Structure Activity Relationship of strigolactones via a novel, quantitative in planta bioassay

Strigolactones (SLs) are plant hormones with various functions in development, responses to stress and interaction with (micro)organisms in the rhizosphere, including seeds of parasitic plants. Their perception for hormonal functions requires an α,β-hydrolase belonging to the D14 clade in higher plants; perception of host-produced SLs in parasitic seeds relies on similar but phylogenetically distinct proteins (D14-like). D14 and D14-like proteins are peculiar receptors, because they cleave SLs before undergoing a conformational change that elicits downstream events. Structure-activity relationship data show that the butenolide D-ring is crucial for bioactivity. We applied a bioisosteric approach to the SLs structure, synthetizing analogues and mimics of natural SLs in which the D-ring was changed from a butenolide to a lactam, and evaluating their bioactivity. This was done by a novel bioassay based on Arabidopsis transgenic lines expressing AtD14 fused to the firefly luciferase, in parallel with the quantification of germination-inducing activity on parasitic seeds. The obtained results showed that the in planta bioassay is robust and quantitative, and thus can be rightly added to the SL-survey toolbox; and also, that modification of the butenolide ring into a lactam significantly hampers the biological activity exhibited by SLs with canonical lactonic D-ring.