Microscopic features of lignin deposition patterns in young apple roots using brightfield and fluorescence imaging

Abstract

Our recent transcriptome analyses have identified that the upregulation of genes involved in phenylpropanoid biosynthesis and laccase-directed lignin formation represent one of the primary defense responses against infection from the necrotrophic oomycete pathogen Pythium ultimum. Taking advantage of lignin autofluorescence in plant tissues and its specific histochemical staining methods, microscopic features of lignin deposition in young apple roots between genotypes and in response to pathogen infection were assessed. The preliminary data indicated that, compared to the Wiesner test, the Maule test is a more efficient lignin staining method for tissue sections of young apple root. Using both brightfield and fluorescence images, elevated intensities of lignin deposition were detected in the parenchyma cells of infected root cortex tissues. The quenched fluorescence intensity due to lignin specific staining corroborated the hypothesis that lignin deposition is a critical component of the detected fluorescence from these sections. The genotype-specific lignin deposition patterns, particularly around vascular bundles, may suggest intrinsic differences in lignin richness and/or monolignol composition between genotypes. Additionally, proanthocyanin deposition was determined to be a less prominent factor in young apple roots. Developing reliable methods to detect anatomical and biochemical changes such as cell wall lignification in response to pathogen infection is crucial for defining the functional role of identified genes and pathways contributing to resistance traits in apple root.

Keywords

• apple root
• lignin detection
• defense response
• resistance trait
• microscopic examination
• pythium ultimum.