Aspartic proteases gene family in rice: Gene structure and expression, predicted protein features and phylogenetic relation

Aspartic proteases gene family: Identification and expression profiles during stem vascular development in tobacco

Aspartic proteases (APs) constitute a large family in plants and are widely involved in diverse biological processes, like chloroplast metabolism, biotic and abiotic stress responses, and reproductive development. In this study, we focused on overall analysis of the APs genes in tobacco. Our analysis included the phylogeny and cis-elements in the cell wall-associated promoters of these genes. To characterize the expression patterns of APs genes in stem vascular development. The tissue expression analysis showed that NtAED3-like was preferentially expressed in the differentiating xylem and phloem cells of the vascular system. Based on histochemical staining analysis showed that the NtAED3-like gene was specifically expressed in stem vascular tissue, root vascular tissue, and petiole vascular tissue. The TdT-mediated dUTP nick-end labeling (TUNEL) assay illustrated a delayed progression of programmed cell death (PCD) within the xylem of the ko-ntaed3a-like mutant, relative to the wild type. The mutant ko-ntaed3a-like exhibited a phenotype of thinning stem circumference and changed in xylem structure and lignin content. In addition, the two-dimension heteronuclear single quantum coherent nuclear magnetic resonance (2D-HSQC) analysis of three milled wood lignins (MWLs) showed that the content of β-O-4 connection in ko-ntaed3a-like decreased slightly compared with wild type. In conclusion, this study provides our understanding of the regulation of vascular tissue development by the NtAED3-like gene in tobacco and provides a better basis for determining the molecular mechanism of the aspartic protease in secondary cell wall (SCW) development.

bHLH-regulated routes in anther development in rice and Arabidopsis

Anther development is a complex process essential for plant reproduction and crop yields. In recent years, significant progress has been made in the identification and characterization of the bHLH transcription factor family involved in anther regulation in rice and Arabidopsis, two extensively studied model plants. Research on bHLH transcription factors has unveiled their crucial function in controlling tapetum development, pollen wall formation, and other anther-specific processes. By exploring deeper into regulatory mechanisms governing anther development and bHLH transcription factors, we can gain important insights into plant reproduction, thereby accelerating crop yield improvement and the development of new plant breeding strategies. This review provides an overview of the current knowledge on anther development in rice and Arabidopsis, emphasizing the critical roles played by bHLH transcription factors in this process. Recent advances in gene expression analysis and functional studies are highlighted, as they have significantly enhanced our understanding of the regulatory networks involved in anther development.

Phylogenetic and AlphaFold predicted structure analyses provide insights for A1 aspartic protease family classification in Arabidopsis

Aspartic proteases are widely distributed in animals, plants, fungi and other organisms. In land plants, A1 aspartic protease family members have been implicated to play important and varied roles in growth, development and defense. Thus a robust classification of this family is important for understanding their gene function and evolution. However, current A1 family members in Arabidopsis are less well classified and need to be re-evaluated. In this paper, 70 A1 aspartic proteases in Arabidopsis are divided into four groups (group I-IV) based on phylogenetic and gene structure analyses of 1200 A1 aspartic proteases which are obtained from 12 Embryophyta species. Group I-III members are further classified into 2, 4 and 7 subgroups based on the AlphaFold predicted structures. Furthermore, unique insights of A1 aspartic proteases have been unraveled by AlphaFold predicted structures. For example, subgroup II-C members have a unique II-C specific motif in the C-extend domain, and subgroup IV is a Spermatophyta conserved group without canonical DTGS/DSGT active sites. These results prove that AlphaFold combining phylogenetic analysis is a promising solution for complex gene family classification.

The grapevine aspartic protease gene family: characterization and expression modulation in response to Plasmopara viticola

Grapevine aspartic proteases gene family is characterized and five VviAPs appear to be involved in grapevine defense against downy mildew. Grapevine (Vitis vinifera L.) is one of the most important crops worldwide. However, it is highly susceptible to the downy mildew disease caused by Plasmopara viticola (Berk. & Curt.) Berl. & De Toni. To minimize the use of fungicides used to control P. viticola, it is essential to gain a deeper comprehension on this pathosystem and proteases have gained particular interest in the past decade. Proteases were shown to actively participate in plant-pathogen interactions, not only in the processes that lead to plant cell death, stress responses and protein processing/degradation but also as components of the recognition and signalling pathways. The aim of this study was to identify and characterize the aspartic proteases (APs) involvement in grapevine defense against P. viticola. A genome-wide search and bioinformatics characterization of the V. vinifera AP gene family was conducted and a total of 81 APs proteins, coded by 65 genes, were found. VviAPs proteins can be divided into three categories, similar to those previously described for other plants. Twelve APs coding genes were selected, and expression analysis was conducted at several time-points after inoculation in both compatible and incompatible interactions. Five grapevine APs may be involved in grapevine tolerance against P. viticola. Our findings provide an overall understanding of the VviAPs gene family and establish better groundwork to further describe the roles of VviAPs in defense against P. viticola.

An aspartic protease 47 causes quantitative recessive resistance to rice black-streaked dwarf virus disease and southern rice black-streaked dwarf virus disease

Rice black-streaked dwarf virus disease (RBSDVD) and southern rice black-streaked dwarf virus disease (SRBSDVD) are the most destructive viral diseases in rice. Progress is limited in breeding due to lack of resistance resource and inadequate knowledge on the underlying functional gene. Using genome-wide association study (GWAS), linkage disequilibrium (LD) decay analyses, RNA-sequencing, and genome editing, we identified a highly RBSDVD-resistant variety and its first functional gene. A highly RBSDVD-resistant variety W44 was identified through extensive evaluation of a diverse international rice panel. Seventeen quantitative trait loci (QTLs) were identified among which qRBSDV6-1 had the largest phenotypic effect. It was finely mapped to a 0.8-1.2 Mb region on chromosome 6, with 62 annotated genes. Analysis of the candidate genes underlying qRBSDV6-1 showed high expression of aspartic proteinase 47 (OsAP47) in a susceptible variety, W122, and a low resistance variety, W44. OsAP47 overexpressing lines exhibited significantly reduced resistance, while the knockout mutants exhibited significantly reduced SRBSDVD and RBSDVD severity. Furthermore, the resistant allele Hap1 of OsAP47 is almost exclusive to Indica, but rare in Japonica. Results suggest that OsAP47 knockout by editing is effective for improving RBSDVD and SRBSDVD resistance. This study provides genetic information for breeding resistant cultivars.

Genome-Wide Analyses of Aspartic Proteases on Potato Genome (Solanum tuberosum): Generating New Tools to Improve the Resistance of Plants to Abiotic Stress

Aspartic proteases are proteolytic enzymes widely distributed in living organisms and viruses. Although they have been extensively studied in many plant species, they are poorly described in potatoes. The present study aimed to identify and characterize S. tuberosum aspartic proteases. Gene structure, chromosome and protein domain organization, phylogeny, and subcellular predicted localization were analyzed and integrated with RNAseq data from different tissues, organs, and conditions focused on abiotic stress. Sixty-two aspartic protease genes were retrieved from the potato genome, distributed in 12 chromosomes. A high number of intronless genes and segmental and tandem duplications were detected. Phylogenetic analysis revealed eight StAP groups, named from StAPI to StAPVIII, that were differentiated into typical (StAPI), nucellin-like (StAPIIIa), and atypical aspartic proteases (StAPII, StAPIIIb to StAPVIII). RNAseq data analyses showed that gene expression was consistent with the presence of cis-acting regulatory elements on StAP promoter regions related to water deficit. The study presents the first identification and characterization of 62 aspartic protease genes and proteins on the potato genome and provides the baseline material for functional gene determinations and potato breeding programs, including gene editing mediated by CRISPR.

