The fruit clusters of Coccoloba uvifera give it its nickname of sea grape.

The fruit clusters of Coccoloba uvifera give it its nickname of sea grape.

© Wikipedia / Hans Hillewaert

Unwavering symbiosis

Research conducted on the introduction of a tree from extreme environments into the sand dunes of Senegal shows its solid relationship with a fungus. This fungus naturally accompanies the tree from its environment of origin, on the other side of the Atlantic.

A faithful and provident partner, Coccoloba uvifera travels with its symbiont... This tree native to the Caribbean coast can cope with extreme environments, thanks to its symbiotic relationship with an ectomycorrhizal fungus?Fungus in a symbiotic relationship with one or more woody plants. “We showed for the first time that its seeds carry spores in their shell, to continue this valuable collaboration in places where they will grow”, explains soil microbiologist Amadou Bâ, co-author of a publication on the subject(1).

In its environment of origin, Coccoloba uvifera - also known as sea grape because of its fruit clusters - grows on beaches, in the intertidal zone. Its roots contain various symbiotic fungi which form a protective sleeve that filters salt and provides the plant with water and nutrients. One of its symbionts plays a major protective role: scleroderma. In return, the sea grape provides its partner with carbon through its photosynthetic activity.

Coccoloba fixes sand dunes

This productive sea grape-scleroderma partnership facilitates the introduction of the tree into challenging environments. During a project involving planting the tree along coastal routes, Amadou Bâ and his colleagues studied its adaptation and interaction with the fungus. “In Senegal, the Water and Forestry department decided to plant them along coastal routes under threat of silting, explains the expert. The trees fix the sand dunes where these routes are established”. The project plans to line 17 km of roadways in the Dakar region with these trees. The water and salt stress to which the sea grape is exposed is similar to that of its environment of origin. The soil is made of salty sand, and the tree will only be watered by sea spray and scarce winter rainfall, for 3 months of the year.

Young colonised roots

Scleroderma bermudense spores are stored in the cavities of the envelope of the Coccoloba uvifera seed.

© IRD/Amadou Bâ

Bloc de texte

To successfully complete the project, the seeds imported from the Antilles germinated in Senegal’s nurseries. They produced seedlings and the scleroderma colonised their young roots, without being consciously introduced. “The spores of the fungus were therefore on the seeds’ natural shell”, points out Amadou Bâ. It should be noted that the sea grape only carries the scleroderma and none of the other associated symbionts in its region of origin, probably because this fungus produces a lot of spores and is therefore more likely to get deposited in large quantities on the fruit teguments. “This spontaneous mode of transmission was hitherto unknown, says Amadou Bâ. Everything suggests that this is a well-established strategy. The pulp of the fruit that fell from the tree in its environment of origin was probably removed by rodents and humans. The seeds, which came into contact with the spores on the ground, carried the material required for the perpetuation of their key fungal partner”. In addition to its originality, this mechanism is economically attractive. It eliminates the costs associated with the production of fungal inoculum3 which up until now was required for germination in nurseries.


Note:

1. S. Sene et al., A pantropically introduced tree is followed by specific ectomycorrhizal symbionts due to pseudo-vertical transmission, The ISME Journal, 26 jan 2018


Contact: amadou.ba@ird.fr