Can we use arbuscular mycorrhizal fungi to

improve resistance to Orobanche cumana in sunflower?

11th World Congress on Parasitic PlantsJohann Louarn (PhD student- Toulouse, France)

The arbuscular mycorrhizal symbiosis

H2O

N

KP

C

� Most common and oldest symbiosis in

the plant kingdom (-450 million years).

� Mutualistic association between soil

fungi (Glomeromycota) and the roots of

80% of land plants. Arbuscular

mycorrhizal fungi

Arbuscule

Plant cell

wall

Molecular dialogue in the arbuscular mycorrhizal symbiosis

� Strigolactones stimulate hyphal branching and metabolism of Arbuscular

Mycorrhizal fungi (Akiyama et al., 2005 ; Besserer et al., 2006).

Fungal

signal

Strigolactone

Orobanche germination stimulants

� Strigolactones induce germination of Striga and

Orobanche seeds (Cook et al., 1966 ; for Review

Xie et al., 2010).

� O.cumana germination can be induced by

GR24 (synthetic strigolactone) and dehydrocostus-

lactone (DCL; Joel et al., 2011).

General case :

Specific case :

dehydrocostus-lactone

StrigolactoneStrigolactone

Arbuscular mycorrhizal symbiosis and Orobanche

� Mycorrhizal symbiosis reduces infection of

sorghum plants by Striga by decreasing the Striga

seed germination rate (Lendzemo et al., 2007).

Is arbuscular mycorrhizal symbiosis a good way to control O.cumana

infection of sunflower?

Non-mycorrhizal G.intraradices

� Root colonization by AM fungi reduces

strigolactone release (López-Ráez et al., 2011) .

Analysis of sunflower root exudates

� Mycorrhizal root exudates induce less O.cumana germination than non-

mycorrhizal roots exudates.

O.cumana germination test

Working hypotheses

Working hypotheses

Do mycorrhizal fungi:

� decrease the root secretion of germination stimulants (strigolactones,DCL…)?

� produce themselves a germination inhibitor?

� induce the root secretion of a germination inhibitor?

Do mycorrhizal roots produce less strigolactones?

� Exudates of mycorrhizal sunflower roots are less active on AM hyphal

branching, suggesting that they contain less strigolactones.

Hyphal branching test on AM fungi

Test

moleculescontrol

spore

Counting of

news hyphal

branches

Root exudate analysis

� Exudate complementation with a synthetic strigolactone (GR24) is not

sufficient to restore O.cumana seed germination to the level of GR24

alone.

� Mycorrhizal sunflower roots hence seem to produce an inhibitor of seed

germination.

O.cumana germination test: complementation with GR24

Analysis of arbuscular mycorrhizal fungal exudates

� Exudates of arbuscular mycorrhizal fungi reduce O.cumana seed

germination in a dose-dependent way.

�These observations were confirmed with another AM fungus, Gigaspora

rosea, and another parasite, Phelipanche ramosa.

O.cumana germination test: fungal exudates added to GR24.

G.i : Glomus intraradices

Effects of fungal exudates on Orobanche seeds

� Exudates of arbuscular mycorrhizal fungi inhibit Orobanche germination

without affecting seed viability.

Viability test: TTC staining.

G.i : Glomus intraradices

Conclusions

� Root colonization with arbuscular

mycorrhizal fungi can reduce

O.cumana seed germination.

�Arbuscular mycorrhizal fungi

produce an inhibitor of Orobanche

seed germination.

�This inhibitor does not affect seed

viability.

Prospects

What is the nature and mechanism of action of the inhitor produced

by arbuscular mycorrhizal fungi?

Does arbuscular mycorrhizal symbiosis affect the production of

dehydrocostus lactone?

What is the contribution of this inhibitor, produced by the AM fungi, to the

potential resistance to O.cumana induced by mycorrhization?

Soizic Rochange

Virginie Puech-

Pages

Guillaume Bécard

Philippe Delavault

Philippe Simier

Alexandra Haouy

Christophe Roux

Coline Balzergue

Francis Carbonne

Jérôme Laparre

Marion Moles

Pierre-Marc Delaux

Jean-Philippe Combier

Damien Formey

Betty Gilbert

Dominique Lauressergues