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.