Plant sesquiterpenes induce hyphal branching in arbuscular mycorrhizal fungi

Abstract

Arbuscular mycorrhizal (AM) fungi form mutualistic, symbiotic associations with the roots of more than 80% of land plants. The fungi are incapable of completing their life cycle in the absence of a host root. Their spores can germinate and grow in the absence of a host, but their hyphal growth is very limited. Little is known about the molecular mechanisms that govern signalling and recognition between AM fungi and their host plants. In one of the first stages of host recognition, the hyphae of AM fungi show extensive branching in the vicinity of host roots before formation of the appressorium, the structure used to penetrate the plant root. Host roots are known to release signalling molecules that trigger hyphal branching, but these branching factors have not been isolated. Here we have isolated a branching factor from the root exudates of Lotus japonicus and used spectroscopic analysis and chemical synthesis to identify it as a strigolactone, 5-deoxy-strigol. Strigolactones are a group of sesquiterpene lactones, previously isolated as seed-germination stimulants for the parasitic weeds Striga and Orobanche. The natural strigolactones 5-deoxy-strigol, sorgolactone and strigol, and a synthetic analogue, GR24, induced extensive hyphal branching in germinating spores of the AM fungus Gigaspora margarita at very low concentrations.

A petunia ABC protein controls strigolactone-dependent symbiotic signalling and branching

“…Subsequently, the shoot, root and Phyto Agar fractions were incubated for 30 min in 50 ml 24% (w/v) trichloroacetic acid at 23 uC. 3 H counts were determined in 3 ml ULTIMA Gold LSC cocktail (Perkin Elmer) with a Liquid Scintillation Analyser Tri-Carb 2900TR (Packard BioScience). The number of disintegrations per minute was computed into a percentage for each fraction and normalized to tissue fresh weight.…”

“…They are mostly exuded from roots and function as signalling compounds in the initiation of arbuscular mycorrhizae 3 , which are plant-fungus symbionts with a global effect on carbon and phosphate cycling 4 . Recently, strigolactones were established to be phytohormones that regulate plant shoot architecture by inhibiting the outgrowth of axillary buds 5,6 .…”

“…In addition to their role in shoot branching, strigolactones are exuded into the rhizosphere under phosphorus-limiting conditions 5 and act as growth stimulants of arbuscular mycorrhizal fungi 3 . To identify efflux carriers of arbuscular-mycorrhiza-promoting factors such as strigolactones, we used a degenerate primer approach ( Supplementary Fig.…”

“…2 Laboratory of Plant Physiology, Wageningen University, 6700 AR Wageningen, The Netherlands. 3 Department of Biology, University of Fribourg, 1700 Fribourg, Switzerland. 4 Centre for Biosystems Genomics, PO Box 98, 6700 AB Wageningen, The Netherlands.…”

Strigolactones fine-tune the root system

“…In addition, mycorrhizal fungi can provide other benefits, such as increased water absorption and root protection from soil heavy metals (Garg and Singla 2011;Porcel et al 2012). An important group of sesquiterpene lactones, commonly named strigolactones, is responsible for stimulating this relationship particularly under nutrient-deficient conditions (Akiyama et al 2005). More recently in evolution, parasitic plants, such as Striga sp., have co-opted strigolactones as a germination signal, thus indicating proximity to host roots.…”