Phytocystatin 6 is a context-dependent, tight-binding inhibitor of Arabidopsis thaliana legumain isoform β

Plant legumains are crucial for processing seed storage proteins and are critical regulators of plant programmed cell death. Although research on legumains boosted recently, little is known about their activity regulation. In our study, we used pull-down experiments to identify AtCYT6 as a natural inhibitor of legumain isoform β (AtLEGβ) in Arabidopsis thaliana. Biochemical analysis revealed that AtCYT6 inhibits both AtLEGβ and papain-like cysteine proteases through two separate cystatin domains. The N-terminal domain inhibits papain-like proteases, while the C-terminal domain inhibits AtLEGβ. Furthermore, we showed that AtCYT6 interacts with legumain in a substrate-like manner, facilitated by a conserved asparagine residue in its reactive center loop. Complex formation was additionally stabilized by charged exosite interactions, contributing to pH-dependent inhibition. Processing of AtCYT6 by AtLEGβ suggests a context-specific regulatory mechanism with implications for plant physiology, development, and programmed cell death. These findings enhance our understanding of AtLEGβ regulation and its broader physiological significance.

Apple vacuolar processing enzyme 4 is regulated by cysteine protease inhibitor and modulates fruit disease resistance

Ring rot is a destructive apple disease caused by Botryosphaeria dothidea. The resistance mechanism of apple plants to B. dothidea remains unclear. Here, we show that APPLE VACUOLAR PROCESSING ENZYME 4 (MdVPE4) is involved in resistance to B. dothidea. MdVPE4 silencing reduced fruit disease resistance, whereas its overexpression improved resistance. Gene expression analysis revealed that MdVPE4 influenced the expression of fruit disease resistance-related genes, such as APPLE POLYGALACTURONASE 1 (MdPG1), APPLE POLYGALACTURONASE INHIBITOR PROTEIN 1 (MdPGIP1), APPLE ENDOCHITINASE 1 (MdCHI1), and APPLE THAUMATIN-LIKE PROTEIN 1 (MdTHA1). The expression of the four genes responding to B. dothidea infection decreased in MdVPE4-silenced fruits. Further analysis demonstrated that B. dothidea infection induced MdVPE4 expression and enzyme activation in apple fruits. Moreover, MdVPE4 activity was modulated by apple cysteine proteinase inhibitor 1 (MdCPI1), which also contributed to resistance towards B. dothidea, as revealed by gene overexpression and silencing analysis. MdCPI1 interacted with MdVPE4 and inhibited its activity. However, MdCPI1 expression was decreased by B. dothidea infection. Taken together, our findings indicate that the interaction between MdVPE4 and MdCPI1 plays an important role in modulating fruit disease resistance to B. dothidea.

MhNRAMP1 From Malus hupehensis Exacerbates Cell Death by Accelerating Cd Uptake in Tobacco and Apple Calli

Excessive cadmium (Cd) damages plants by causing cell death. The present study discusses the function of natural resistance-associated macrophage protein (NRAMP) on cell death caused by Cd in Malus hupehensis. MhNRAMP1 was isolated from M. hupehensis roots, and its protein was located in the cell membrane as a transmembrane protein characterized by hydrophobicity. MhNRAMP1 expression in the roots was induced by Cd stress and calcium (Ca) deficiency. MhNRAMP1 overexpression increased Cd concentration in yeasts and enhanced their sensitivity to Cd. Phenotypic comparisons of plants under Cd stress revealed that the growth of transgenic tobacco and apple calli overexpressing MhNRAMP1 was worse than that of the wild type (WT). The Cd²⁺ influx of transgenic tobacco roots and apple calli was higher, and the recovery time of the Cd²⁺ influx to a stable state in transgenic apple calli was longer than that of the WT. Cd accumulation and the percentage of apoptotic cells in transgenic lines were higher. Correspondingly, the caspase-1-like and vacuolar processing enzyme (VPE) activities and MdVPEγ expression were higher in transgenic apple calli, but the expression levels of genes that inhibit cell death were lower than those in the WT under Cd stress. Moreover, the Cd translocation from the roots to leaves was increased after MhNRAMP1 overexpression, but the Cd translocation from the leaves to seeds was not affected. These results suggest that MhNRMAP1 exacerbated Cd-induced cell death, which was accomplished by mediating Cd²⁺ uptake and accumulation, as well as stimulating VPE.

Vacuolar processing enzyme 4 contributes to maternal control of grain size in barley by executing programmed cell death in the pericarp

The angiosperm embryo and endosperm are limited in space because they grow inside maternal seed tissues. The elimination of cell layers of the maternal seed coat by programmed cell death (PCD) could provide space and nutrition to the filial organs. Using the barley (Hordeum vulgare L.) seed as a model, we elucidated the role of vacuolar processing enzyme 4 (VPE4) in cereals by using an RNAi approach and targeting the enzymatic properties of the recombinant protein. A comparative characterization of transgenic versus wild-type plants included transcriptional and metabolic profiling, flow cytometry, histology and nuclear magnetic imaging of grains. The recombinant VPE4 protein exhibited legumain and caspase-1 properties in vitro. Pericarp disintegration was delayed in the transgenic grains. Although the VPE4 gene and enzymatic activity was decreased in the early developing pericarp, storage capacity and the size of the endosperm and embryo were reduced in the mature VPE4-repressed grains. The persistence of the pericarp in the VPE4-affected grains constrains endosperm and embryo growth and leads to transcriptional reprogramming, perturbations in signalling and adjustments in metabolism. We conclude that VPE4 expression executes PCD in the pericarp, which is required for later endosperm filling, and argue for a role of PCD in maternal control of seed size in cereals.

