Recombinant Oryza sativa subsp. indica CASP-like protein OsI_01913 (OsI_01913)

What is OsI_01913 and what is its function in rice plants?

OsI_01913, also known as CASP-like protein 1E1 or OsCASPL1E1, is a member of the Casparian strip membrane domain protein (CASP) family in rice (Oryza sativa subsp. indica) with UniProt ID B8A7Z5 . This protein likely plays a crucial role in the formation of Casparian strips (CS) in endodermal cells of rice roots, similar to other CASP family members . Casparian strips are specialized cell wall modifications that form barriers in the endodermis, controlling the movement of water and solutes between soil and vascular tissue. Based on comparative analysis with related proteins like OsCASP1, OsI_01913 likely forms part of a transmembrane scaffold that recruits lignin biosynthetic enzymes for CS formation and influences suberin deposition . The protein appears to be involved in maintaining ion homeostasis and potentially in plant responses to environmental stresses, particularly salt stress, which is consistent with the general role of CASP family proteins in nutrient uptake regulation and stress adaptation .

How is recombinant OsI_01913 typically produced for research applications?

Recombinant OsI_01913 protein for research applications is produced using an E. coli expression system with an N-terminal His tag to facilitate purification . The production process involves cloning the full-length OsI_01913 gene sequence (encoding amino acids 1-215) into an appropriate expression vector, transforming E. coli cells, inducing protein expression under controlled conditions, and purifying the His-tagged protein using affinity chromatography . Quality control ensures purity greater than 90% as determined by SDS-PAGE . The purified protein is typically lyophilized and supplied as a powder in Tris/PBS-based buffer with 6% Trehalose at pH 8.0 . For experimental use, the protein should be reconstituted in deionized sterile water to a concentration of 0.1-1.0 mg/mL, with the addition of 5-50% glycerol (recommended default is 50%) for long-term storage at -20°C/-80°C . This standardized production process ensures consistency and reliability for research applications investigating OsI_01913 function.

How does OsI_01913 contribute to Casparian strip formation in rice roots?

While the specific role of OsI_01913 in Casparian strip formation hasn't been directly characterized in the available research, its function can be inferred from studies on related CASP proteins. In Arabidopsis, CASP proteins form a transmembrane scaffold that recruits lignin biosynthetic enzymes for Casparian strip formation . Studies on OsCASP1, a related protein in rice, demonstrate that it orchestrates Casparian strip formation and suberin deposition in rice roots . Loss of OsCASP1 function leads to delayed CS formation, uneven lignin deposition, and altered suberin patterns .

Given that OsI_01913 is classified as a CASP-like protein, it likely contributes to CS formation in specific tissues or developmental stages of rice roots . The CASP_like-I subfamily, to which OsI_01913 probably belongs, has been suggested to function similarly to AtCASP1, AtCASP3, and OsCASP1 in endodermal Casparian strip formation and selective mineral uptake . The protein likely participates in a complex with other CASP family members to establish the precise localization of lignin deposition in endodermal cell walls, creating the apoplastic barrier that regulates nutrient and water movement into the vascular system.

What are the optimal storage and handling conditions for recombinant OsI_01913 protein?

To maintain the stability and activity of recombinant OsI_01913 protein for experimental applications, researchers should follow these specific handling protocols :

The reconstitution protocol involves briefly centrifuging the vial before opening, reconstituting the protein in deionized sterile water to 0.1-1.0 mg/mL, and adding glycerol to a final concentration of 5-50% (with 50% being the recommended default) . The reconstituted protein should be aliquoted to avoid repeated freeze-thaw cycles, which can significantly degrade protein quality and experimental reproducibility . Researchers should verify protein purity through SDS-PAGE before experimental use, with expected purity greater than 90% . These stringent storage and handling conditions are crucial for maintaining the native conformation and functional properties of OsI_01913 in biochemical and cellular assays.

What visualization techniques can be used to study OsI_01913's role in Casparian strip formation?

