Functional characterization of the plant ubiquitin regulatory X (UBX) domain-containing protein AtPUX7 in Arabidopsis thaliana

CDC48 in plants and its emerging function in plant immunity

Protein homeostasis, namely the balance between protein synthesis and degradation, must be finely controlled to ensure cell survival, notably through the ubiquitin-proteasome system (UPS). In all species, including plants, homeostasis is disrupted by biotic and abiotic stresses. A key player in the maintenance of protein balance, the protein CDC48, shows emerging functions in plants, particularly in response to biotic stress. In this review on CDC48 in plants, we detail its highly conserved structure, describe a gene expansion that is only present in Viridiplantae, discuss its various functions and regulations, and finally highlight its recruitment, still not clear, during the plant immune response.

GA signaling expands: The plant UBX domain-containing protein 1 is a binding partner for the GA receptor

The plant Ubiquitin Regulatory X (UBX) domain-containing protein 1 (PUX1) functions as a negative regulator of gibberellin (GA) signaling. GAs are plant hormones that stimulate seed germination, the transition to flowering, and cell elongation and division. Loss of Arabidopsis (Arabidopsis thaliana) PUX1 resulted in a "GA-overdose" phenotype including early flowering, increased stem and root elongation, and partial resistance to the GA-biosynthesis inhibitor paclobutrazol during seed germination and root elongation. Furthermore, GA application failed to stimulate further stem elongation or flowering onset suggesting that elongation and flowering response to GA had reached its maximum. GA hormone partially repressed PUX1 protein accumulation, and PUX1 showed a GA-independent interaction with the GA receptor GA-INSENSITIVE DWARF-1 (GID1). This suggests that PUX1 is GA regulated and/or regulates elements of the GA signaling pathway. Consistent with PUX1 function as a negative regulator of GA signaling, the pux1 mutant caused increased GID1 expression and decreased accumulation of the DELLA REPRESSOR OF GA1-3, RGA. PUX1 is a negative regulator of the hexameric AAA+ ATPase CDC48, a protein that functions in diverse cellular processes including unfolding proteins in preparation for proteasomal degradation, cell division, and expansion. PUX1 binding to GID1 required the UBX domain, a binding motif necessary for CDC48 interaction. Moreover, PUX1 overexpression in cell culture not only stimulated the disassembly of CDC48 hexamer but also resulted in co-fractionation of GID1, PUX1, and CDC48 subunits in velocity sedimentation assays. Based on our results, we propose that PUX1 and CDC48 are additional factors that need to be incorporated into our understanding of GA signaling.

Targeting plant UBX proteins: AI-enhanced lessons from distant cousins

Across all eukaryotic kingdoms, ubiquitin regulatory X (UBX) domain-containing adaptor proteins control the segregase cell division control protein 48 (CDC48), and thereby also control cellular proteostasis and adaptation. The structures and biological roles of UBX proteins in animals and fungi have garnered considerable attention. However, their counterparts in plants remain markedly understudied. Since 2021, the artificial intelligence (AI)-based algorithm AlphaFold has provided predictions of protein structural features that can be highly accurate. Predictions of the proteomes of all major model organisms are now freely accessible to the entire research community through user-friendly web interfaces. We propose that the combination of cross-kingdom comparison with AF analysis produces a wealth of testable hypotheses to inspire and guide experimental research on plant UBX domain-containing (PUX) proteins.

ATG8-Interacting Motif: Evolution and Function in Selective Autophagy of Targeting Biological Processes

Autophagy is an evolutionarily conserved vacuolar process functioning in the degradation of cellular components for reuse. In plants, autophagy is generally activated upon stress and its regulation is executed by numbers of AuTophaGy-related genes (ATGs), of which the ATG8 plays a dual role in both biogenesis of autophagosomes and recruitment of ATG8-interacting motif (AIM) anchored selective autophagy receptors (SARs). Such motif is either termed as AIM or ubiquitin-interacting motif (UIM), corresponding to the LC3-interacting region (LIR)/AIM docking site (LDS) or the UIM docking site (UDS) of ATG8, respectively. To date, dozens of AIM or UIM containing SARs have been characterized. However, the knowledge of these motifs is still obscured. In this review, we intend to summarize the current understanding of SAR proteins and discuss the conservation and diversification of the AIMs/UIMs, expectantly providing new insights into the evolution of them in various biological processes in plants.