Expression in Escherichia coli, Refolding, and Purification of Plant Aspartic Proteases

Aspartic proteases (APs) are widely distributed in plants. The large majority of genes encoding putative APs exhibit distinct features when compared with the so-called typical APs, and have been grouped as atypical and nucellin-like APs. Remarkably, a diverse pattern of enzymatic properties, subcellular localizations, and biological functions are emerging for these proteases, illustrating the functional complexity among plant pepsin-like proteases. However, many key questions regarding the structure-function relationships of plant APs remain unanswered. Therefore, the expression of these enzymes in heterologous systems is a valuable strategy to unfold the unique features/biochemical properties among members of this family of proteases. Here, we describe our protocol for the production and purification of recombinant plant APs, using a procedure where the protein is refolded from inclusion bodies by dialysis. This method allows the production of untagged versions of the target protease, which has revealed to be critical to disclose differences in processing/activation requirements between plant APs. The protocol includes protein expression, washing and solubilization of inclusion bodies, refolding by dialysis, and a protein purification method. Specific considerations on critical aspects of the refolding process and further suggestions for evaluation of the final recombinant product are also provided.

Identification of Proteases and Protease Inhibitors in Seeds of the Recalcitrant Forest Tree Species Quercus ilex

Proteases and protease inhibitors have been identified in the recalcitrant species Quercus ilex using in silico and wet methods, with focus on those present in seeds during germination. In silico analyses showed that the Q. ilex transcriptome database contained 2,240 and 97 transcripts annotated as proteases and protease inhibitors, respectively. They belonged to the different families according to MEROPS, being the serine and metallo ones the most represented. The data were compared with those previously reported for other Quercus species, including Q. suber, Q. lobata, and Q. robur. Changes in proteases and protease inhibitors alongside seed germination in cotyledon and embryo axis tissues were assessed using proteomics and in vitro and in gel activity assays. Shotgun (LC-MSMS) analysis of embryo axes and cotyledons in nonviable (NV), mature (T1) and germinated (T3) seeds allowed the identification of 177 proteases and 12 protease inhibitors, mostly represented by serine and metallo types. Total protease activity, as determined by in vitro assays using azocasein as substrate, was higher in cotyledons than in embryo axes. There were not differences in activity among cotyledon samples, while embryo axis peaked at germinated T4 stage. Gel assays revealed the presence of protease activities in at least 10 resolved bands, in the Mr range of 60-260 kDa, being some of them common to cotyledons and embryo axes in either nonviable, mature, and germinated seeds. Bands showing quantitative or qualitative changes upon germination were observed in embryo axes but not in cotyledons at Mr values of 60-140 kDa. Proteomics shotgun analysis of the 10 bands with protease activity supported the results obtained in the overall proteome analysis, with 227 proteases and 3 protease inhibitors identified mostly represented by the serine, cysteine, and metallo families. The combined use of shotgun proteomics and protease activity measurements allowed the identification of tissue-specific (e.g., cysteine protease inhibitors in embryo axes of mature acorns) and stage-specific proteins (e.g., those associated with mobilization of storage proteins accumulated in T3 stage). Those proteins showing differences between nonviable and viable seeds could be related to viability, and those variables between mature and germinated could be associated with the germination process. These differences are observed mostly in embryo axes but not in cotyledons. Among them, those implicated in mobilization of reserve proteins, such as the cathepsin H cysteine protease and Clp proteases, and also the large number of subunits of the CNS and 26S proteasome complex differentially identified in embryos of the several stages suggests that protein degradation via CNS/26S plays a major role early in germination. Conversely, aspartic proteases such as nepenthesins were exclusively identified in NV seeds, so their presence could be used as indicator of nonviability.

Role of proteases in the response of plants to drought

Plants are sessile organisms that, to survive they develop response mechanisms under water deficit conditions. Plant proteases play an essential role in a diversity of biological processes, among them tolerance to drought stress. Proteolysis is a critical regulator of stomatal development. Plant proteases are involved in the crosstalk among phytohormones and adjustment of stomatal aperture. Plant proteases are also related to the increment in reactive oxygen species (ROS) production detected in the plant biochemical response to drought. Plant proteases mitigate this process by degrading damaged, denatured, and aggregated proteins, remobilizing amino acids, and generating molecules involved in signal transductions. Although many roles for proteases have been proposed, molecular bases that regulate these mechanisms remain unknown. In this review, we summarize the current knowledge on the participation of proteases in the signaling pathways of plants in response to water deficit and their relationship with plant stress tolerance.

Transcription Factor GmWRKY46 Enhanced Phosphate Starvation Tolerance and Root Development in Transgenic Plants

Phosphorus (P) is one of the essential macronutrients, whose deficiency limits the growth and development of plants. In this study, we investigated the possible role of GmWRKY46 in the phosphate (Pi) starvation stress tolerance of soybean. GmWRKY46 belonged to the group III subfamily of the WRKY transcription factor family, which was localized in the nucleus and had transcriptional activator activity. GmWRKY46 could be strongly induced by Pi starvation, especially in soybean roots. Overexpression of GmWRKY46 significantly enhanced tolerance to Pi starvation and lateral root development in transgenic Arabidopsis. RNA-seq analysis showed that overexpression of GmWRKY46 led to change in many genes related to energy metabolisms, stress responses, and plant hormone signal transduction in transgenic Arabidopsis. Among these differential expression genes, we found that overexpression of AtAED1 alone could enhance the tolerance of transgenic Arabidopsis to Pi starvation. Y1H and ChIP-qPCR analyses showed that GmWRKY46 could directly bind to the W-box motif of the AtAED1 promoter in vitro and in vivo. Furthermore, results from intact soybean composite plants with GmWRKY46 overexpression showed that GmWRKY46 was involved in hairy roots development and subsequently affected plant growth and Pi uptake. These results provide a basis for the molecular genetic breeding of soybean tolerant to Pi starvation.

Barley Nepenthesin-Like Aspartic Protease HvNEP-1 Degrades Fusarium Phytase, Impairs Toxin Production, and Suppresses the Fungal Growth

Nepenthesins are categorized under the subfamily of the nepenthesin-like plant aspartic proteases (PAPs) that form a distinct group of atypical PAPs. This study describes the effect of nepenthesin 1 (HvNEP-1) protease from barley (Hordeum vulgare L.) on fungal histidine acid phosphatase (HAP) phytase activity. Signal peptide lacking HvNEP-1 was expressed in Pichia pastoris and biochemically characterized. Recombinant HvNEP-1 (rHvNEP-1) strongly inhibited the activity of Aspergillus and Fusarium phytases, which are enzymes that release inorganic phosphorous from phytic acid. Moreover, rHvNEP-1 suppressed in vitro fungal growth and strongly reduced the production of mycotoxin, 15-acetyldeoxynivalenol (15-ADON), from Fusarium graminearum. The quantitative PCR analysis of trichothecene biosynthesis genes (TRI) confirmed that rHvNEP-1 strongly repressed the expression of TRI4, TRI5, TRI6, and TRI12 in F. graminearum. The co-incubation of rHvNEP-1 with recombinant F. graminearum (rFgPHY1) and Fusarium culmorum (FcPHY1) phytases induced substantial degradation of both Fusarium phytases, indicating that HvNEP-1-mediated proteolysis of the fungal phytases contributes to the HvNEP-1-based suppression of Fusarium.

