Strigolactones Are Involved in Root Response to Low Phosphate Conditions in Arabidopsis

Strigolactones (SLs) are plant hormones that suppress lateral shoot branching, and act to regulate root hair elongation and lateral root formation. Here, we show that SLs are regulators of plant perception of or response to low inorganic phosphate (Pi) conditions. This regulation is mediated by MORE AXILLARY GROWTH2 (MAX2) and correlated with transcriptional induction of the auxin receptor TRANSPORT INHIBITOR RESPONSE1 (TIR1). Mutants of SL signaling (max2-1) or biosynthesis (max4-1) showed reduced response to low Pi conditions relative to the wild type. In max4-1, but not max2-1, the reduction in response to low Pi was compensated by the application of a synthetic strigolactone GR24. Moreover, AbamineSG, which decreases SL levels in plants, reduced the response to low Pi in the wild type, but not in SL-signaling or biosynthesis mutants. In accordance with the reduced response of max2-1 to low Pi relative to the wild type, several phosphate-starvation response and phosphate-transporter genes displayed reduced induction in max2-1, even though Pi content in max2-1 and the wild type were similar. Auxin, but not ethylene, was sufficient to compensate for the reduced max2-1 response to low Pi conditions. Moreover, the expression level of TIR1 was induced under low Pi conditions in the wild type, but not in max2-1. Accordingly, the tir1-1 mutant showed a transient reduction in root hair density in comparison with the wild type under low Pi conditions. Therefore, we suggest that the response of plants to low Pi is regulated by SLs; this regulation is transmitted via the MAX2 component of SL signaling and is correlated with transcriptional induction of the TIR1 auxin receptor.