The chaperone-like protein Cdc48 regulates ubiquitin-proteasome system in plants

The degradation of misfolded proteins is mainly mediated by the ubiquitin-proteasome system (UPS). UPS can be assisted by the protein Cdc48 but the relationship between UPS and Cdc48 in plants has been poorly investigated. Here, we analysed the regulation of UPS by Cdc48 in tobacco thanks to two independent cell lines overexpressing Cdc48 constitutively and plant leaves overexpressing Cdc48 transiently. In the cell lines, the accumulation of ubiquitinated proteins was affected both quantitatively and qualitatively and the number of proteasomal subunits was modified, while proteolytic activities were unchanged. Similarly, the over-expression of Cdc48 in planta impacted the accumulation of ubiquitinated proteins. A similar process occurred in leaves overexpressing transiently Rpn3, a proteasome subunit. Cdc48 being involved in plant immunity, its regulation of UPS was also investigated in response to cryptogein, an elicitor of immune responses. In the cell lines stably overexpressing Cdc48 and in leaves transiently overexpressing Cdc48 and/or Rpn3, cryptogein triggered a premature cell death while no increase of the proteasomal activity occurred. Overall, this study highlights a role for Cdc48 in ubiquitin homeostasis and confirms its involvement, as well as that of Rpn3, in the processes underlying the hypersensitive response.

Versatile control of the CDC48 segregase by the plant UBX-containing (PUX) proteins

In plants, AAA-adenosine triphosphatase (ATPase) Cell Division Control Protein 48 (CDC48) uses the force generated through ATP hydrolysis to pull, extract, and unfold ubiquitylated or sumoylated proteins from the membrane, chromatin, or protein complexes. The resulting changes in protein or RNA content are an important means for plants to control protein homeostasis and thereby adapt to shifting environmental conditions. The activity and targeting of CDC48 are controlled by adaptor proteins, of which the plant ubiquitin regulatory X (UBX) domain-containing (PUX) proteins constitute the largest family. Emerging knowledge on the structure and function of PUX proteins highlights that these proteins are versatile factors for plant homeostasis and adaptation that might inspire biotechnological applications.

Recent advances in the regulation of plant immunity by S-nitrosylation

S-nitrosylation, the addition of a nitric oxide (NO) moiety to a reactive protein cysteine (Cys) thiol, to form a protein S-nitrosothiol (SNO), is emerging as a key regulatory post-translational modification (PTM) to control the plant immune response. NO also S-nitrosylates the antioxidant tripeptide, glutathione, to form S-nitrosoglutathione (GSNO), both a storage reservoir of NO bioactivity and a natural NO donor. GSNO and, by extension, S-nitrosylation, are controlled by GSNO reductase1 (GSNOR1). The emerging data suggest that GSNOR1 itself is a target of NO-mediated S-nitrosylation, which subsequently controls its selective autophagy, regulating cellular protein SNO levels. Recent findings also suggest that S-nitrosylation may be deployed by pathogen-challenged host cells to counteract the effect of delivered microbial effector proteins that promote pathogenesis and by the pathogens themselves to augment virulence. Significantly, it also appears that S-nitrosylation may regulate plant immune functions by controlling SUMOylation, a peptide-based PTM. In this context, global SUMOylation is regulated by S-nitrosylation of SUMO conjugating enzyme 1 (SCE1) at Cys139. This redox-based PTM has also been shown to control the function of a key zinc finger transcriptional regulator during the establishment of plant immunity. Here, we provide an update of these recent advances.

Proximity labeling proteomics reveals critical regulators for inner nuclear membrane protein degradation in plants

The inner nuclear membrane (INM) selectively accumulates proteins that are essential for nuclear functions; however, overaccumulation of INM proteins results in a range of rare genetic disorders. So far, little is known about how defective, mislocalized, or abnormally accumulated membrane proteins are actively removed from the INM, especially in plants and animals. Here, via analysis of a proximity-labeling proteomic profile of INM-associated proteins in Arabidopsis, we identify critical components for an INM protein degradation pathway. We show that this pathway relies on the CDC48 complex for INM protein extraction and 26S proteasome for subsequent protein degradation. Moreover, we show that CDC48 at the INM may be regulated by a subgroup of PUX proteins, which determine the substrate specificity or affect the ATPase activity of CDC48. These PUX proteins specifically associate with the nucleoskeleton underneath the INM and physically interact with CDC48 proteins to negatively regulate INM protein degradation in plants.

