Reflectance spectroscopy for QTL mapping of amounts and types of epicuticular waxes on onion leaves

Authors: MUNAIZ, EDUARDO - University Of Wisconsin; TOWNSEND, PHILIP - University Of Wisconsin; Havey, Michael

Submitted to: Molecules
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2020
Publication Date: 7/29/2020
Citation: Munaiz, E.D., Townsend, P.A., Havey, M.J. 2020. Reflectance spectroscopy for QTL mapping of amounts and types of epicuticular waxes on onion leaves. Molecules. 2020, 25(15), 3454. https://doi.org/10.3390/molecules25153454.
DOI: https://doi.org/10.3390/molecules25153454

Interpretive Summary: Epicuticular waxes on the surface of plant leaves are important for tolerance to abiotic stresses and plant-parasite interactions. In onion (Allium cepa L.), variation for amounts and types of epicuticular waxes are significantly associated with less feeding damage by the insect Thrips tabaci (thrips). Epicuticular wax profiles are measured using used gas chromatography mass spectrometry (GCMS), which is a labor intensive and relatively expensive approach. Biochemical spectroscopy is a non-destructive tool for measurement and analysis of physiological and chemical features of plants. This study used GCMS and full-range biochemical spectroscopy to characterize epicuticular waxes on seven onion accessions with visually glossy (low wax), semi-glossy (intermediate wax), or waxy (copious wax) foliage, as well as a segregating family from the cross of glossy and waxy onions. In agreement with previous studies, GCMS revealed that the three main waxes on leaves of wild type waxy onion were the ketone hentriacontanone-16 (H16) and fatty alcohols octacosanol-1 (Oct) and triacontanol-1 (Tri). The glossy cultivar ‘Odourless Greenleaf’ had a unique phenotype with essentially no H16 and Tri and higher amounts of Oct and the fatty alcohol hexacosanol-1 (Hex). Hyperspectral reflectance profiles were measured on leaves of the onion accessions and segregating family, and partial least-squares regression (PLSR) was utilized to generate a spectral coefficient for every wavelength and prediction models for amounts of the three major wax components. PLSR predictions were robust with independent validation coefficients of determination at 0.72, 0.70 and 0.42 for H16, Oct, and Tri, respectively. The predicted amounts of H16, Oct and Tri are the result of an additive effect of multiple spectral features of different intensities. Variation of reflectance for Oct, and Tri revealed unique spectral features at 645 nm and 730 nm, respectively, and at 437 nm and 437 nm differentiating these two fatty alcohols from H16. Reflectance spectroscopy successfully revealed a major quantitative trait locus (QTL) for amounts of H16, Oct and Tri in the segregating family, agreeing with previous genetic studies. This study demonstrates that hyperspectral signatures can be used for non-destructive measurement of major waxes on onion leaves as a basis for rapid plant assessment in support of developing thrips-resistant onions.

Technical Abstract: Epicuticular waxes on the surface of plant leaves are important for tolerance to abiotic stresses and plant-parasite interactions. In onion (Allium cepa L.), variation for amounts and types of epicuticular waxes are significantly associated with less feeding damage by the insect Thrips tabaci (thrips). Epicuticular wax profiles are measured using used gas chromatography mass spectrometry (GCMS), which is a labor intensive and relatively expensive approach. Biochemical spectroscopy is a non-destructive tool for measurement and analysis of physiological and chemical features of plants. This study used GCMS and full-range biochemical spectroscopy to characterize epicuticular waxes on seven onion accessions with visually glossy (low wax), semi-glossy (intermediate wax), or waxy (copious wax) foliage, as well as a segregating family from the cross of glossy and waxy onions. In agreement with previous studies, GCMS revealed that the three main waxes on leaves of wild type waxy onion were the ketone hentriacontanone-16 (H16) and fatty alcohols octacosanol-1 (Oct) and triacontanol-1 (Tri). The glossy cultivar ‘Odourless Greenleaf’ had a unique phenotype with essentially no H16 and Tri and higher amounts of Oct and the fatty alcohol hexacosanol-1 (Hex). Hyperspectral reflectance profiles were measured on leaves of the onion accessions and segregating family, and partial least-squares regression (PLSR) was utilized to generate a spectral coefficient for every wavelength and prediction models for amounts of the three major wax components. PLSR predictions were robust with independent validation coefficients of determination at 0.72, 0.70 and 0.42 for H16, Oct, and Tri, respectively. The predicted amounts of H16, Oct and Tri are the result of an additive effect of multiple spectral features of different intensities. Variation of reflectance for Oct, and Tri revealed unique spectral features at 645 nm and 730 nm, respectively, and at 437 nm and 437 nm differentiating these two fatty alcohols from H16. Reflectance spectroscopy successfully revealed a major quantitative trait locus (QTL) for amounts of H16, Oct and Tri in the segregating family, agreeing with previous genetic studies. This study demonstrates that hyperspectral signatures can be used for non-destructive measurement of major waxes on onion leaves as a basis for rapid plant assessment in support of developing thrips-resistant onions.