Strigolactone (SL) phytohormones play critical roles in controlling plant architecture and managing plant interactions with soil microflora1. While approximately 25 natural SLs have been isolated from plants2, over one hundred are predicted to exist3. All share a common structural core; a tricyclic lactone (rings ABC) connected via an enol ether bridge to an α,β-unsaturated furanone moiety (ring D).
Structures vary due to the presence of side groups on the A- and B-rings, while the C- and D-rings, important for bioactivity, are highly conserved. Fragmentation of the D-ring, to generate an m/z 97 product ion, provides a useful mechanism to detect SLs by mass spectrometry.
Typically, SLs are analysed using triple quadrupole MS and multiple reaction monitoring (MRM) methods monitoring transitions for Q1 [M+H]+ to Q3 [M+H –D-ring]+. This works well for the detection of known SLs but is an ineffective tool for the identification of unknown SLs for which the Q1 ions are unknown. Screening strategies utilising precursor ion and neutral loss survey scan experiments targeting the loss of m/z 97 in Q3 are also ineffective due to the lower sensitivity of these scan modes in triple quadrupole MS and the low concentration of endogenous SLs.
To detect both known and novel SLs in new plant species and tissues, we have developed a generic MRM-dependent enhanced product ion (EPI) method to screen all possible ions in Q1 with m/z 97 in Q3. Detection of such a transition triggers an EPI experiment that gives a fragmentation spectrum generated from the Q1 ion. Comparison of the EPI spectra with published data provides confidence of compound identification when SL standards are not available and provides a mechanism by which novel SLs maybe identified.
We have applied this technique to studies to understand the relationship between SLs and plant nutrient stress and the production of inhibitors of microbial nitrogen cycling in soil.