ATG8-Binding UIM Proteins Define a New Class of Autophagy Adaptors and Receptors

During autophagy, vesicle dynamics and cargo recruitment are driven by numerous adaptors and receptors that become tethered to the phagophore through interactions with lipidated ATG8/LC3 decorating the expanding membrane. Most currently described ATG8-binding proteins exploit a well-defined ATG8-interacting motif (AIM, or LC3-interacting region [LIR]) that contacts a hydrophobic patch on ATG8 known as the LIR/AIM docking site (LDS). Here we describe a new class of ATG8 interactors that exploit ubiquitin-interacting motif (UIM)-like sequences for high-affinity binding to an alternative ATG8 interaction site. Assays with candidate UIM-containing proteins together with unbiased screens identified a large collection of UIM-based ATG8 interactors in plants, yeast, and humans. Analysis of a subset also harboring ubiquitin regulatory X (UBX) domains revealed a role for UIM-directed autophagy in clearing non-functional CDC48/p97 complexes, including some impaired in human disease. With this new class of adaptors and receptors, we greatly extend the reach of selective autophagy and identify new factors regulating autophagic vesicle dynamics.

Toward the understanding of the role of CDC48, a major component of the protein quality control, in plant immunity

The evolutionally conserved chaperone-like protein CDC48 (cell division cycle 48) is a major component of ubiquitin-dependent protein degradation pathways in animal and yeast and, more generally, of the protein quality control machinery. In plants, CDC48 plays essential regulatory functions in development and the possibly that it contributes to protein degradation through the ubiquitin-proteasome system (UPS) and the endoplasmic reticulum-associated protein degradation (ERAD) system has been reported. In this review we described recent findings highlighting a role for CDC48 in plant immunity. First data indicated that CDC48 is S-nitrosylated in plant cells undergoing an immune response, regulates the turnover of immune receptors and mediates the degradation of viral proteins. Furthermore its overexpression was associated to an exacerbated hypersensitive-like cell death. We also designed and reported here the first CDC48 interactome. The corresponding data confirm the closed interaction of CDC48 with components of the UPS and shed light on its putative regulatory function of S-adenosyl-methionine synthesis and metabolism. More generally, these investigations further support the concept that plant cells facing pathogen attack finely regulate the protein quality control machinery.

Polyamine Function in Plants: Metabolism, Regulation on Development, and Roles in Abiotic Stress Responses

Polyamines (PAs) are low molecular weight aliphatic nitrogenous bases containing two or more amino groups. They are produced by organisms during metabolism and are present in almost all cells. Because they play important roles in diverse plant growth and developmental processes and in environmental stress responses, they are considered as a new kind of plant biostimulant. With the development of molecular biotechnology techniques, there is increasing evidence that PAs, whether applied exogenously or produced endogenously via genetic engineering, can positively affect plant growth, productivity, and stress tolerance. However, it is still not fully understood how PAs regulate plant growth and stress responses. In this review, we attempt to cover these information gaps and provide a comprehensive and critical assessment of the published literature on the relationships between PAs and plant flowering, embryo development, senescence, and responses to several (mainly abiotic) stresses. The aim of this review is to summarize how PAs improve plants' productivity, and to provide a basis for future research on the mechanism of action of PAs in plant growth and development. Future perspectives for PA research are also suggested.

Functional characterization of the chaperon-like protein Cdc48 in cryptogein-induced immune response in tobacco

Cdc48, a molecular chaperone conserved in different kingdoms, is a member of the AAA+ family contributing to numerous processes in mammals including proteins quality control and degradation, vesicular trafficking, autophagy and immunity. The functions of Cdc48 plant orthologues are less understood. We previously reported that Cdc48 is regulated by S-nitrosylation in tobacco cells undergoing an immune response triggered by cryptogein, an elicitin produced by the oomycete Phytophthora cryptogea. Here, we inv estigated the function of NtCdc48 in cryptogein signalling and induced hypersensitive-like cell death. NtCdc48 was found to accumulate in elicited cells at both the protein and transcript levels. Interestingly, only a small proportion of the overall NtCdc48 population appeared to be S-nitrosylated. Using gel filtration in native conditions, we confirmed that NtCdc48 was present in its hexameric active form. An immunoprecipitation-based strategy following my mass spectrometry analysis led to the identification of about a hundred NtCdc48 partners and underlined its contribution in cellular processes including targeting of ubiquitylated proteins for proteasome-dependent degradation, subcellular trafficking and redox regulation. Finally, the analysis of cryptogein-induced events in NtCdc48-overexpressing cells highlighted a correlation between NtCdc48 expression and hypersensitive cell death. Altogether, this study identified NtCdc48 as a component of cryptogein signalling and plant immunity.

Intron-Mediated Enhancement: A Tool for Heterologous Gene Expression in Plants?

