memory in Arabidopsis ^[9]. In addition, nbr1 mutants are hypersensitive to oxidative, drought and salt stress relative to WT plants ^{[10][4][6][7]}. Similar results were observed in tomato plants in which NBR1 was silenced by VIGS. ATGs and NBR1 gene silencing triggered the accumulation of ubiquitinated insoluble proteins and decreased the number of autophagosomes under cold and heat stress ^[11][12]. In poplars, PagNBR1 is also induced by salt stress. PagNBR1 overexpressing poplars displayed more salt stress tolerance by accelerating antioxidant system activity and autophagy activity ^[13]. These results demonstrate the important roles of NBR1 in resisting stress conditions via the autophagy pathway.

3. Atg8-Interacting Proteins 1/2/3 (ATI1/2/3)

Atg8-Interacting Proteins 1/2/3 (ATI1/2/3) are AIM-motif-containing proteins identified through a yeast two-hybrid screen for proteins interacting with ATG8. ATI1 and ATI2 are homologous in that each contain two AIM motifs and a transmembrane domain. These two proteins define a newly identified stress-induced compartment that moves along the ER network and is subsequently transported to the vacuole in Arabidopsis plants ^[14][15]. Salt stress promotes ATI1 protein accumulation. Arabidopsis that are deficient in both homologs (ATI-KD) display increased sensitivity to salt treatment both at the seedling stage and in older plants but no effect was observed on germination. The authors found that ATI1 may play a role in the elimination of damaged plastid and ER proteins produced during salt stress ^[16]. ATI3 proteins contain a WxxL motif at the C-terminus required for ATG8 interaction. ATI3 homologs are found in dicots but not in other organisms, including monocots. The ati3 mutant plants display hypersensitivity to HS, and the interaction of ATI3 and ATG8 is increased under HS ^[17].

Dominant suppressor of Kar 2 (DSK2), a ubiquitin-binding receptor, is another ATG8-interacting protein with an AIM motif. DSK2-RNAi Arabidopsis plants display increased sensitivity to drought stress and increased levels of BES1 (BRI1-EMS SUPPRESSOR 1) relative to the WT. BES1 is a master regulator of the brassinosteroid (BR) pathway. DSK2 can be phosphorylated by another negative regulator of the BR pathway, BIN2, which promotes DSK2-ATG8 interaction ^[18]. These results suggest that DSK2, acting as an autophagy receptor, specifically directs BES1 degradation through the autophagy pathway under drought stress conditions. This link between BR signaling and autophagy means that these processes regulate the plant stress response together. Phytohormones play important roles in plant growth, development, abiotic and biotic stress responses. Phytohormone signals and the autophagy pathway jointly regulate plant responses to abiotic stress ^[19]; however, the relationship between phytohormones and autophagy in plant abiotic stress regulation remains unclear.

4. Tryptophan-rich sensory protein/translocator (TSPO)

Tryptophan-rich sensory protein/translocator (TSPO) is a type of tryptophan-rich sensory protein/peripheral-type benzodiazepine receptor (TspO/MBR) domain-containing membrane protein, which also has an AIM motif that interacts with ATG8 ^[20]. The expression of AtTSPO is induced by osmotic and salt stress ^[21][22]. AtTSPO overexpression makes plants hypersensitive to salt stress, possibly because AtTSPO can bind the plasma membrane aquaporin AtPIP2;7 and regulate its degradation through the autophagy pathway, and the overdegradation of aquaporins impairs cell water status ^[23]. In addition, similar to NBR1, TSPO is degraded via the autophagy pathway in a manner dependent on ATG5 and the PI3K complex ^[20]. Another protein from Medicago, cold acclimation-specific 31 (MtCas31), can also regulate MtPIP2;7 stability through the autophagy pathway and participate in drought stress regulation. MtCAS31 directly interacts with MtATG8a in the AIM-like motifs YXXXI and MtPIP2;7, supporting its function in autophagic degradation. The overexpression of MtCAS31 promotes autophagy and MtPIP2;7 degradation under drought stress. These results demonstrate that MtCAS31 functions as a positive regulator of drought stress in Medicago and participates in the drought-induced autophagic degradation of MtPIP2;7 as a cargo receptor ^[24]. The selected substrates of AtTSPO and MtCAS31 are from the same family of proteins but do not overlap, which may be related to the meticulous regulation of autophagy.

5. Constitutively stressed 1 (COST1)

Constitutively stressed 1 (COST1), a plant-specific gene, can also interact with ATG8E but does not have a typical AIM or UIM motif. The cost1 mutant exhibits decreased growth and increased drought tolerance, together with constitutive autophagy and increased expression of drought response genes. COST1 overexpression confers drought hypersensitivity and reduces autophagy. COST1 co-localizes with ATG8E and NBR1 in autophagosomes and directly affects the ATG8E protein level, indicating that it plays a pivotal role in the direct regulation of autophagy ^[25][26]. The above results illustrate that COST1 is a negative regulator of both drought resistance and autophagy. The increased drought tolerance of the cost1 mutant is due to autophagy activation ^[25][26][27]. The majority of the autophagy receptors/adaptors identified to date are ATG8-interacting proteins and thus ATG8 is an essential regulator of autophagy; however, the regulation of autophagy is complicated, and autophagy plays a role in almost every part of the plant life cycle. ATG8-independent receptors/adaptors must exist and require further exploration.