Reactive Carbonyl Species Mediate Isothiocyanate Signaling Pathway in Arabidopsis thaliana Guard Cells

Our previous results demonstrated that depletion of glutathione (GSH) rather than elevation of levels of reactive oxygen species (ROS) is highly correlated with the decrease in stomatal aperture induced by isothiocyanates (ITCs), although ROS is considered a key second messenger in stomatal closure, suggesting that another signal component regulates stomatal apertures along with GSH depletion. This study, using Arabidopsis, clarified that reactive carbonyl species (RCS), especially acrolein and 4-hydroxy-(E)-2-nonenal, are determinants of stomatal aperture responses to ITCs. All tested ITCs, allyl isothiocyanate (AITC), sulforaphane (SFN), benzyl isothiocyanate (BITC), and phenethyl isothiocyanate (PEITC), significantly induced stomatal closure, which was inhibited by the RCS scavengers, carnosine and pyridoxamine. The RCS scavengers suppressed ITC-induced depletion of GSH but not elevation of ROS levels. All tested ITCs (AITC, SFN, BITC, and PEITC) increased levels of RCS and non-RCS aldehydes in the epidermal tissues. However, acrolein, 4-hydroxy-(E)-2-nonenal, crotonaldehyde, and (E)-2-pentenal induced stomatal closure at 10 and 100 μM, whereas propionaldehyde, butyraldehyde, and n-pentanal did not at concentrations up to 100 μM. Acrolein and 4-hydroxy-(E)-2-nonenal more effectively induced stomatal closure and GSH depletion than crotonaldehyde and (E)-2-pentenal did. The contents of RCS were more strongly correlated with GSH levels and stomatal closure than with ROS levels. These results suggest that RCS, especially acrolein and 4-hydroxy-(E)-2-nonenal, acts as key regulators of stomatal closure in guard cells in response to ITCs.