Abstract
The biosynthesis of the recently identified novel class of plant hormones, strigolactones, is up-regulated upon phosphate deficiency in many plant species. It is generally accepted that the evolutionary origin of strigolactone up-regulation is their function as a rhizosphere signal that stimulates hyphal branching of arbuscular mycorrhizal fungi. In this work, we demonstrate that this induction is conserved in Arabidopsis (Arabidopsis thaliana), although Arabidopsis is not a host for arbuscular mycorrhizal fungi. We demonstrate that the increase in strigolactone production contributes to the changes in shoot architecture observed in response to phosphate deficiency. Using high-performance liquid chromatography, column chromatography, and multiple reaction monitoring-liquid chromatography-tandem mass spectrometry analysis, we identified two strigolactones (orobanchol and orobanchyl acetate) in Arabidopsis and have evidence of the presence of a third (5-deoxystrigol). We show that at least one of them (orobanchol) is strongly reduced in the putative strigolactone biosynthetic mutants more axillary growth1 (max1) and max4 but not in the signal transduction mutant max2. Orobanchol was also detected in xylem sap and up-regulated under phosphate deficiency, which is consistent with the idea that root-derived strigolactones are transported to the shoot, where they regulate branching. Moreover, two additional putative strigolactone-like compounds were detected in xylem sap, one of which was not detected in root exudates. Together, these results show that xylem-transported strigolactones contribute to the regulation of shoot architectural response to phosphate-limiting conditions.
Highlights
The biosynthesis of the recently identified novel class of plant hormones, strigolactones, is up-regulated upon phosphate deficiency in many plant species
It has been postulated that all strigolactones are derived from a single carotenoid substrate through oxidative cleavage performed by a carotenoid cleavage dioxygenase (CCD), the product of which is subsequently oxidized by cytochrome P450s (Matusova et al, 2005)
To assess if the induction by Arabidopsis root exudates of germination of P. ramosa seed is increased by phosphate deficiency, 4-week-old Arabidopsis plants were subjected to phosphate starvation for 2 weeks, in parallel with fully fertilized plants
Summary
The biosynthesis of the recently identified novel class of plant hormones, strigolactones, is up-regulated upon phosphate deficiency in many plant species. Two additional putative strigolactone-like compounds were detected in xylem sap, one of which was not detected in root exudates Together, these results show that xylem-transported strigolactones contribute to the regulation of shoot architectural response to phosphate-limiting conditions. The growth of the primary root is reduced and the outgrowth of the lateral roots near the soil surface is stimulated such that phosphate-rich areas that are usually found in the top layers of the soil can be explored (Al-Ghazi et al, 2003) In addition to these changes in root system architecture, resources are redirected from the shoot to the root (Lopez-Bucio et al, 2002), contributing to the change in the root-toshoot ratio, enabling the plant to better cope with its environment (Bonser et al, 1996). It has been shown that other phytohormones that are produced in the roots, such as abscisic acid and cytokinins, are transported through the xylem (Hartung et al, 2002; Sakakibara, 2006), making the xylem a good candidate for strigolactone transport
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