Two sides of the same story in grapevine-pathogen interactions

Proteases are an integral part of plant defence systems, and their role in plant-pathogen interactions is unequivocal. Emerging evidence suggests that different protease families contribute to the establishment not only of hypersensitive response, priming, and signalling, but also of recognition events through complex proteolytic cascades. Moreover, they play a crucial role in pathogen/microbe-associated molecular pattern (PAMP/MAMP)-triggered immunity as well as in effector-triggered immunity. However, despite important advances in our understanding of the role of proteases in plant defence, the contribution of proteases to pathogen defence in grapevine remains poorly understood. In this review, we summarize current knowledge of the main grapevine pathosystems and explore the role of serine, cysteine, and aspartic proteases from both the host and pathogen point of views.

A novel glycine-rich domain protein, GRDP1, functions as a critical feedback regulator for controlling cell death and disease resistance in rice

Lesion mimic mutants constitute a valuable genetic resource for unraveling the signaling pathways and molecular mechanisms governing the programmed cell death and defense responses of plants. Here, we identified a lesion mimic mutant, spl-D, from T-DNA insertion rice lines. The mutant exhibited higher accumulation of H2O2, spontaneous cell death, decreased chlorophyll content, up-regulation of defense-related genes, and enhanced disease resistance. The causative gene, OsGRDP1, encodes a cytosol- and membrane-associated glycine-rich domain protein. OsGRDP1 was expressed constitutively in all of the organs of the wild-type plant, but was up-regulated throughout plant development in the spl-D mutant. Both the overexpression and knockdown (RNAi) of OsGRDP1 resulted in the lesion mimic phenotype. Moreover, the intact-protein level of OsGRDP1 was reduced in the spotted leaves from both overexpression and RNAi plants, suggesting that the disruption of intact OsGRDP1 is responsible for lesion formation. OsGRDP1 interacted with an aspartic proteinase, OsAP25. In the spl-D and overexpression plants, proteinase activity was elevated, and lesion formation was partially suppressed by an aspartic proteinase inhibitor. Taken together, our results reveal that OsGRDP1 is a critical feedback regulator, thus contributing to the elucidation of the mechanism underlying cell death and disease resistance.

Defense and Offense Strategies: The Role of Aspartic Proteases in Plant-Pathogen Interactions

Plant aspartic proteases (APs; E.C.3.4.23) are a group of proteolytic enzymes widely distributed among different species characterized by the conserved sequence Asp-Gly-Thr at the active site. With a broad spectrum of biological roles, plant APs are suggested to undergo functional specialization and to be crucial in developmental processes, such as in both biotic and abiotic stress responses. Over the last decade, an increasing number of publications highlighted the APs' involvement in plant defense responses against a diversity of stresses. In contrast, few studies regarding pathogen-secreted APs and AP inhibitors have been published so far. In this review, we provide a comprehensive picture of aspartic proteases from plant and pathogenic origins, focusing on their relevance and participation in defense and offense strategies in plant-pathogen interactions.

Genome-wide identification, evolution and expression analysis of the aspartic protease gene family during rapid growth of moso bamboo (Phyllostachys edulis) shoots

BACKGROUND

Aspartic proteases (APs) are a class of aspartic peptidases belonging to nine proteolytic enzyme families whose members are widely distributed in biological organisms. APs play essential functions during plant development and environmental adaptation. However, there are few reports about APs in fast-growing moso bamboo.

RESULT

In this study, we identified a total of 129 AP proteins (PhAPs) encoded by the moso bamboo genome. Phylogenetic and gene structure analyses showed that these 129 PhAPs could be divided into three categories (categories A, B and C). The PhAP gene family in moso bamboo may have undergone gene expansion, especially the members of categories A and B, although homologs of some members in category C have been lost. The chromosomal location of PhAPs suggested that segmental and tandem duplication events were critical for PhAP gene expansion. Promoter analysis revealed that PhAPs in moso bamboo may be involved in plant development and responses to environmental stress. Furthermore, PhAPs showed tissue-specific expression patterns and may play important roles in rapid growth, including programmed cell death, cell division and elongation, by integrating environmental signals such as light and gibberellin signals.

CONCLUSION

Comprehensive analysis of the AP gene family in moso bamboo suggests that PhAPs have experienced gene expansion that is distinct from that in rice and may play an important role in moso bamboo organ development and rapid growth. Our results provide a direction and lay a foundation for further analysis of plant AP genes to clarify their function during rapid growth.

Genome-wide identification of the aspartic protease gene family and their response under powdery mildew stress in wheat

Aspartic proteases (APs) are one of the four main protease super families. In plants, they are involved in many biological processes, such as biotic and abiotic stress resistance, protein processing and degradation, senescence, and programmed cell death. By performing a database (TGACv1) search and domain prediction, we identified 263 wheat AP (TaAP) proteins and observed 38 TaAP genes exhibiting alternative splicing. Moreover, by constructing a phylogenetic tree, we found that the TaAP proteins can be divided into three families and have a certain close evolutionary relationship to Arabidopsis thaliana and rice AP proteins. Transcriptome analysis showed that 29 genes in the TaAP family were up-regulated after being induced by powdery mildew. The expression of TaAP224 showed the most significant difference in transcriptome and qRT-PCR analyses. Subsequently, the promoters of these 29 genes were analysed, and we found that they contained multiple disease resistance and hormone elements, such as WRKY71OS, a common disease resistance element that is also involved in the GA signalling pathway and inhibits starch hydrolysis. The comprehensive annotation and expression profiling performed in this study increased our understanding of the TaAP family genes in wheat growth and development, and the results can be used as a basis for further study of candidate TaAP genes involved in powdery mildew resistance mechanisms.

Auxin apical dominance governed by the OsAsp1-OsTIF1 complex determines distinctive rice caryopses development on different branches

In rice (Oryza sativa), caryopses located on proximal secondary branches (CSBs) have smaller grain size and poorer grain filling than those located on apical primary branches (CPBs), greatly limiting grain yield. However, the molecular mechanism responsible for developmental differences between CPBs and CSBs remains elusive. In this transcriptome-wide expression study, we identified the gene Aspartic Protease 1 (OsAsp1), which reaches an earlier and higher transcriptional peak in CPBs than in CSBs after pollination. Disruption of OsAsp1 expression in the heterozygous T-DNA line asp1-1^+/–eliminated developmental differences between CPBs and CSBs. OsAsp1 negatively regulated the transcriptional inhibitor of auxin biosynthesis, OsTAA1 transcriptional inhibition factor 1 (OsTIF1), to preserve indole-3-acetic acid (IAA) apical dominance in CPBs and CSBs. IAA also facilitated OsTIF1 translocation from the endoplasmic reticulum (ER) to the nucleus by releasing the interaction of OsTIF1 with OsAsp1 to regulate caryopses IAA levels via a feedback loop. IAA promoted transcription of OsAsp1 through MADS29 to maintain an OsAsp1 differential between CPBs and CSBs during pollination. Together, these findings provide a mechanistic explanation for the distributed auxin differential between CPBs and CSBs to regulate distinct caryopses development in different rice branches and potential targets for engineering yield improvement in crops.

