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

Authors: Zhu, Yanmin; Rainbow, Jordan; ZHOU, ZHE - Institute Of Animal Science (CAAS)

Submitted to: Fruit Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/13/2023
Publication Date: 2/1/2024
Citation: Zhu, Y., Rainbow, J.L., Zhou, Z. 2024. Microscopic features of lignin deposition patterns in young apple roots using brightfield and fluorescence imaging. Fruit Research. 4. Article e007. https://doi.org/10.48130/frures-0023-0045.
DOI: https://doi.org/10.48130/frures-0023-0045

Interpretive Summary: Our recent transcriptome analyses have identified several notable transcriptome changes during apple root defense activation against infection from Pythium ultimum. One of the major defense tactics is the upregulation of genes involved in phenylpropanoid biosynthesis, secondary metabolite transportation and laccase-directed lignin formation and cell wall fortification. Guided by the findings from these transcriptome analyses, this study primarily examines the relationship between lignin deposition and resistance traits in apple roots. Taking advantage of lignin autofluorescence in plant tissues and available lignin specific histochemical staining methods, microscopic features of lignin deposition in young apple roots were assessed between genotypes and in response to pathogen infection. 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 roots. Using both brightfield and fluorescence images, elevated intensity of lignin deposition was detected in the parenchyma cells of infected root cortex tissues. The quenched fluorescence intensity due to lignin specific staining substantiated the notion that detected fluorescence is primarily emitted from lignin in the cell walls of young apple root. The genotype-specific lignin deposition patterns, particularly around vascular bundles, may suggest intrinsic differences in lignin richness and/or monolignol composition between genotypes. Additionally, deposition of proanthocyanin, a fluorescent flavonoid compound in cell walls, was determined to be not a 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 crucially important for defining the functional role of specific candidate genes and pathways for their contribution to resistance traits in apple roots.

Technical Abstract: Our recent transcriptome analyses have uncovered a multi-phase and multi-layer defense tactic in apple root towards infection from a necrotrophic oomycete pathogen Pythium ultimum. Some of the most notable transcriptome changes during apple root defense activation include upregulation of genes involved in phenylpropanoid biosynthesis, secondary metabolite transportation and laccase-directed lignin formation which may lead to cell wall fortification. 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 intensity of lignin deposition was 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 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.