Many plant promoters were characterized and used for transgene expression in plants. Even though these promoters drive high levels of transgene expression in plants, the expression patterns are rarely constitutive but restricted to some tissues and developmental stages. In terms of crop improvement not only the enhancement of expression per se but, in particular, tissue-specific and spatial expression of genes plays an important role. Introns were used to boost expression in transgenic plants in the field of crop improvement for a long time. However, the mechanism behind this so called intron-mediated enhancement (IME) is still largely unknown. This review highlights the complexity of IME on the levels of its regulation and modes of action and gives an overview on IME methodology, examples in fundamental research and models of proposed mechanisms. In addition, the application of IME in heterologous gene expression is discussed.

Structure and functions of the chaperone-like p97/CDC48 in plants

BACKGROUND

The chaperone-like p97 is a member of the AAA+ ATPase enzyme family that contributes to numerous cellular activities. P97 has been broadly studied in mammals (VCP/p97) and yeasts (CDC48: Cell Division Cycle 48/p97) and numerous investigations highlighted that this protein is post-translationally regulated, is structured in homohexamer and interacts with partners and cofactors that direct it to distinct cellular signalization pathway including protein quality control and degradation, cell cycle regulation, genome stability, vesicular trafficking, autophagy and immunity.

SCOPE OF REVIEW

p97 is also conserved in plants (CDC48) but its functions are less understood. In the present review we intended to present the state of the art of the structure, regulation and functions of CDC48 in plants.

MAJOR CONCLUSIONS

Evidence accumulated underline that CDC48 plays a crucial role in development, cell cycle regulation and protein turnover in plants. Furthermore, its involvement in plant immunity has recently emerged and first interacting partners have been identified, shedding light on its putative cellular activities.

GENERAL SIGNIFICANCE

Identification of emerging functions of CDC48 in plants opens new roads of research in immunity and provides new insights into the mechanisms of protein quality control.

Biotrophy at Its Best: Novel Findings and Unsolved Mysteries of the Arabidopsis-Powdery Mildew Pathosystem

It is generally accepted in plant-microbe interactions research that disease is the exception rather than a common outcome of pathogen attack. However, in nature, plants with symptoms that signify colonization by obligate biotrophic powdery mildew fungi are omnipresent. The pervasiveness of the disease and the fact that many economically important plants are prone to infection by powdery mildew fungi drives research on this interaction. The competence of powdery mildew fungi to establish and maintain true biotrophic relationships renders the interaction a paramount example of a pathogenic plant-microbe biotrophy. However, molecular details underlying the interaction are in many respects still a mystery. Since its introduction in 1990, the Arabidopsis-powdery mildew pathosystem has become a popular model to study molecular processes governing powdery mildew infection. Due to the many advantages that the host Arabidopsis offers in terms of molecular and genetic tools this pathosystem has great capacity to answer some of the questions of how biotrophic pathogens overcome plant defense and establish a persistent interaction that nourishes the invader while in parallel maintaining viability of the plant host.

NO signaling in plant immunity: a tale of messengers

Nitric oxide (NO) is a free radical gas involved in a myriad of plant physiological processes including immune responses. How NO mediates its biological effects in plant facing microbial pathogen attack is an unresolved question. Insights into the molecular mechanisms by which it propagates signals reveal the contribution of this simple gas in complex signaling pathways shared with reactive oxygen species (ROS) and the second messenger Ca(2+). Understanding of the subtle cross-talks operating between these signals was greatly improved by the recent identification and the functional analysis of proteins regulated through S-nitrosylation, a major NO-dependent post-translational protein modification. Overall, these findings suggest that NO is probably an important component of the mechanism coordinating and regulating Ca(2+) and ROS signaling in plant immunity.

Endoplasmic reticulum-mediated protein quality control in Arabidopsis

A correct three-dimensional structure is crucial for the physiological functions of a protein, yet the folding of proteins to acquire native conformation is a fundamentally error-prone process. Eukaryotic organisms have evolved a highly conserved endoplasmic reticulum-mediated protein quality control (ERQC) mechanism to monitor folding processes of secretory and membrane proteins, allowing export of only correctly folded proteins to their physiological destinations, retaining incompletely/mis-folded ones in the ER for additional folding attempts, marking and removing terminally misfolded ones via a unique multiple-step degradation process known as ER-associated degradation (ERAD). Most of our current knowledge on ERQC and ERAD came from genetic and biochemical investigations in yeast and mammalian cells. Recent studies in the reference plant Arabidopsis thaliana uncovered homologous components and similar mechanisms in plants for monitoring protein folding and for retaining, repairing, and removing misfolded proteins. These studies also revealed critical roles of the plant ERQC/ERAD systems in regulating important biochemical/physiological processes, such as abiotic stress tolerance and plant defense. In this review, we discuss our current understanding about the molecular components and biochemical mechanisms of the plant ERQC/ERAD system in comparison to yeast and mammalian systems.