Rice is a major food crop and an important model plant. Compared with caryopses on apical primary branches (CPBs) of rice, those located on proximal secondary branches (CSBs) display smaller grains and poor grain filling, which greatly limit rice yield potential fulfilment, especially among ‘super’ rice cultivars. In this study, we demonstrated that high indole-3-acetic (IAA) levels upregulated Aspartic Protease 1 (OsAsp1) transcription via MADS29 post-pollination to produce higher OsAsp1 levels in CPBs than in CSBs. OsAsp1 then interacted with OsTAA1 transcriptional inhibition factor 1 (OsTIF1) in the endoplasmic reticulum (ER) to dispel OsTIF1 transcriptional inhibition of OsTAA1, causing IAA content to peak in CPBs at 5 days after fertilisation (DAF). IAA facilitated OsTIF1 translocation from the ER to the nucleus by reducing its interaction with OsAsp1 as feedback regulation of IAA levels in caryopses. Thus, differential auxin levels between CPBs and CSBs are determined by the OsAsp1-OsTIF1 complex, and are essential for the distinct development of CPBs and CSBs, providing potential targets for engineering yield improvement in crops.

Molecular cloning, expression and characterization of aspartyl protease inhibitor from Ancylostoma ceylanicum

Aspartyl protease inhibitors (APIs) from parasitic intestinal nematodes are highly immunogenic and have been suggested as potential vaccine antigens. Ac-API-1 from Ancylostoma caninum showed strong immunogenicity and its polyclonal antibodies could specifically recognize the excretory/secretory products of adult worms. However, little is known about molecular characteristics and biological function of API from Ancylostoma ceylanicum (Ace-API). In this study, the Ace-API mature peptide coding sequence was cloned and expressed, and molecular characteristics of its full length sequence were analyzed. Ace-API cDNA was 684 bp in length, which encoded 228 amino acids. The similarity of the Ace-API amino acid sequence to Ac-API-1 and Adu-API-1 was 96.93% and 96.49%, respectively, and they clustered together in the phylogenetic tree. Escheria coli-expressed recombinant protein was mainly soluble in the supernatant of bacterial cell lysate. Western blot showed that Ace-API protein had good reactivity to the serum of infected dogs. Pepsin inhibition assay revealed that the recombinant protein had inhibitory activity on pepsin. Immunofluorescence results demonstrated that Ace-API was mainly localized to the epidermis, excretory glands, and pseudocoelomic fluid of the adult. Using the quantitative real-time PCR, the expression of Ace-api mRNA in adults was significantly higher than that in the third stage (L3) larvae. Together, these data indicate that Ace-API is secreted extracellularly by the parasite, and might play a role in protecting the parasite against the proteolytic digestion by the host proteases, which stimulate further studies to explore this protein as a potential hookworm vaccine candidate.

Apoplastic Proteases: Powerful Weapons against Pathogen Infection in Plants

Plants associate with diverse microbes that exert beneficial, neutral, or pathogenic effects inside the host. During the initial stages of invasion, the plant apoplast constitutes a hospitable environment for invading microbes, providing both water and nutrients. In response to microbial infection, a number of secreted proteins from host cells accumulate in the apoplastic space, which is related to microbial association or colonization processes. However, the molecular mechanisms underlying plant modulation of the apoplast environment and how plant-secreted proteases are involved in pathogen resistance are still poorly understood. Recently, several studies have reported the roles of apoplastic proteases in plant resistance against bacteria, fungi, and oomycetes. On the other hand, microbe-secreted proteins directly and/or indirectly inhibit host-derived apoplastic proteases to promote infection. These findings illustrate the importance of apoplastic proteases in plant-microbe interactions. Therefore, understanding the protease-mediated apoplastic battle between hosts and pathogens is of fundamental importance for understanding plant-pathogen interactions. Here, we provide an overview of plant-microbe interactions in the apoplastic space. We define the apoplast, summarize the physical and chemical properties of these structures, and discuss the roles of plant apoplastic proteases and pathogen protease inhibitors in host-microbe interactions. Challenges and future perspectives for research into protease-mediated apoplastic interactions are discussed, which may facilitate the engineering of resistant crops.

Roles of Plant-Specific Inserts in Plant Defense

Ubiquitously expressed in plants, the plant-specific insert (PSI) of typical plant aspartic proteases (tpAPs) has been associated with plant development, stress response, and defense processes against invading pathogens. Despite sharing high sequence identity, structural studies revealed possible different mechanisms of action among species. The PSI induces signaling pathways of defense hormones in vivo and demonstrates broad-spectrum activity against phytopathogens in vitro. Recent characterization of the PSI-tpAP relationship uncovered novel, nonconventional intracellular protein transport pathways and improved tpAP production yields for industrial applications. In spite of research to date, relatively little is known about the structure-function relationships of PSIs. A comprehensive understanding of their biological roles may benefit plant protection strategies against virulent phytopathogens.

Molecular Properties and New Potentials of Plant Nepenthesins

Nepenthesins are aspartic proteases (APs) categorized under the A1B subfamily. Due to nepenthesin-specific sequence features, the A1B subfamily is also named nepenthesin-type aspartic proteases (NEPs). Nepenthesins are mostly known from the pitcher fluid of the carnivorous plant Nepenthes, where they are availed for the hydrolyzation of insect protein required for the assimilation of insect nitrogen resources. However, nepenthesins are widely distributed within the plant kingdom and play significant roles in plant species other than Nepenthes. Although they have received limited attention when compared to other members of the subfamily, current data indicates that they have exceptional molecular and biochemical properties and new potentials as fungal-resistance genes. In the current review, we provide insights into the current knowledge on the molecular and biochemical properties of plant nepenthesins and highlights that future focus on them may have strong potentials for industrial applications and crop trait improvement.

Impact of Autophagy on Gene Expression and Tapetal Programmed Cell Death During Pollen Development in Rice

Autophagy has recently been shown to be required for tapetal programmed cell death (PCD) and pollen maturation in rice. A transcriptional regulatory network is also known to play a key role in the progression of tapetal PCD. However, the relationship between the gene regulatory network and autophagy in rice anther development is mostly unknown. Here, we comprehensively analyzed the effect of autophagy disruption on gene expression profile during the tapetal PCD in rice anther development using high-throughput RNA sequencing. Expression of thousands of genes, including specific transcription factors and several proteases required for tapetal degradation, fluctuated synchronously at specific stages during tapetal PCD progression in the wild-type anthers, while this fluctuation showed significant delay in the autophagy-deficient mutant Osatg7-1. Moreover, gene ontology enrichment analysis in combination with self-organizing map clustering as well as pathway analysis revealed that the expression patterns of a variety of organelle-related genes as well as genes involved in carbohydrate/lipid metabolism were affected in the Osatg7-1 mutant during pollen maturation. These results suggest that autophagy is required for proper regulation of gene expression and quality control of organelles and timely progression of tapetal PCD during rice pollen development.

Genome-wide characterization of aspartic protease (AP) gene family in Populus trichocarpa and identification of the potential PtAPs involved in wood formation

BACKGROUND

Aspartic protease (AP) is one of four large proteolytic enzyme families that are involved in plant growth and development. Little is known about the AP gene family in tree species, although it has been characterized in Arabidopsis, rice and grape. The AP genes that are involved in tree wood formation remain to be determined.

