Uncovering legumain genes in rice

OsVPE2, a Member of Vacuolar Processing Enzyme Family, Decreases Chilling Tolerance of Rice

Chilling is a major abiotic stress affecting rice growth, development and geographical distribution. Plant vacuolar processing enzymes (VPEs) contribute to the seed storage protein processing and mediate the programmed cell death by abiotic and biotic stresses. However, little is known about the roles of plant VPEs in cold stress responses and tolerance regulation. Here, we found that OsVPE2 was a chilling-responsive gene. The early-indica rice variety Xiangzaoxian31 overexpressing OsVPE2 was more sensitive to chilling stress, whereas the OsVPE2-knockout mutants generated by the CRISPR-Cas9 technology exhibited significantly enhanced chilling tolerance at the seedling stage without causing yield loss. Deficiency of OsVPE2 reduces relative electrolyte leakage, accumulation of toxic compounds such as reactive oxygen species and malondialdehyde, and promotes antioxidant enzyme activities under chilling stress conditions. It was indicated that OsVPE2 mediated the disintegration of vacuoles under chilling stress, accompanied by the entry of swollen mitochondria into vacuoles. OsVPE2 suppressed the expression of genes that have a positive regulatory role in antioxidant process. Moreover, haplotype analysis suggested that the natural variation in the OsVPE2 non-coding region may endow OsVPE2 with different expression levels, thereby probably conferring differences in cold tolerance between japonica and indica sub-population. Our results thus reveal a new biological function of the VPE family in regulating cold resistance, and suggest that the gene editing or natural variations of OsVPE2 can be used to create cold tolerant rice varieties with stable yield.

Vacuolar Processing Enzymes Modulating Susceptibility Response to Fusarium oxysporum f. sp. cubense Tropical Race 4 Infections in Banana

Fusarium oxysporum f. sp. cubense tropical race 4 (FocTR4) is a destructive necrotrophic fungal pathogen afflicting global banana production. Infection process involves the activation of programmed cell death (PCD). In this study, seven Musa acuminata vacuolar processing enzyme (MaVPE1-MaVPE7) genes associated with PCD were successfully identified. Phylogenetic analysis and tissue-specific expression categorized these MaVPEs into the seed and vegetative types. FocTR4 infection induced the majority of MaVPE expressions in the susceptible cultivar "Berangan" as compared to the resistant cultivar "Jari Buaya." Consistently, upon FocTR4 infection, high caspase-1 activity was detected in the susceptible cultivar, while low level of caspase-1 activity was recorded in the resistant cultivar. Furthermore, inhibition of MaVPE activities via caspase-1 inhibitor in the susceptible cultivar reduced tonoplast rupture, decreased lesion formation, and enhanced stress tolerance against FocTR4 infection. Additionally, the Arabidopsis VPE-null mutant exhibited higher tolerance to FocTR4 infection, indicated by reduced sporulation rate, low levels of H₂O₂ content, and high levels of cell viability. Comparative proteomic profiling analysis revealed increase in the abundance of cysteine proteinase in the inoculated susceptible cultivar, as opposed to cysteine proteinase inhibitors in the resistant cultivar. In conclusion, the increase in vacuolar processing enzyme (VPE)-mediated PCD played a crucial role in modulating susceptibility response during compatible interaction, which facilitated FocTR4 colonization in the host.

Disruption of the Interfacial Membrane Leads to Magnaporthe oryzae Effector Re-location and Lifestyle Switch During Rice Blast Disease