Several specialized visualization techniques can be employed to investigate OsI_01913's role in Casparian strip formation, based on methodologies described for related CASP proteins :

• Berberine-aniline blue staining provides a powerful approach for visualizing Casparian strips through clearing with lactic acid saturated with chloral hydrate followed by staining . This technique allows both surface observation and cross-sectional analysis, with autofluorescence of cell walls detected as blue . In wild-type rice, the fluorescence of the sclerenchyma appears stronger than that of the endodermis, providing a baseline for comparison with OsI_01913 mutants .

• Phloroglucinol staining specifically detects lignin deposition in the endodermis and Casparian strips . This method can reveal if OsI_01913 mutations alter lignification patterns, such as causing ectopic lignin deposition in the outer peripheral walls of endodermal cells rather than just the radial walls .

• Propidium iodide (PI) penetration assay directly measures barrier function by incubating small lateral roots with 10 μg/ml PI for 10 minutes . Quantification of PI penetration into the stele provides a functional assessment of Casparian strip integrity, revealing where the apoplastic barrier is compromised in OsI_01913 mutants .

• Cross-sectional analysis combined with fluorescence microscopy allows detailed observation of CS structure in different zones of the root . This approach enables precise spatial characterization of how OsI_01913 affects CS development and can be quantified to provide statistical comparisons between wild-type and mutant plants.

These complementary visualization techniques, when applied to compare wild-type plants with OsI_01913 knockouts or overexpression lines, provide comprehensive insights into the protein's role in Casparian strip formation and function.

What approaches are recommended for generating OsI_01913 mutants for functional studies?

Based on successful strategies used for related CASP proteins, several approaches can be employed to generate OsI_01913 mutants for functional studies :

• CRISPR/Cas9 gene editing represents the most precise and efficient approach for creating OsI_01913 knockouts . This technique involves designing guide RNAs targeting exon 1 or other critical regions of the OsI_01913 gene, transforming rice calli with the CRISPR/Cas9 construct, and screening for mutations . When applying this approach to OsCASP1, researchers generated sixteen mutant lines, though viability varied significantly with only one surviving line . This highlights the importance of creating multiple independent lines when targeting OsI_01913.

• Natural mutant screening can identify variants with altered OsI_01913 function from diverse rice populations . This approach was successful in identifying the Oscasp1-3 mutant as a natural variant derived from high-generation progeny of specific rice lines . Screening EMS-mutagenized populations or existing rice germplasm collections can potentially identify OsI_01913 mutations with interesting phenotypes.

• Map-based cloning can identify OsI_01913 mutations responsible for observed phenotypes through genetic mapping . This approach requires crossing plants displaying relevant phenotypes (such as altered root development or stress sensitivity) with reference cultivars and analyzing segregation patterns.

Verification of successful mutagenesis requires a multifaceted approach including PCR-based genotyping, RT-qPCR expression analysis, and detailed phenotypic characterization focusing on root development, Casparian strip formation, and responses to environmental stresses. Creating multiple independent mutant lines with different genetic backgrounds is crucial for distinguishing OsI_01913-specific effects from background-dependent variations.

What complementation strategies can confirm the specific functions of OsI_01913?

To definitively attribute observed phenotypes to OsI_01913 function, several complementation strategies can be employed based on successful approaches used with related CASP proteins :

• Native promoter-gene constructs provide the most physiologically relevant complementation approach . For OsI_01913, researchers should create a construct containing the OsI_01913 promoter (the complete intergenic region upstream of the coding sequence) fused to the OsI_01913 gene . This construct can be cloned into a suitable vector (such as pCAMBIA-1300) using techniques like In-Fusion cloning and transformed into calli derived from OsI_01913 mutant plants . Successful complementation would restore wild-type phenotypes, confirming that the observed defects specifically result from loss of OsI_01913 function.

• Promoter-gene-reporter constructs enable simultaneous complementation and expression pattern analysis . Creating an OsI_01913pro:OsI_01913-GUS construct allows visualization of OsI_01913 expression in different tissues while testing functional rescue . This approach provides insights into where and when OsI_01913 functions during development and under different environmental conditions.

• Cross-species complementation can reveal functional conservation between rice OsI_01913 and related proteins from other species. Introducing OsI_01913 into Arabidopsis casp mutants, or vice versa, would determine whether these proteins can functionally substitute for each other despite evolutionary divergence.