RESULTS

A total of 67 AP genes were identified in Populus trichocarpa (PtAP) and classified into three categories (A, B and C). Chromosome mapping analysis revealed that two-thirds of the PtAP genes were located in genome duplication blocks, indicating the expansion of the AP family by segmental duplications in Populus. The microarray data from the Populus eFP browser demonstrated that PtAP genes had diversified tissue expression patterns. Semi-qRT-PCR analysis further determined that more than 10 PtAPs were highly or preferentially expressed in the developing xylem. When the involvement of the PtAPs in wood formation became the focus, many SCW-related cis-elements were found in the promoters of these PtAPs. Based on PtAP_promoter::GUS techniques, the activities of PtAP66 promoters were observed only in fiber cells, not in the vessels of stems as the xylem and leaf veins developed in the transgenic Populus tree, and strong GUS signals were detected in interfascicular fiber cells, roots, anthers and sepals of PtAP17_promoter::GUS transgenic plants. Intensive GUS activities in various secondary tissues implied that PtAP66 and PtAP17 could function in wood formation. In addition, most of the PtAP proteins were predicted to contain N- and (or) O-glycosylation sites, and the integration of PNGase F digestion and western blotting revealed that the PtAP17 and PtAP66 proteins were N-glycosylated in Populus.

CONCLUSIONS

Comprehensive characterization of the PtAP genes suggests their functional diversity during Populus growth and development. Our findings provide an overall understanding of the AP gene family in trees and establish a better foundation to further describe the roles of PtAPs in wood formation.

An atypical aspartic protease modulates lateral root development in Arabidopsis thaliana

Few atypical aspartic proteases (APs) present in plants have been functionally studied to date despite having been implicated in developmental processes and stress responses. Here we characterize a novel atypical AP that we name Atypical Aspartic Protease in Roots 1 (ASPR1), denoting its expression in Arabidopsis roots. Recombinant ASPR1 produced by transient expression in Nicotiana benthamiana was active and displayed atypical properties, combining optimum acidic pH, partial sensitivity to pepstatin, pronounced sensitivity to redox agents, and unique specificity preferences resembling those of fungal APs. ASPR1 overexpression suppressed primary root growth and lateral root development, implying a previously unknown biological role for an AP. Quantitative comparison of wild-type and aspr1 root proteomes revealed deregulation of proteins associated with both reactive oxygen species and auxin homeostasis in the mutant. Together, our findings on ASPR1 reinforce the diverse pattern of enzymatic properties and biological roles of atypical APs and raise exciting questions on how these distinctive features impact functional specialization among these proteases.

Atypical and nucellin-like aspartic proteases: emerging players in plant developmental processes and stress responses

Members of the pepsin-like family (A1) of aspartic proteases (APs) are widely distributed in plants. A large number of genes encoding putative A1 APs are found in different plant genomes, the vast majority of which exhibit distinct features when compared with the so-called typical APs (and, therefore, grouped as atypical and nucellin-like APs). These features include the absence of the plant-specific insert; an unusually high number of cysteine residues; the nature of the amino acids preceding the first catalytic aspartate; and unexpected localizations. The over-representation of atypical and nucellin-like APs in plants is suggestive of greater diversification of protein functions and a more regulatory role for these APs, as compared with the housekeeping function generally attributed to typical APs. New functions have been uncovered for non-typical APs, with proposed roles in biotic and abiotic stress responses, chloroplast metabolism, and reproductive development, clearly suggesting functional specialization and tight regulation of activity. Furthermore, unusual enzymatic properties have also been documented for some of these proteases. Here, we give an overview of the current knowledge on the distinctive features and functions of both atypical and nucellin-like APs, and discuss this emerging pattern of functional complexity and specialization among plant pepsin-like proteases.

Proteomics Analysis Reveals That Caspase-Like and Metacaspase-Like Activities Are Dispensable for Activation of Proteases Involved in Early Response to Biotic Stress in Triticum aestivum L

Plants, including Triticum aestivum L., are constantly attacked by various pathogens which induce immune responses. Immune processes in plants are tightly regulated by proteases from different families within their degradome. In this study, a wheat degradome was characterized. Using profile hidden Markov model (HMMer) algorithm and Pfam database, comprehensive analysis of the T. aestivum genome revealed a large number of proteases (1544 in total) belonging to the five major protease families: serine, cysteine, threonine, aspartic, and metallo-proteases. Mass-spectrometry analysis revealed a 30% difference between degradomes of distinct wheat cultivars (Khakasskaya and Darya), and infection by biotrophic (Puccinia recondita Rob. ex Desm f. sp. tritici) or necrotrophic (Stagonospora nodorum) pathogens induced drastic changes in the presence of proteolytic enzymes. This study shows that an early immune response to biotic stress is associated with the same core of proteases from the C1, C48, C65, M24, M41, S10, S9, S8, and A1 families. Further liquid chromatography-mass spectrometry (LC-MS) analysis of the detected protease-derived peptides revealed that infection by both pathogens enhances overall proteolytic activity in wheat cells and leads to activation of proteolytic cascades. Moreover, sites of proteolysis were identified within the proteases, which probably represent targets of autocatalytic activation, or hydrolysis by another protease within the proteolytic cascades. Although predicted substrates of metacaspase-like and caspase-like proteases were similar in biotrophic and necrotrophic infections, proteolytic activation of proteases was not found to be associated with metacaspase-like and caspase-like activities. These findings indicate that the response of T. aestivum to biotic stress is regulated by unique mechanisms.

Whole-grain oats (Avena sativa L.) as a carrier of lactic acid bacteria and a supplement rich in angiotensin I-converting enzyme inhibitory peptides through solid-state fermentation

This study explored a novel strategy to develop solid-state whole-grain oats as a novel carrier of lactic acid bacteria and a nutraceutical supplement rich in ACE inhibitory peptides. Oats were fermented by Lactobacillus plantarum B1-6, Rhizopus oryzae, or a combination of L. plantarum B1-6 and R. oryzae. L. plantarum showed a much better growth performance in oats when it was combined with R. oryzae than when it was cultured alone, as evidenced by an increase in viable cell count to 9.70 log cfu g-1 after 72 h of fermentation. The coinoculated fermented oats (CFO) and the R. oryzae-fermented oats (RFO) were then selected for investigations on protein hydrolysis and on the functional properties of the released bioactive peptides. The results showed that the soluble protein contents changed from 7.05 mg g-1 to 14.43 and 10.21 mg g-1 for CFO and RFO, respectively. However, the degree of hydrolysis and the content of peptides with molecular masses less than 10 000 Da indicated that the CFO proteins can be degraded to a greater degree. As analyzed by electrophoresis and reversed-phase high-performance liquid chromatography, the protein and peptide profiles of CFO and RFO demonstrated that the proteins from CFO were more obviously hydrolyzed and more small peptides were obtained. In addition, both CFO and RFO presented higher ACE inhibitory activities than unfermented oats, whereas the protein extracts from CFO exerted a lower IC50 value of 0.42 mg protein per mL compared with the protein extracts from the other samples. This research has broadened our knowledge on the development of whole-grain oat products as a probiotic carrier and on the difference between mixed solid-state fermentation (SSF) and fungi SSF in terms of protein degradation and the capacity to release ACE inhibitory peptides. Our approach could be used to obtain probiotic food products and probably to develop oats as a potential therapeutic ingredient targeting hypertension.