To cause the devastating rice blast disease, the hemibiotrophic fungus Magnaporthe oryzae produces invasive hyphae (IH) that are enclosed in a plant-derived interfacial membrane, known as the extra-invasive hyphal membrane (EIHM), in living rice cells. Little is known about when the EIHM is disrupted and how the disruption contributes to blast disease. Here we show that the disruption of the EIHM correlates with the hyphal growth stage in first-invaded susceptible rice cells. Our approach utilized GFP that was secreted from IH as an EIHM integrity reporter. Secreted GFP (sec-GFP) accumulated in the EIHM compartment but appeared in the host cytoplasm when the integrity of the EIHM was compromised. Live-cell imaging coupled with sec-GFP and various fluorescent reporters revealed that the loss of EIHM integrity preceded shrinkage and eventual rupture of the rice vacuole. The vacuole rupture coincided with host cell death, which was limited to the invaded cell with presumed closure of plasmodesmata. We report that EIHM disruption and host cell death are landmarks that delineate three distinct infection phases (early biotrophic, late biotrophic, and transient necrotrophic phases) within the first-invaded cell before reestablishment of biotrophy in second-invaded cells. M. oryzae effectors exhibited infection phase-specific localizations, including entry of the apoplastic effector Bas4 into the host cytoplasm through the disrupted EIHM during the late biotrophic phase. Understanding how infection phase-specific cellular dynamics are regulated and linked to host susceptibility will offer potential targets that can be exploited to control blast disease.

Genome-Wide Identification and Characterization of Vacuolar Processing Enzyme Gene Family and Diverse Expression Under Stress in Apple (Malus × Domestic)

Vacuolar processing enzymes (VPEs) play an important role in stress resistance and development of plants. Despite their diverse roles, little information is available in apple (Malus × domestic). This study firstly presents the genome-wide identification of VPE family genes in apple, resulting in 20 family members those are unevenly distributed across six out of the 17 chromosomes. Phylogenetic analysis assigned these genes into four groups. Analysis of exon-intron junctions and motifs of each candidate gene revealed high levels of conservation within and between phylogenetic groups. Cis-element including w box, ABRE, LTR, and TC-rich repeats were found in promoters of MdVPEs. NCBI-GEO database shown that the expression of MdVPEs exhibited diverse patterns in different tissues as well as the infection of Pythium ultimum and Apple Stem Grooving Virus. Furthermore, qRT-PCR showed that MdVPE genes were responsive to salt, cadmium, low-temperature, and drought. Overexpression of MDP0000172014, which was strongly induced by salt and drought stress, significantly decreased Arabidopsis tolerance to salt stress. The genome-wide identification and characterization of MdVPEs in apple provided basic information for the potential utilization of MdVPEs in stress resistance.

De novo assembly of Vriesea carinata leaf transcriptome to identify candidate cysteine-proteases

Vriesea carinata is an endemic bromeliad from the Brazilian Atlantic Forest. It has trichome and tank system in their leaves which allows to absorb water and nutrients. It belongs to Bromeliaceae family, which includes several species highly enriched of cysteine-proteases (CysPs). These proteolytic enzymes regulate processes as senescence, cell differentiation, pathogen-linked programmed cell death and mobilization of proteins. Although, their biological importance, there are not genomic resources in V. carinata that can help to identify and understand their molecular mechanisms involved in different biological processes. Thus high-throughput transcriptome sequencing of V. carinata is necessary to generate sequences for the purpose of gene discovery and functional genomic studies. In the present study, we sequenced and assembled the V. carinata transcriptome to the identification of CysPs. A total of 43,232 contigs were assembled for the leaf tissue. BLAST analysis indicated that 23,803 contigs exhibited similarity to non-redundant Viridiplantae proteins. 28.24% of the contigs were classified into the COG database, and gene ontology categorized them into 61 functional groups. A metabolic pathway analysis with KEGG revealed 9679 contigs assigned to 31 metabolic pathways. Among 16 full-length CysPs identified, 11 were evaluated in respect to their expression patterns in the leaf apex, base and inflorescence tissues. The results showed differential expression levels of legumain, metacaspase, pyroglutamyl and papain-like CysPs depending of the leaf region. These results provide a global overview of V. carinata gene functions and expression activities of CysPs in those tissues.