For rigorous complementation analysis, researchers should examine multiple independent transgenic lines and quantify the degree of phenotypic rescue across various parameters including root development, Casparian strip formation, ion homeostasis, and stress tolerance. Statistical comparison between wild-type, mutant, and complemented lines provides conclusive evidence for OsI_01913's specific functions.

How does OsI_01913 compare structurally and functionally with other CASP family proteins?

Comparative analysis of OsI_01913 with other CASP family proteins reveals important structural and functional relationships that inform our understanding of this protein's role in rice :

Structurally, OsI_01913 contains 215 amino acids, placing it within the size range observed for rice CASP proteins (153-421 amino acids) . Like other CASP proteins, it likely contains multiple transmembrane domains that anchor it in the plasma membrane at specific locations during Casparian strip formation . Motif analysis of CASP proteins shows that members of the same subfamilies share conserved sequence motifs, with CASP and CASP_like-I subfamilies (to which OsI_01913 likely belongs) typically containing specific motifs (1, 3, 4, 6, 8, and 9) . These conserved domains likely mediate protein-protein interactions and membrane localization essential for CASP function.

Evolutionarily, CASP proteins in rice and Arabidopsis are organized into distinct subfamilies that reflect their functional specializations . The variation in subfamily composition between rice and Arabidopsis suggests differential adaptation of CASP functions in these species . Collinearity analysis identified six homologous CASP gene pairs between rice and Arabidopsis, including cases where single genes in one species correspond to multiple genes in the other, indicating complex evolutionary relationships .

Functionally, OsI_01913 likely shares roles with other CASP family members in Casparian strip formation and potentially in environmental stress responses . Related proteins like OsCASP1 are involved in Casparian strip formation, suberin deposition, and salt stress tolerance . The close evolutionary relationship between CASP and CASP_like-I subfamilies suggests OsI_01913 may participate in similar processes, potentially with tissue-specific or stress-specific functions .

How is OsI_01913 expression regulated in response to environmental stresses?

While direct information about OsI_01913 expression under environmental stresses is limited, analysis of CASP gene family regulation provides important insights that likely apply to OsI_01913 :

Promoter analysis of CASP genes in rice and Arabidopsis has revealed numerous cis-acting regulatory elements associated with stress responses and hormone signaling . These include elements responsive to drought (MBS), low temperature (LTR), defense and stress (TC-rich repeats), and various plant hormones including auxin, abscisic acid, salicylic acid, gibberellin, ethylene, and methyl jasmonate . The presence of multiple hormone-responsive elements in many CASP genes suggests complex transcriptional regulation integrating diverse environmental signals . For instance, some CASP genes contain responsive elements for abscisic acid, salicylic acid, methyl jasmonate, and ethylene simultaneously, indicating sophisticated stress-response capabilities .

Salt stress significantly impacts CASP expression patterns. OsCASP1, a related protein, shows increased expression under salt stress, particularly in the steles of rice roots . This upregulation correlates with the protein's role in maintaining ion homeostasis during salt stress . Given that loss of OsCASP1 function leads to reduced salt tolerance, similar regulatory mechanisms might control OsI_01913 expression under salt stress .

Nutrient availability also appears to influence CASP gene expression. RT-qPCR results have identified several OsCASP_like genes (including OsCASP_like2/3/13/17/21/30) as candidate genes responding to ion deficiency conditions . This suggests that OsI_01913 might be similarly regulated by nutrient status, consistent with the role of Casparian strips in controlling nutrient uptake.

Understanding OsI_01913's specific expression patterns would require analyzing its promoter region for stress-responsive elements and performing expression studies under various environmental conditions using techniques like RT-qPCR and reporter gene constructs.

How do mutations in OsI_01913 affect root barrier properties and stress tolerance?