Expression, activation and processing of a novel plant milk-clotting aspartic protease in Pichia pastoris

Galium verum, also known as Lady's Bedstraw or Cheese Rennet, is an herbaceous perennial plant traditionally used in cheese-making. We used RACE PCR to isolate novel enzymes from Galium verum with the ability to clot milk. This approach generated two cDNA sequences (named preprogaline A and B) encoding proteins displaying the typical plant aspartic protease primary structure. Preprogaline B was expressed in the yeast Pichia pastoris, after deleting and replacing its original signal peptide with the yeast α-factor signal peptide from Saccharomyces cerevisiae. The secreted recombinant protein was obtained by growing P. pastoris in YPD medium and had the ability to clot milk. The mature form of progaline B is a heterodimeric glycosylated enzyme, with a molecular weight of approximately 48 kDa, that contains a heavy (30.7 kDa) and a light (13.5 kDa) polypeptide chains linked by disulfide bonds. Western blot analysis revealed that progaline B is activated by the acidification of the yeast culture medium and that enzymatic activation requires two steps. First the precursor protein is cleaved into two polypeptide chains by partial removal of the plant-specific insert (PSI) present in plant aspartic proteases; this is later followed by propeptide removal. By altering the pH of the P. pastoris culture medium, we were able to obtain either active or inactive forms of the enzyme. Recombinant progaline B displayed a κ-casein hydrolysis pattern analogous to those produced by the animal and microbial coagulants currently used in the dairy industry, but it exhibited a different digestion profile on α- and β-caseins. The plant protease progaline B displays milk-clotting activities suitable for the production of novel dairy products.

Genome resequencing and transcriptome profiling reveal structural diversity and expression patterns of constitutive disease resistance genes in Huanglongbing-tolerant Poncirus trifoliata and its hybrids

Huanglongbing (HLB) is the most destructive bacterial disease of citrus worldwide. While most citrus varieties are susceptible to HLB, Poncirus trifoliata, a close relative of Citrus, and some of its hybrids with Citrus are tolerant to HLB. No specific HLB tolerance genes have been identified in P. trifoliata but recent studies have shown that constitutive disease resistance (CDR) genes were expressed at much higher levels in HLB-tolerant Poncirus hybrids and the expression of CDR genes was modulated by Candidatus Liberibacter asiaticus (CLas), the pathogen of HLB. The current study was undertaken to mine and characterize the CDR gene family in Citrus and Poncirus and to understand its association with HLB tolerance in Poncirus. We identified 17 CDR genes in two citrus genomes, deduced their structures, and investigated their phylogenetic relationships. We revealed that the expansion of the CDR family in Citrus seems to be due to segmental and tandem duplication events. Through genome resequencing and transcriptome sequencing, we identified eight CDR genes in the Poncirus genome (PtCDR1-PtCDR8). The number of SNPs was the highest in PtCDR2 and the lowest in PtCDR7. Most of the deletion and insertion events were observed in the UTR regions of Citrus and Poncirus CDR genes. PtCDR2 and PtCDR8 were in abundance in the leaf transcriptomes of two HLB-tolerant Poncirus genotypes and were also upregulated in HLB-tolerant, Poncirus hybrids as revealed by real-time PCR analysis. These two CDR genes seem to be good candidate genes for future studies of their role in citrus-CLas interactions.

OsDi19-4 acts downstream of OsCDPK14 to positively regulate ABA response in rice

The drought-induced 19 protein family consists of several atypical Cys2/His2-type zinc finger proteins in plants and plays an important role in abiotic stress. In this study, we found that overexpressing OsDi19-4 in rice altered the expression of a series of abscisic acid (ABA)-responsive genes, resulting in strong ABA-hypersensitive phenotypes including ABA-induced seed germination inhibition, early seedling growth inhibition and stomatal closure. On the contrary, OsDi19-4 knockdown lines were less sensitive to ABA. Additionally, OsCDPK14 was identified to interact with OsDi19-4 and be responsible for the phosphorylation of OsDi19-4, and the phosphorylation of OsDi19-4 was further enhanced after the treatment of ABA. Apart from these, OsDi19-4 was shown to directly bind to the promoters of OsASPG1 and OsNAC18 genes, two ABA-responsive genes, and regulate their expression. Transient expression assays confirmed the direct regulation role of OsDi19-4, and the regulation was further enhanced by the increased phosphorylation of OsDi19-4 after the treatment of ABA. Taken together, these data demonstrate that OsDi19-4 acts downstream of OsCDPK14 to positively regulate ABA response by modulating the expression of ABA-responsive genes in rice.

Cupincin: A Unique Protease Purified from Rice (Oryza sativa L.) Bran Is a New Member of the Cupin Superfamily

Cupin superfamily is one of the most diverse super families. This study reports the purification and characterization of a novel cupin domain containing protease from rice bran for the first time. Hypothetical protein OsI_13867 was identified and named as cupincin. Cupincin was purified to 4.4 folds with a recovery of 4.9%. Cupincin had an optimum pH and temperature of pH 4.0 and 60°C respectively. Cupincin was found to be a homotrimer, consisting of three distinct subunits with apparent molecular masses of 33.45 kDa, 22.35 kDa and 16.67 kDa as determined by MALDI-TOF, whereas it eluted as a single unit with an apparent molecular mass of 135.33 ± 3.52 kDa in analytical gel filtration and migrated as a single band in native page, suggesting its homogeneity. Sequence identity of cupincin was deduced by determining the amino-terminal sequence of the polypeptide chains and by and de novo sequencing. For understanding the hydrolysing mechanism of cupincin, its three-dimensional model was developed. Structural analysis indicated that cupincin contains His313, His326 and Glu318 with zinc ion as the putative active site residues, inhibition of enzyme activity by 1,10-phenanthroline and atomic absorption spectroscopy confirmed the presence of zinc ion. The cleavage specificity of cupincin towards oxidized B-chain of insulin was highly specific; cleaving at the Leu₁₅-Tyr₁₆ position, the specificity was also determined using neurotensin as a substrate, where it cleaved only at the Glu₁-Tyr₂ position. Limited proteolysis of the protease suggests a specific function for cupincin. These results demonstrated cupincin as a completely new protease.

Identification of altered metabolic pathways of γ-irradiated rice mutant via network-based transcriptome analysis

In order to develop rice mutants for crop improvement, we applied γ-irradiation mutagenesis and selected a rice seed color mutant (MT) in the M14 targeting-induced local lesions in genome lines. This mutant exhibited differences in germination rate, plant height, and root length in seedlings compared to the wild-type plants. We found 1645 different expressed probes of MT by microarray hybridization. To identify the modified metabolic pathways, we conducted integrated genomic analysis such as weighted correlation network analysis with a module detection method of differentially expressed genes (DEGs) in MT on the basis of large-scale microarray transcriptional profiling. These modules are largely divided into three subnetworks and mainly exhibit overrepresented gene ontology functions such as oxidation-related function, ion-binding, and kinase activity (phosphorylation), and the expressional coherences of module genes mainly exhibited in vegetative and maturation stages. Through a metabolic pathway analysis, we detected the significant DEGs involved in the major carbohydrate metabolism (starch degradation), protein degradation (aspartate protease), and signaling in sugars and nutrients. Furthermore, the accumulation of amino acids (asparagine and glutamic acid), sucrose, and starch in MT were affected by gamma rays. Our results provide an effective approach for identification of metabolic pathways associated with useful agronomic traits in mutation breeding.