Integrative omics analysis in Pandanus odorifer (Forssk.) Kuntze reveals the role of Asparagine synthetase in salinity tolerance

Pandanus odorifer (Forssk) Kuntze grows naturally along the coastal regions and withstands salt-sprays as well as strong winds. A combination of omics approaches and enzyme activity studies was employed to comprehend the mechanistic basis of high salinity tolerance in P. odorifer. The young seedlings of P. odorifer were exposed to 1 M salt stress for up to three weeks and analyzed using RNAsequencing (RNAseq) and LC-MS. Integrative omics analysis revealed high expression of the Asparagine synthetase (AS) (EC 6.3.5.4) (8.95 fold) and remarkable levels of Asparagine (Asn) (28.5 fold). This indicated that salt stress promoted Asn accumulation in P. odorifer. To understand this further, the Asn biosynthesis pathway was traced out in P. odorifer. It was noticed that seven genes involved in Asn bisynthetic pathway namely glutamine synthetase (GS) (EC 6.3.1.2) glutamate synthase (GOGAT) (EC 1.4.1.14), aspartate kinase (EC 2.7.2.4), pyruvate kinase (EC 2.7.1.40), aspartate aminotransferase (AspAT) (EC 2.6.1.1), phosphoenolpyruvate carboxylase (PEPC) (EC 4.1.1.31) and AS were up-regulated under salt stress. AS transcripts were most abundant thereby showed its highest activity and thus were generating maximal Asn under salt stress. Also, an up-regulated Na+/H+ antiporter (NHX1) facilitated compartmentalization of Na+ into vacuoles, suggesting P. odorifer as salt accumulator species.

Genetic Dissection of Grain Nutritional Traits and Leaf Blight Resistance in Rice

Colored rice is rich in nutrition and also a good source of valuable genes/quantitative trait loci (QTL) for nutrition, grain quality, and pest and disease resistance traits for use in rice breeding. Genome-wide association analysis using high-density single nucleotide polymorphism (SNP) is useful in precisely detecting QTLs and genes. We carried out genome-wide association analysis in 152 colored rice accessions, using 22,112 SNPs to map QTLs for nutritional, agronomic, and bacterial leaf blight (BLB) resistance traits. Wide variations and normal frequency distributions were observed for most of the traits except anthocyanin content and BLB resistance. The structural and principal component analysis revealed two subgroups. The linkage disequilibrium (LD) analysis showed 74.3% of the marker pairs in complete LD, with an average LD distance of 1000 kb and, interestingly, 36% of the LD pairs were less than 5 Kb, indicating high recombination in the panel. In total, 57 QTLs were identified for ten traits at p < 0.0001, and the phenotypic variance explained (PVE) by these QTLs varied from 9% to 18%. Interestingly, 30 (53%) QTLs were co-located with known or functionally-related genes. Some of the important candidate genes for grain Zinc (Zn) and BLB resistance were OsHMA9, OsMAPK6, OsNRAMP7, OsMADS13, and OsZFP252, and Xa1, Xa3, xa5, xa13 and xa26, respectively. Red rice genotype, Sayllebon, which is high in both Zn and anthocyanin content, could be a valuable material for a breeding program for nutritious rice. Overall, the QTLs identified in our study can be used for QTL pyramiding as well as genomic selection. Some of the novel QTLs can be further validated by fine mapping and functional characterization. The results show that pigmented rice is a valuable resource for mineral elements and antioxidant compounds; it can also provide novel alleles for disease resistance as well as for yield component traits. Therefore, large opportunities exist to further explore and exploit more colored rice accessions for use in breeding.

Structural analyses of Arabidopsis thaliana legumain γ reveal differential recognition and processing of proteolysis and ligation substrates

