Role of mitochondria in the response of arbuscular mycorrhizal fungi to strigolactones

Abstract

The plant signals strigolactones activate seed germination of the parasitic weeds (Striga and Orobanche), growth of arbuscular mycorrhizal (AM) fungi and have recently been described as a new class of plant hormones that inhibit shoot branching. In AM fungi, the synthetic strigolactone analogue GR24 rapidly stimulates mitochondrial metabolism (within minutes) and biogenesis (within one hour). New gene expression, more active nuclear division and cell proliferation occur later (within days). By using pharmacological approaches to inhibit the mitochondrial ATP synthesis, various steps of the respiratory chain and the mitochondrial protein translation, we further describe the mechanisms underlying the mitochondrial response to GR24. We show with SHAM and KCN inhibition treatments that the respiratory chain of Gigaspora rosea is branched and includes an alternative oxydase. The two electron transports can be used for GR24 activation of hyphal branching but only the alternative one is used for spore germination. By using the inhibitors Oligomycin, Rotenone, Antimycine A and KCN, we show that indirect (proton pumping) and direct inhibition of ATP synthase does not completely abolish the activation of hyphal branching by GR24. However, hyphal branching was totally inhibited with the suppression of mitochondrial biogenesis, confirming the essential role played by mitochondria to amplify the strigolactone response of AM fungi.Strigolactones are plant signal molecules initially characterized as seed germination stimulants of the parasitic weeds Orobanche and Striga. 1 Recent findings have revealed that natural strigolactones or the synthetic strigolactone analogue GR24 can also elicit hyphal branching of arbuscular mycorrhizal (AM) fungi, a cellular response typically observed when these fungi grow close to a living root. 2 Moreover when plants are affected in strigolactone production their capacity to be mycorrhized is strongly reduced. 3,4 An additional role of strigolactones, as possible novel plant hormones inhibiting shoot branching, has recently been discovered. 4,5 The cellular modes of action of strigolactones on seeds of parasitic weeds and on plant axillary buds are not known. In previous studies we showed, on the AM fungus Gigaspora rosea, that GR24 elicits a fast (within minutes) synthesis and utilisation of NADH by mitochondria. This boost of mitochondrial activity was correlated, after one hour of GR24 treatment, with an enhancement of cellular ATP production. This enhancement of oxydative metabolism precedes active mitochondrial proliferation. 6,7 Taken together, these results suggested that mitochondrial activation is a key event required for switching the fungus to a pre-symbiotic stage, characterized by a more active nuclear proliferation, an upregulation of several genes and an activated cell proliferation (the so called branching response). 6 In order to clarify the role of mitochondria in the GR24 response, we carried out a series of pharmacological experiments where different steps of the oxidative p...

The synthetic strigolactone GR24 influences the growth pattern of phytopathogenic fungi

“…By this, the response of pathogenic fungi differs from the response of symbiotic AM fungi. SLs have been found to stimulate hyphal growth of AM fungi, causing intense branching activity characterized by cell proliferation, amplification of nucleus number per mycelial length and expression of genes related to mitochondrial activity and increased respiration (Giovannetti et al 1996;Buée et al 2000;Besserer et al 2006Besserer et al , 2009. BF from carrot hairy-root cultures and the SL analogs GR24 and GR7 were active on the phylogenetically distant AM fungi, Gigaspora sp.…”

Plant Microbes Symbiosis: Applied Facets

“…13.2 ). The extensive work of Besserer et al ( 2006Besserer et al ( , 2008Besserer et al ( , 2009 later corroborated this fi nding. The extensive work of Besserer et al ( 2006Besserer et al ( , 2008Besserer et al ( , 2009 later corroborated this fi nding.…”

Genomics of Arbuscular Mycorrhizal Fungi

“…These responses are conserved in AM fungi as R. irregularis and G. rosea both responded in the same way. It was shown that these responses are dependent on an NADH dehydrogenase as well as an alternative oxidase in the respiratory chain (Besserer et al, 2008, Besserer, Bécard, Roux, & Séjalon-Delmas, 2009). Both enzymes are involved in GR24 response, while germination relies on the second only.…”