Sweet potato SPAP1 is a typical aspartic protease and participates in ethephon-mediated leaf senescence

Plant aspartic proteases are generally divided into three categories: typical, nucellin-like, and atypical aspartic proteases based on their gene and protein structures. In this report, a full-length cDNA SPAP1 was cloned from sweet potato leaves, which contained 1515 nucleotides (504 amino acids) and exhibited high amino acid sequence identity (ca. 51-72%) with plant typical aspartic proteases, including tomato LeAspP, potato StAsp, and wheat WAP2. SPAP1 also contained conserved DTG and DSG amino acid residues within its catalytic domain and plant specific insert (PSI) at the C-terminus. The cDNA corresponding to the mature protein (starting from the 66th to 311th amino acid residues) without PSI domain was constructed with pET30a expression vector for fusion protein and antibody production. RT-PCR and protein blot hybridization showed that SPAP1 expression level was the highest in L3 mature leaves, then gradually declined until L5 completely yellow leaves. Ethephon, an ethylene-releasing compound, also enhanced SPAP1 expression at the time much earlier than the onset of leaf senescence. Exogenous application of SPAP1 fusion protein promoted ethephon-induced leaf senescence, which could be abolished by pre-treatment of SPAP1 fusion protein with (a) 95 °C for 5 min, (b) aspartic protease inhibitor pepstatin A, and (c) anti-SPAP1 antibody, respectively. Exogenous SPAP1 fusion protein, whereas, did not significantly affect leaf senescence under dark. These data conclude that sweet potato SPAP1 is a functional typical aspartic protease and participates in ethephon-mediated leaf senescence. The SPAP1-promoted leaf senescence and its activity are likely not associated with the PSI domain. Interaction of ethephon-inducible components for effective SPAP1 promotion on leaf senescence is also suggested.

Comparative transcriptome analyses reveal core parasitism genes and suggest gene duplication and repurposing as sources of structural novelty

The origin of novel traits is recognized as an important process underlying many major evolutionary radiations. We studied the genetic basis for the evolution of haustoria, the novel feeding organs of parasitic flowering plants, using comparative transcriptome sequencing in three species of Orobanchaceae. Around 180 genes are upregulated during haustorial development following host attachment in at least two species, and these are enriched in proteases, cell wall modifying enzymes, and extracellular secretion proteins. Additionally, about 100 shared genes are upregulated in response to haustorium inducing factors prior to host attachment. Collectively, we refer to these newly identified genes as putative “parasitism genes.” Most of these parasitism genes are derived from gene duplications in a common ancestor of Orobanchaceae and Mimulus guttatus, a related nonparasitic plant. Additionally, the signature of relaxed purifying selection and/or adaptive evolution at specific sites was detected in many haustorial genes, and may play an important role in parasite evolution. Comparative analysis of gene expression patterns in parasitic and nonparasitic angiosperms suggests that parasitism genes are derived primarily from root and floral tissues, but with some genes co-opted from other tissues. Gene duplication, often taking place in a nonparasitic ancestor of Orobanchaceae, followed by regulatory neofunctionalization, was an important process in the origin of parasitic haustoria.

Response of an aspartic protease gene OsAP77 to fungal, bacterial and viral infections in rice

BACKGROUND

Aspartic protease (APs) plays important roles in plant growth, development and biotic and abiotic stresses. We previously reported that the expression of a rice AP gene (OsAP77, Os10g0537800) was induced by probenazole (PBZ), a chemical inducer of disease resistance. In this study we examined some characteristics of this gene in response to fungal, bacterial and viral pathogens.

RESULTS

To elucidate the spatial and temporal expression of OsAP77, the chimeric gene was constructed carrying the structural gene encoding β-glucuronidase (GUS) driven by the OsAP77 promoter. This construct was introduced into rice and the transgenic lines were tested to analyze gene expression by fungal, bacterial and viral infections. Inoculation with Magnaporthe oryzae or Xanthomonas oryzae pv. oryzae revealed the enhanced GUS activities in vascular tissues surrounding the symptom sites by each pathogen. Moreover, GUS activity also increased after inoculation with Cucumber mosaic virus (CMV). Transgenic plants immersed in a solution containing salicylic acid (SA), isonicotinic acid (INA), hydrogen peroxide (H2O2) or abscisic acid (ABA) showed an increased level of GUS activity exclusively in vascular tissues. RT-PCR analysis showed that OsAP77 was induced not only by infection with these pathogens, but also after treatment with SA, INA, H2O2 or ABA. A knockout mutant line of OsAP77 by the insertion of Tos17 after inoculation with M. oryzae, X. oryzae pv. oryzae or CMV showed an enhanced susceptibility compared to wild type.

CONCLUSION

These results suggest that the expression of OsAP77 is induced by pathogen infection and defense related signaling molecules in a vascular tissue specific manner and that this gene has a positive role of defense response against fungal, bacterial and viral infections.

Genome-wide identification, evolutionary and expression analysis of the aspartic protease gene superfamily in grape

Background

Aspartic proteases (APs) are a large family of proteolytic enzymes found in almost all organisms. In plants, they are involved in many biological processes, such as senescence, stress responses, programmed cell death, and reproduction. Prior to the present study, no grape AP gene(s) had been reported, and their research on woody species was very limited.

Results

In this study, a total of 50 AP genes (VvAP) were identified in the grape genome, among which 30 contained the complete ASP domain. Synteny analysis within grape indicated that segmental and tandem duplication events contributed to the expansion of the grape AP family. Additional analysis between grape and Arabidopsis demonstrated that several grape AP genes were found in the corresponding syntenic blocks of Arabidopsis, suggesting that these genes arose before the divergence of grape and Arabidopsis. Phylogenetic relationships of the 30 VvAPs with the complete ASP domain and their Arabidopsis orthologs, as well as their gene and protein features were analyzed and their cellular localization was predicted. Moreover, expression profiles of VvAP genes in six different tissues were determined, and their transcript abundance under various stresses and hormone treatments were measured. Twenty-seven VvAP genes were expressed in at least one of the six tissues examined; nineteen VvAPs responded to at least one abiotic stress, 12 VvAPs responded to powdery mildew infection, and most of the VvAPs responded to SA and ABA treatments. Furthermore, integrated synteny and phylogenetic analysis identified orthologous AP genes between grape and Arabidopsis, providing a unique starting point for investigating the function of grape AP genes.

Conclusions

The genome-wide identification, evolutionary and expression analyses of grape AP genes provide a framework for future analysis of AP genes in defining their roles during stress response. Integrated synteny and phylogenetic analyses provide novel insight into the functions of less well-studied genes using information from their better understood orthologs.

OsAP65, a rice aspartic protease, is essential for male fertility and plays a role in pollen germination and pollen tube growth

Aspartic proteases (APs) comprise a large proteolytic enzyme family widely distributed in animals, microbes, viruses, and plants. The rice genome encodes 96 APs, of which only a few have been functionally characterized. Here, the identification and characterization of a novel AP gene, OsAP65, which plays an indispensable role in pollen tube growth in rice, is reported. The T-DNA insertion line of OsAP65 caused severe segregation distortion. In the progeny derived from an individual heterozygous for the T-DNA insertion, the wild type and T-DNA-carrying heterozygote segregated at a ratio close to 1:1, while homozygotes of disrupted OsAP65 (OsAP65-/-) were not recovered. Reciprocal crosses between heterozygotes and wild-type plants demonstrated that the mutant alleles could not be transmitted through the male gamete. Examination of the anthers from heterozygous plants revealed that the mutant pollen matured normally, but did not germinate or elongate. OsAP65 was expressed in various tissues and the transcript level in heterozygous plants was about half of the amount measured in the wild-type plants. The subcellular localization showed that OsAP65 is a pre-vacuolar compartment (PVC) protein. These results indicated that OsAP65 was essential for rice pollen germination and tube growth.