Legumain is a dual-function protease-peptide ligase whose activities are of great interest to researchers studying plant physiology and to biotechnological applications. However, the molecular mechanisms determining the specificities for proteolysis and ligation are unclear because structural information on the substrate recognition by a fully activated plant legumain is unavailable. Here, we present the X-ray structure of Arabidopsis thaliana legumain isoform γ (AtLEGγ) in complex with the covalent peptidic Ac-YVAD chloromethyl ketone (CMK) inhibitor targeting the catalytic cysteine. Mapping of the specificity pockets preceding the substrate-cleavage site explained the known substrate preference. The comparison of inhibited and free AtLEGγ structures disclosed a substrate-induced disorder-order transition with synergistic rearrangements in the substrate-recognition sites. Docking and in vitro studies with an AtLEGγ ligase substrate, sunflower trypsin inhibitor (SFTI), revealed a canonical, protease substrate-like binding to the active site-binding pockets preceding and following the cleavage site. We found the interaction of the second residue after the scissile bond, P2'-S2', to be critical for deciding on proteolysis versus cyclization. cis-trans-Isomerization of the cyclic peptide product triggered its release from the AtLEGγ active site and prevented inadvertent cleavage. The presented integrative mechanisms of proteolysis and ligation (transpeptidation) explain the interdependence of legumain and its preferred substrates and provide a rational framework for engineering optimized proteases, ligases, and substrates.

Rice bifunctional phytocystatin is a dual modulator of legumain and papain-like proteases

Phytocystatins are well-known inhibitors of C1A cysteine proteinases. However, previous research has revealed legumain (C13) protease inhibition via a carboxy-extended phytocystatin. Among the 12 phytocystatins genes in rice, OcXII is the only gene possessing this carboxy-terminal extension. The specific legumain inhibition activity was confirmed, in our work, using a recombinant OcXII harboring only the carboxy-terminal domain and this part did not exhibit any effect on papain-like activities. Meanwhile, rice plants silenced at the whole OcXII gene presented higher legumain and papain-like proteolytic activities, resulting in a faster initial seedling growth. However, when germinated under stressful alkaline conditions, OcXII-silenced plants exhibited impaired root formation and delayed shoot growth. Interestingly, the activity of OcXII promoter gene was detected in the rice seed scutellum region, and decreases with seedling growth. Seeds from these plants also exhibited slower growth at germination under ABA or alkaline conditions, while maintaining very high levels of OcXII transcriptional activation. This likely reinforces the proteolytic control necessary for seed germination and growth. In addition, increased legumain activity was detected in OcXII RNAi plants subjected to a fungal elicitor. Overall, the results of this study highlight the association of OcXII with not only plant development processes, but also with stress response pathways. The results of this study reinforce the bifunctional ability of carboxy-extended phytocystatins in regulating legumain proteases via its carboxy-extended domain and papain-like proteases by its amino-terminal domain.

Gene Cloning, Expression and Enzyme Activity of Vitis vinifera Vacuolar Processing Enzymes (VvVPEs)

Vacuolar processing enzymes (VPEs) have received considerable attention due to their caspase-1-like activity and ability to regulate programmed cell death (PCD), which plays an essential role in the development of stenospermocarpic seedless grapes ovules. To characterize VPEs and the relationship between stenospermocarpic grapes and the VPE gene family, we identified 3 Vitis vinifera VPE genes (VvβVPE, VvγVPE, and VvδVPE) from the PN40024 grape genome and cloned the full-length complementary DNAs (cDNAs) from the ‘Vitis vinifera cv. Pinot Noir’ and ‘Vitis vinifera cv. Thompson Seedless’ varietals. Each of the VPEs contained a typical catalytic dyad [His (177), Cys (219)] and substrate binding pocket [Arg (112), Arg (389), Ser (395)], except that Ser (395) in the VvγVPE protein sequence was replaced with alanine. Phylogenetic analysis of 4 Arabidopsis thaliana and 6 Vitis vinifera VPEs revealed that the 10 VPEs form 3 major branches. Furthermore, the 6 grapevine VPEs share a similar gene structure, with 9 exons and 8 introns. The 6 grapevine VPEs are located on 3 different chromosomes. We also tested the enzymatic activity of recombinant VPEs expressed in the Pichia Pastoris expression system and found that the VvVPEs exhibit cysteine peptidase activity. Tissue-specific expression analysis showed that VvδVPE is only expressed in flowers, buds and ovules, that VvγVPE is expressed in various tissues, and that VvβVPE was expressed in roots, flowers, buds and ovules. The results of quantitative real-time PCR (qRT-PCR) suggested that VvβVPE in seeded grapes increased significantly at 30 days after full-bloom (DAF), close to the timing of endosperm abortion at 32 DAF. These results suggested that VvβVPE is related to ovule abortion in seedless grapes. Our experiments provide a new perspective for understanding the mechanism of stenospermocarpic seedlessness and represent a useful reference for the further study of VPEs.