Plant Vacuoles

Plant vacuoles are multifunctional organelles. On the one hand, most vegetative tissues develop lytic vacuoles that have a role in degradation. On the other hand, seed cells have two types of storage vacuoles: protein storage vacuoles (PSVs) in endosperm and embryonic cells and metabolite storage vacuoles in seed coats. Vacuolar proteins and metabolites are synthesized on the endoplasmic reticulum and then transported to the vacuoles via Golgi-dependent and Golgi-independent pathways. Proprotein precursors delivered to the vacuoles are converted into their respective mature forms by vacuolar processing enzyme, which also regulates various kinds of programmed cell death in plants. We summarize two types of vacuolar membrane dynamics that occur during defense responses: vacuolar membrane collapse to attack viral pathogens and fusion of vacuolar and plasma membranes to attack bacterial pathogens. We also describe the chemical defense against herbivores brought about by the presence of PSVs in the idioblast myrosin cell.

The Life and Death of a Plant Cell

Like all eukaryotic organisms, plants possess an innate program for controlled cellular demise termed programmed cell death (PCD). Despite the functional conservation of PCD across broad evolutionary distances, an understanding of the molecular machinery underpinning this fundamental program in plants remains largely elusive. As in mammalian PCD, the regulation of plant PCD is critical to development, homeostasis, and proper responses to stress. Evidence is emerging that autophagy is key to the regulation of PCD in plants and that it can dictate the outcomes of PCD execution under various scenarios. Here, we provide a broad and comparative overview of PCD processes in plants, with an emphasis on stress-induced PCD. We also discuss the implications of the paradox that is functional conservation of apoptotic hallmarks in plants in the absence of core mammalian apoptosis regulators, what that means, and whether an equivalent form of death occurs in plants.

The Life and Death of a Plant Cell

Like all eukaryotic organisms, plants possess an innate program for controlled cellular demise termed programmed cell death (PCD). Despite the functional conservation of PCD across broad evolutionary distances, an understanding of the molecular machinery underpinning this fundamental program in plants remains largely elusive. As in mammalian PCD, the regulation of plant PCD is critical to development, homeostasis, and proper responses to stress. Evidence is emerging that autophagy is key to the regulation of PCD in plants and that it can dictate the outcomes of PCD execution under various scenarios. Here, we provide a broad and comparative overview of PCD processes in plants, with an emphasis on stress-induced PCD. We also discuss the implications of the paradox that is functional conservation of apoptotic hallmarks in plants in the absence of core mammalian apoptosis regulators, what that means, and whether an equivalent form of death occurs in plants.

Plant life needs cell death, but does plant cell death need Cys proteases?

Caspases are key regulators of apoptosis in animals. This correlation has driven plant researchers for decades to look for caspases regulating programmed cell death (PCD) in plants. These studies revealed caspase-like activities, caspase-related proteases, and cysteine (Cys) proteases regulating PCD in plants, but identified no caspases and no conserved, apoptosis-like death pathway. Here, we critically review the evidence for Cys proteases implicated in PCD in plants. We discuss the role of papain-like Cys proteases, vacuolar processing enzymes, and metacaspases in PCD during the development of tracheary elements, seed coat, suspensor, and tapetum, and during the hypersensitive response. There are several convincing cases where these Cys proteases are required for PCD, but this requirement is often not conserved across different plant species. There are also cases where Cys proteases contribute to the speed, but not the timing of PCD, while other Cys proteases are nonessential for PCD, but have other roles, e.g., in the clearance of cell remains after PCD. These data illustrate the need for caution when generalizing the role of Cys proteases in regulating PCD in plants, and call for studies that further investigate plant Cys proteases and other PCD regulators.

Two vacuole-mediated defense strategies in plants

As plants lack immune cells, each cell has to defend itself against invading pathogens. Plant cells have a large central vacuole that accumulates a variety of hydrolytic enzymes and antimicrobial compounds, raising the possibility that vacuoles play a role in plant defense. However, how plants use vacuoles to protect against invading pathogens is poorly understood. Recently, we characterized two vacuole-mediated defense strategies associated with programmed cell death (PCD). In one strategy, vacuolar processing enzyme (VPE) mediated the disruption of the vacuolar membrane, resulting in the release of vacuolar contents into the cytoplasm in response to viral infection. In the other strategy, proteasome-dependent fusion of the central vacuole with the plasma membrane caused the discharge of vacuolar antibacterial protease and cell death-promoting contents from the cell in response to bacterial infection. Intriguingly, both strategies relied on enzymes with caspase-like activities: the vacuolar membrane-collapse system required VPE, which has caspase-1-like activity, and the membrane-fusion system required a proteasome that has caspase-3-like activity. Thus, plants may have evolved a cellular immune system that involves vacuolar membrane collapse to prevent the systemic spread of viral pathogens, and membrane fusion to inhibit the proliferation of bacterial pathogens.