Abstract
In this paper we review the membrane transport processes that are involved in the transfer of mineral nutrients and organic carbon between the symbiotic partners in mycorrhizas. In particular, we reassess the prevailing hypothesis that transfer in vesicular-arbuscular (VA) mycorrhizas occurs simultaneously and bidirectionally across the same interface and that arbuscules are the main sites of transfer. Using cytochemical techniques, we and our collaborators have reexamined the distribution of ATPases in the arbuscular and intercellular hyphal interfaces in VA mycorrhizas formed between roots of Allium cepa (onion) and the fungus Glomus intraradices. The results showed that H+-ATPases have different localisation on plant and fungal membranes in arbuscular and hyphal interfaces (Gianinazzi-Pearson et al. 1991). While some arbuscular interfaces had H+-ATPase activity on both fungal and plant membranes, in most cases the fungal membrane lacked this activity. In contrast, the plasma membranes of intercellular hyphae always had H+-ATPase and the adjacent root cells did not. This suggests that the different interfaces in a VA mycorrhiza may have different functions. We propose that passive loss of P from the arbuscules is associated with active uptake by the energised (ATPase-bearing) plant membrane and that passive loss of carbohydrate from the root cells is followed by active uptake by the intercellular hyphae. If this model is correct, then variations in "mycorrhizal efficiency" (i.e. the extent to which mycorrhizal plants grow better than non-mycorrhizal controls) might be determined by differences in the numbers of active arbuscules as a proportion of the total fungal biomass within the root.As a first step towards investigating this possibility, we have developed methods for measuring the surface areas of arbuscular and hyphal interfaces in different fungus-host combinations, Glomus spp./ Allium porrum (leek). We have also measured fluxes of P from fungus to plant and have been able to partition these between the arbuscular and total (arbuscular plus hyphal) interfaces. The implications of this work, and suggestions for future investigations of the molecular mechanisms involved in nutrient transfer in mycorrhizas, are discussed. Key words: Mycorrhizas/Glomus intraradices/ATPases/Allium cepa.
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Claims 6
Claim · model-written summary
ATPase localization differed between plant and fungal membranes and between arbuscular and hyphal interfaces.
- Taxon
- Allium cepa
Allium cepae1 - Interactor
- Glomus intraradicesRhizophagus intraradicese1symbiont
- Gene
- H+-ATPasese0
- Outcome
- H+-ATPases have different localisatione0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“H+-ATPases have different localisation on plant and fungal membranes in arbuscular and hyphal interfaces”
e1“roots of Allium cepa (onion) and the fungus Glomus intraradices”
Claim · model-written summary
Some arbuscular interfaces had ATPase activity on both membranes, whereas most fungal membranes lacked it.
- Taxon
- Allium cepaAllium cepae1
- Interactor
- Glomus intraradicesRhizophagus intraradicese1symbiont
- Outcome
- H+-ATPase activity on both fungal and plant membranese0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“While some arbuscular interfaces had H+-ATPase activity on both fungal and plant membranes, in most cases the fungal membrane lacked this activity”
e1“roots of Allium cepa (onion) and the fungus Glomus intraradices”
Claim · model-written summary
Intercellular hyphal membranes always had ATPase activity while adjacent root cells did not.
- Taxon
- Allium cepaAllium cepae1
- Interactor
- Glomus intraradicesRhizophagus intraradicese1symbiont
- Outcome
- H+-ATPase and the adjacent root cells did note0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the plasma membranes of intercellular hyphae always had H+-ATPase and the adjacent root cells did not”
e1“roots of Allium cepa (onion) and the fungus Glomus intraradices”
Claim · model-written summary
The authors propose passive phosphorus loss from arbuscules followed by active plant uptake.
- Taxon
- Allium cepaAllium cepae1
- Interactor
- Glomus intraradicesRhizophagus intraradicese1symbiont
- Compound
- Pe0
- Outcome
- active uptakee0
- Qualifier
- passive loss of P from the arbusculese0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“passive loss of P from the arbuscules is associated with active uptake by the energised (ATPase-bearing) plant membrane”
e1“roots of Allium cepa (onion) and the fungus Glomus intraradices”
Claim · model-written summary
The authors propose passive carbohydrate loss from root cells followed by active uptake by intercellular hyphae.
- Taxon
- Allium cepaAllium cepae1
- Interactor
- Glomus intraradicesRhizophagus intraradicese1symbiont
- Compound
- carbohydratee0
- Outcome
- active uptake by the intercellular hyphaee0
- Qualifier
- passive loss of carbohydrate from the root cellse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“passive loss of carbohydrate from the root cells is followed by active uptake by the intercellular hyphae”
e1“roots of Allium cepa (onion) and the fungus Glomus intraradices”
Claim · model-written summary
Methods were developed to measure arbuscular and hyphal interface areas in leek mycorrhizas.
- Taxon
- Allium porrum
Allium ampeloprasume0 - Outcome
- surface areas of arbuscular and hyphal interfacese0
GlomusNot reportedValue
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“developed methods for measuring the surface areas of arbuscular and hyphal interfaces in different fungus-host combinations, Glomus spp./ Allium porrum (leek)”