Characterization of the legumains encoded by the genome of Theobroma cacao L

Legumains are cysteine proteases related to plant development, protein degradation, programmed cell death, and defense against pathogens. In this study, we have identified and characterized three legumains encoded by Theobroma cacao genome through in silico analyses, three-dimensional modeling, genetic expression pattern in different tissues and as a response to the inoculation of Moniliophthora perniciosa fungus. The three proteins were named TcLEG3, TcLEG6, and TcLEG9. Histidine and cysteine residue which are part of the catalytic site were conserved among the proteins, and they remained parallel in the loop region in the 3D modeling. Three-dimensional modeling showed that the propeptide, which is located in the terminal C region of legumains blocks the catalytic cleft. Comparing dendrogram data with the relative expression analysis, indicated that TcLEG3 is related to the seed legumain group, TcLEG6 is related with the group of embryogenesis activities, and protein TcLEG9, with processes regarding the vegetative group. Furthermore, the expression analyses proposes a significant role for the three legumains during the development of Theobroma cacao and in its interaction with M. perniciosa.

The Amborella vacuolar processing enzyme family

Most vacuolar proteins are synthesized on rough endoplasmic reticulum as proprotein precursors and then transported to the vacuoles, where they are converted into their respective mature forms by vacuolar processing enzymes (VPEs). In the case of the seed storage proteins, this process is of major importance, as it conditions the establishment of vigorous seedlings. Toward the goal of identifying proteome signatures that could be associated with the origin and early diversification of angiosperms, we previously characterized the 11S-legumin-type seed storage proteins from Amborella trichopoda, a rainforest shrub endemic to New Caledonia that is also the probable sister to all other angiosperms (Amborella Genome Project, 2013). In the present study, proteomic and genomic approaches were used to characterize the VPE family in this species. Three genes were found to encode VPEs in the Amborella's genome. Phylogenetic analyses showed that the Amborella sequences grouped within two major clades of angiosperm VPEs, indicating that the duplication that generated the ancestors of these clades occurred before the most recent common ancestor of living angiosperms. A further important duplication within the VPE family appears to have occurred in common ancestor of the core eudicots, while many more recent duplications have also occurred in specific taxa, including both Arabidopsis thaliana and Amborella. An analysis of natural genetic variation for each of the three Amborella VPE genes revealed the absence of selective forces acting on intronic and exonic single-nucleotide polymorphisms among several natural Amborella populations in New Caledonia.

Vacuolar processing enzyme in plant programmed cell death

Vacuolar processing enzyme (VPE) is a cysteine proteinase originally identified as the proteinase responsible for the maturation and activation of vacuolar proteins in plants, and it is known to be an ortholog of animal asparaginyl endopeptidase (AEP/VPE/legumain). VPE has been shown to exhibit enzymatic properties similar to that of caspase 1, which is a cysteine protease that mediates the programmed cell death (PCD) pathway in animals. Although there is limited sequence identity between VPE and caspase 1, their predicted three-dimensional structures revealed that the essential amino-acid residues for these enzymes form similar pockets for the substrate peptide YVAD. In contrast to the cytosolic localization of caspases, VPE is localized in vacuoles. VPE provokes vacuolar rupture, initiating the proteolytic cascade leading to PCD in the plant immune response. It has become apparent that the VPE-dependent PCD pathway is involved not only in the immune response, but also in the responses to a variety of stress inducers and in the development of various tissues. This review summarizes the current knowledge on the contribution of VPE to plant PCD and its role in vacuole-mediated cell death, and it also compares VPE with the animal cell death executor caspase 1.