EAT1 promotes tapetal cell death by regulating aspartic proteases during male reproductive development in rice

Programmed cell death is essential for the development of multicellular organisms, yet pathways of plant programmed cell death and its regulation remain elusive. Here we report that ETERNAL TAPETUM 1, a basic helix-loop-helix transcription factor conserved in land plants, positively regulates programmed cell death in tapetal cells in rice anthers. eat1 exhibits delayed tapetal cell death and aborted pollen formation. ETERNAL TAPETUM 1 directly regulates the expression of OsAP25 and OsAP37, which encode aspartic proteases that induce programmed cell death in both yeast and plants. Expression and genetic analyses revealed that ETERNAL TAPETUM 1 acts downstream of TAPETUM DEGENERATION RETARDATION, another positive regulator of tapetal programmed cell death, and that ETERNAL TAPETUM 1 can also interact with the TAPETUM DEGENERATION RETARDATION protein. This study demonstrates that ETERNAL TAPETUM 1 promotes aspartic proteases triggering plant programmed cell death, and reveals a dynamic regulatory cascade in male reproductive development in rice.

Chlapsin, a chloroplastidial aspartic proteinase from the green algae Chlamydomonas reinhardtii

Aspartic proteinases have been extensively characterized in land plants but up to now no evidences for their presence in green algae group have yet been reported in literature. Here we report on the identification of the first (and only) typical aspartic proteinase from Chlamydomonas reinhardtii. This enzyme, named chlapsin, was shown to maintain the primary structure organization of typical plant aspartic proteinases but comprising distinct features, such as similar catalytic motifs DTG/DTG resembling those from animal and microbial counterparts, and an unprecedentedly longer plant specific insert domain with an extra segment of 80 amino acids, rich in alanine residues. Our results also demonstrated that chlapsin accumulates in Chlamydomonas chloroplast bringing this new enzyme to a level of uniqueness among typical plant aspartic proteinases. Chlapsin was successfully expressed in Escherichia coli and it displayed the characteristic enzymatic properties of typical aspartic proteinases, like optimum activity at acidic pH and complete inhibition by pepstatin A. Another difference to plant aspartic proteinases emerged as chlapsin was produced in an active form without its putative prosegment domain. Moreover, recombinant chlapsin showed a restricted enzymatic specificity and a proteolytic activity influenced by the presence of redox agents and nucleotides, further differentiating it from typical plant aspartic proteinases and anticipating a more specialized/regulated function for this Chlamydomonas enzyme. Taken together, our results revealed a pattern of complexity for typical plant aspartic proteinases in what concerns sequence features, localization and biochemical properties, raising new questions on the evolution and function of this vast group of plant enzymes.

Expression profile analysis of the polygalacturonase-inhibiting protein genes in rice and their responses to phytohormones and fungal infection

Polygalacturonase-inhibiting proteins (PGIPs) are typically leucine-rich repeat (LRR) proteins that can inhibit the activity of fungal polygalacturonases (PGs). In this study, two new Ospgip genes, named Ospgip6 and Ospgip7 with consensus sequence of ten imperfect LRR motif located on rice chromosomes 8 and 9, were identified using BLAST analysis. Both of them appear to be extracellular glycoproteins. To have a global view of the dynamic gene expression pattern, seven Ospgip genes were first analyzed using the Affymetrix rice genome array data from online resource. All of these seven Ospgip genes showed variable expression patterns among tissues/organs. In order to further investigate the potential function of these Ospgip genes, the responses of Ospgip genes to the treatment of various phytohormones (abscisic acid, brassinosteroid, gibberellic acid, 3-indole acetic acid, jasmonic acid, kinetin, naphthalene acetic acid and salicylic acid) as well as fungal infection were analyzed by real-time PCR using time course array. Generally, all the Ospgip genes were slightly up-regulated in the indica rice cultivar Minghui 63 under GA(3), KT and NAA treatments (except Ospgip2, which was down-regulated under KT treatment). In the japonica rice cultivar Zhonghua 11, Ospgip genes were regulated by most treatments with the response time variability. We also analyzed putative cis-elements in the promoter regions of Ospgip genes. This dataset provided a versatile resource to understand the regulatory network of Ospgip genes during the process of phytohormones treatment and fungal infection in the model monocotyledonous plant, rice, and could aid in the transgenic breeding against rice fungal diseases.

KEY MESSAGE

All the seven Ospgip genes showed variable expression patterns in Minghui 63 and their expressions were regulated by different phytohormone treatments or fungal infection in Minghui 63 and Zhonghua 11.

Molecular basis underlying the S5-dependent reproductive isolation and compatibility of indica/japonica rice hybrids

The S5 locus regulates spikelet fertility of indica/japonica hybrid rice (Oryza sativa). There are three alleles at the S5 locus, including an indica allele (S5i), a japonica allele (S5j), and a wide-compatibility allele (S5n). This study analyzed the molecular basis for S5-dependent reproductive isolation and compatibility of indica/japonica rice hybrids. Three S5 alleles were expressed at extremely low levels, and only in the ovary. S5n was more similar to S5i in both RNA and protein expression profiles. The S5 locus was not essential for embryo sac development, although deleterious interactions between S5i and S5j resulted in reduced rates of spikelet fertility. The yeast two-hybrid system was used to test direct interactions between S5-encoded proteins. The results indicated that the S5i- and S5j-encoded eukaryotic aspartyl proteases formed both homodimers and heterodimers, whereas the S5n-encoded aspartyl protease was incapable of dimerization. Site-directed mutagenesis revealed that a single amino acid difference between S5i- and S5j-encoded aspartyl proteases (phenylalanine/leucine at residue 273) was primarily responsible for embryo sac abortion. The S5 locus may have promoted the subspeciation of indica and japonica, but it also enables gene flow between them.

Plant protein-coding gene families: emerging bioinformatics approaches

Protein-coding gene families are sets of similar genes with a shared evolutionary origin and, generally, with similar biological functions. In plants, the size and role of gene families has been only partially addressed. However, suitable bioinformatics tools are being developed to cluster the enormous number of sequences currently available in databases. Specifically, comparative genomic databases promise to become powerful tools for gene family annotation in plant clades. In this review, I evaluate the data retrieved from various gene family databases, the ease with which they can be extracted and how useful the extracted information is.

Rice functional genomics research: progress and implications for crop genetic improvement

Rice is a staple food crop and has become a reference of monocot plant for functional genomic research. With the availability of high quality rice genome sequence, there has been rapid accumulation of functional genomic resources, including: large mutant libraries by T-DNA insertion, transposon tagging, and chemical mutagenesis; global expression profiles of the genes in the entire life cycle of rice growth and development; full-length cDNAs for both indica and japonica rice; sequences from resequencing large numbers of diverse germplasm accessions. Such resource development has greatly accelerated gene cloning. By the end of 2010, over 600 genes had been cloned using various methods. Many of the genes control agriculturally useful traits such as yield, grain quality, resistances to biotic and abiotic stresses, and nutrient-use efficiency, thus have potential utility in crop genetic improvement. This review was aimed to provide a comprehensive summary of such progress. We also presented our perspective for future studies.