While direct studies on OsI_01913 mutations are not detailed in the available research, findings from related CASP proteins provide a framework for understanding potential effects :

Suberin deposition is also affected by CASP mutations. Loss of OsCASP1 function alters the expression of genes involved in suberin biosynthesis and disrupts the normal deposition of suberin in the endodermis and sclerenchyma . When visualized with berberine-aniline blue staining, OsCASP1 mutants show abnormal fluorescence patterns in the sclerenchyma compared to wild type . OsI_01913 mutations might similarly affect suberin localization, potentially compromising root barrier function.

The functional consequences of altered barrier properties include impaired ion homeostasis and reduced stress tolerance. OsCASP1 mutants exhibit withered leaves, fewer tillers, and decreased salt stress tolerance due to disrupted ion balance . The propidium iodide penetration assay revealed compromised barrier function in these mutants, allowing increased movement of solutes through the apoplastic pathway . OsI_01913 mutations would likely produce similar phenotypes, though potentially with tissue-specific or stress-specific variations depending on the protein's precise expression pattern and function.

To fully characterize how OsI_01913 mutations affect root barrier properties and stress tolerance, researchers would need to examine lignin and suberin deposition patterns, measure ion content in different tissues, and assess plant performance under various stress conditions.

How does OsI_01913 interact with other proteins during Casparian strip formation?

The protein interaction network of OsI_01913 during Casparian strip formation has not been directly characterized, but studies on related CASP proteins provide valuable insights into potential interaction mechanisms :

CASP proteins in Arabidopsis form a transmembrane scaffold that recruits lignin biosynthetic enzymes to specific locations in the endodermal cell wall during Casparian strip formation . This process involves precise spatial organization of multiple proteins to establish the apoplastic barrier. Given the structural similarities between OsI_01913 and other CASP proteins, it likely participates in similar protein complexes in rice.

The conserved motifs identified in CASP and CASP_like-I subfamily proteins (to which OsI_01913 likely belongs) potentially mediate specific protein-protein interactions . These motifs (1, 3, 4, 6, 8, and 9) are largely conserved across rice and Arabidopsis CASP proteins, suggesting functional importance in protein complex formation . OsI_01913 likely utilizes these conserved regions to interact with other CASPs and with enzymes involved in lignin biosynthesis.

Loss of OsCASP1 function alters the expression of genes involved in Casparian strip formation, including other CASP family members . In different genetic backgrounds, CASP gene expression patterns vary in response to OsCASP1 mutation, with some genes showing upregulation and others downregulation . This suggests complex regulatory interactions among CASP family members, potentially including OsI_01913.

To elucidate OsI_01913's specific interaction partners, researchers would need to employ techniques such as co-immunoprecipitation, yeast two-hybrid assays, or bimolecular fluorescence complementation. Identifying the protein complex components would provide crucial insights into how OsI_01913 contributes to Casparian strip formation and potentially to stress responses in rice.

How can researchers quantify Casparian strip integrity changes resulting from OsI_01913 manipulation?

To rigorously quantify changes in Casparian strip integrity resulting from OsI_01913 manipulation, researchers should employ multiple complementary approaches :

• Propidium Iodide (PI) penetration assay provides a direct functional measurement of barrier integrity. Roots are incubated with 10 μg/ml PI for 10 minutes, then imaged using confocal microscopy . Researchers can quantify the area of stele where PI penetration is blocked versus allowed, comparing percentages between wild-type and OsI_01913 mutant plants . This approach directly assesses the functional consequence of altered Casparian strip development.

• Berberine-aniline blue fluorescence quantification enables structural assessment of Casparian strips. After staining root cross-sections, fluorescence intensity can be measured in the endodermis and sclerenchyma using standardized imaging parameters . The ratio of endodermal to sclerenchyma fluorescence provides a quantitative comparison between genotypes, with significant changes indicating altered Casparian strip formation.

• Lignin distribution analysis using phloroglucinol staining allows visualization and quantification of lignification patterns . Image analysis software can quantify staining intensity across different cell layers and cell wall regions, revealing whether OsI_01913 manipulation causes ectopic lignin deposition or changes in lignification timing .

• Developmental categorization classifies roots into different categories based on Casparian strip development stages . Researchers can calculate the percentage of roots in each category for wild-type and OsI_01913 mutant plants, analyzing the distributions using appropriate statistical tests (e.g., one-way ANOVA with Tukey test) . This approach provides insights into developmental timing effects of OsI_01913 manipulation.

