In order to pinpoint mechanisms used by hosts to control arbuscule formation, the researchers focused on a mutant strain of the plant Lotus japonicus, a wild legume related to beans, peas and lentils, that features a perturbed arbuscule formation. "In this strain, we identified the gene affected by the mutation as RAM1, the product of which is required for the activation of other genes, and hence for the production of the proteins they encode," says Gutjahr. "These proteins, in turn, are very probably required to permit arbuscule formation to proceed. Their precise functions remain to be characterised in upcoming projects."
Shrub-like arbuscules | in root cells of Lotus japonicus, a wild legume, stained with a fluorescent dye. Scale bar = 25 µm.
The RAM1 gene itself is strongly activated during arbuscule formation, so the mechanisms responsible for its own activation are of great interest. Gutjahr and her colleagues have now shown that two different regulatory proteins are required to induce the gene: the transcription factor CYCLOPS, which was already known to play a key role in the regulation of root symbioses; plus a protein called DELLA, that was also familiar, albeit in another context. DELLA forms part of a signal transduction pathway that is activated by plant hormones called gibberellins, which are essential for the control of plant physiology and growth.
"To our surprise, we found that, in the context of RAM1 activation, CYCLOPS and DELLA interact with one another directly," Priya Pimprikar explains, a doctoral student in Gutjahr's group and first author of the current study. Caroline Gutjahr: "With this interaction, we believe we have identified, for the first time, one of the central nodes upon which information relating to symbiosis on the one hand, and plant physiology on the other, converges, thus enabling the plant to determine the extent of root colonisation in accordance with its current need for phosphate, for instance."
The symbiosis between plants and mycorrhizal fungi is also of economic and ecological significance, as intensive farming requires enormous amounts of artificial fertilisers each year to maintain the fertility of the soil. These fertilisers contain mineral phosphates that are only available in finite quantities, some estimates even suggest that reserves may run out within the next 100 years. "Arbuscular mycorrhiza can help reduce the need for the application of phosphates in agriculture," Gutjahr eplains. "And a better understanding of the mechanisms underlying arbuscule formation is an important prerequisite for the success of plant breeding efforts to enhance the efficiency of this type of symbiosis."
(© Ludwig Maximilians University of Munich LMU, AcademiaNet)