Imaging of Multiple Plant Hormones in Roots of Rice (Oryza sativa) Using Nanoparticle-Assisted Laser Desorption/Ionization Mass Spectrometry

Abstract

Plant hormones can act in synergistic and antagonistic ways in response to biotic and abiotic stresses and in plant growth and development. Thus, a technique is needed to simultaneously determine the distributions and concentrations of several plant hormones. Previously, we reported that localizations of two plant hormones [cytokinin (CK) and abscisic acid (ABA)] can be simultaneously visualized in a plant tissue using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS). In MALDI-MS, however, self-ionization of an organic matrix occasionally interferes with ionizations of small molecules (<500 m/z) including most plant hormones. Another technique, nanoparticle-assisted laser desorption/ionization (Nano-PALDI), can avoid matrix self-ionization using nanoparticles to assist the ionization of analytes. Here, we compared the ionization efficiencies of common plant hormones by MALDI-MS and Nano-PALDI-MS. For the comparison, we prepared a standard mix of seven plant hormones [ABA, auxin (IAA), brassinosteroid (Br), two CKs (trans-zeatin, tZ, and 6-(γ,γ-dimethylallylamino) purine, iP), jasmonic acid, and salicylic acid (SA)], an ethylene precursor (1-aminocyclopropane-1-carboxylic acid, ACC), and a heavy hydrogen-labeled ABA (D6-ABA). Basic MALDI-MS detected all compounds except IAA, Br, and D6-ABA, while Nano-PALDI-MS detected all nine compounds. By Nano-PALDI-MS imaging (MSI), each of the abovementioned hormones and ACC were also detected in root cross sections of rice which were incubated in the hormone mix for 2 h. In the elongation zone of untreated roots, Nano-PALDI-MSI revealed high levels of ABA and CKs in the outer part of roots and much lower levels in the stele, but Br, SA, and ACC were broadly distributed in the cross section. IAA seemed to be distributed in the epidermis, cortex, and stele. Multiple-hormone imaging using Nano-PALDI-MS has great potential for investigating the roles of hormone signaling in crop development and stress responses.

Sensors for the quantification, localization and analysis of the dynamics of plant hormones

“…Different approaches to follow this dynamism have been undertaken. Imaging mass spectrometry has been developed as a powerful method to detect the distribution of multiple hormones simultaneously (Shiono et al ., 2017; Shiono and Taira, 2020), but, for now, the spatial resolution of imaging mass spectrometry is comparatively low. As a complementary technology, biosensors that provide extremely high spatiotemporal information are in great demand.…”

Ammonium regulates the development of pine roots through hormonal crosstalk and differential expression of transcription factors in the apex

“…The MS image was reconstructed from the obtained MS spectra with a mass bin width of m/z ± 0.1 from the exact mass using flexImaging 4.0 (Bruker Daltonik GmbH). The accurate mass of the ions was used for image generation, and mass accuracy and root-mean-square error (RMSE) were automatically calculated by the imaging software to avoid false-positive signals (Shiono and Taira, 2020). Comparisons of MS images were derived from the relative intensity for each signal normalized by the highest intensity spot on the slide.…”

Mass spectrometry imaging for direct visualization of components in plants tissues

“…Some inorganic matrixes have been reported have strong affinity to specific substances [54][55][56][57]. In recent years, many researches about MSI of components in plants tissues by inorganic matrixes have been reported [53,[58][59][60]. However, most inorganic nanomaterials have high electrical conductivity and chemical stability, and longterm use may cause some damage to the mass spectrometer, such as short circuit [61].…”

“…Sublimation method provides higher spatial resolution than spraying because it can obtain fine crystals, but its extraction is inefficient than spraying method because there is no solvent involved in the process. For inorganic matrix deposition, spraying is often used [58], but NPs are often clustered and thus affect the analysis. Imprinting [62] has been applied in recent years, mainly used for inorganic matrix deposition.…”