Combining these quantitative methods creates a comprehensive assessment of how OsI_01913 affects Casparian strip integrity, encompassing both structural development and functional barrier properties. These measurements should be correlated with phenotypic observations and physiological parameters to understand the broader implications of altered Casparian strip integrity.

What bioinformatic tools are most effective for analyzing OsI_01913 and related CASP proteins?

Several specialized bioinformatic tools and approaches are particularly valuable for analyzing OsI_01913 and related CASP proteins :

These bioinformatic approaches provide a foundation for understanding OsI_01913's sequence features, evolutionary context, potential functions, and regulatory mechanisms, guiding experimental design for functional characterization.

How should researchers reconcile contradictory findings about OsI_01913 function?

When faced with contradictory findings regarding OsI_01913 function, researchers should implement a systematic approach to reconciliation based on practices established in CASP protein research :

• Genetic background effects can significantly influence experimental outcomes. Research on OsCASP1 demonstrated that "the genetic background may influence the expression of the genes involved in CS formation" . When analyzing OsI_01913 function, researchers should use multiple mutant lines in diverse genetic backgrounds with appropriate controls for each. Contradictory results may reflect genuine biological variation rather than experimental error.

• Developmental stage variations must be carefully controlled. Casparian strip formation proceeds through distinct developmental stages, and the timing may vary between genetic backgrounds . Researchers should precisely document the developmental stage being examined and ensure comparisons occur at equivalent stages. Timeline experiments tracking CS formation over time can resolve contradictions related to developmental timing.

• Methodological standardization is essential for meaningful comparisons. Different staining techniques (berberine-aniline blue, phloroglucinol, PI penetration) may yield different results when examining CS formation . Researchers should employ multiple complementary methods and standardize protocols across laboratories to minimize method-based discrepancies.

• Functional redundancy within the CASP family may mask phenotypes in single mutants. The rice genome contains multiple CASP family members with potentially overlapping functions . Creating double or triple knockouts combining OsI_01913 mutation with mutations in related genes may reveal functions not apparent in single mutants.

• Environmental conditions significantly impact CASP gene expression and function. Given that CASP genes respond to environmental stresses and hormones , inconsistent results might stem from subtle variations in growth conditions. Researchers should meticulously document and control environmental parameters, testing OsI_01913 function under both standard and stress conditions.

By systematically addressing these factors and combining structural, functional, and molecular analyses, researchers can work toward a coherent understanding of OsI_01913 function despite initial contradictions.

What statistical approaches should be used when analyzing OsI_01913 experimental data?

• Analysis of Variance (ANOVA) with appropriate post-hoc tests should be applied when comparing multiple experimental groups. One-way ANOVA with Tukey's test effectively determines significant differences between wild-type, OsI_01913 mutant, and complemented lines . This approach is particularly suitable for analyzing continuous variables such as root length, fluorescence intensity, or ion content.

• Categorical data analysis using chi-square tests or Fisher's exact test is appropriate when categorizing roots based on Casparian strip development or barrier function. The percentage of small lateral roots (SLRs) in different categories can be compared between genotypes to identify significant shifts in development . This approach provides insights into how OsI_01913 manipulation affects developmental processes.

• Quantitative phenotype analysis requires appropriate descriptive statistics and visualization. Box plots or scatter plots with error bars (showing standard deviation or standard error) help identify patterns and outliers in continuous data. For meaningful statistical power, experiments should include sufficient biological replicates (typically n ≥ 10).

• Gene expression analysis of OsI_01913 and related genes requires normalization to reference genes and calculation of relative expression levels. Fold changes between experimental conditions should be tested for significance using t-tests or ANOVA . When analyzing multiple genes, researchers should apply multiple testing correction (e.g., Bonferroni or Benjamini-Hochberg) to control false discovery rates.

• Reproducibility assessment through multiple biological replicates is essential. Experiments should be repeated independently to ensure consistent results, with both biological and technical replicates incorporated into the experimental design. Power analysis helps determine appropriate sample sizes for detecting expected effect sizes.