Researchers from Moscow State University, led by MV Lomonosov, together with colleagues from the Main Botanical Garden and NV Tsitsina of the Russian Academy of Sciences, demonstrated that special bacteria living on the roots of leafless orchids assist plants in acquiring nutrients and surviving long periods without moisture. The findings were published in the journal Microorganisms, shedding light on the remarkable partnerships within orchid ecosystems.
Orchids form one of the largest and most diverse families of flowering plants. They are predominantly tropical, and a striking 70 percent of species grow as epiphytes, meaning they live on the trunks and crowns of trees. This lifestyle exposes them to limited soil moisture and intense ultraviolet radiation, compelling a unique adaptation strategy to thrive in such environments.
In response to aridity, many orchids have developed a frugal metabolism. Their leaves have become reduced to tiny scales, while their aerial roots assume a crucial role. These roots are enveloped by a moisture-absorbing fabric called velamen, which also provides protection against ultraviolet light and physical damage, enabling efficient water uptake from the air and surrounding microclimates.
Velamen hosts a diverse community of fungi and bacteria, forming a dynamic microbiome that influences orchid nutrition. Earlier work from Moscow State University revealed that bacteria inhabiting the aerial roots of leafy orchids help fix atmospheric nitrogen, converting it into forms usable by the plant. Some of these microbes also secrete auxins, a class of plant hormones that regulate growth and development in orchids, contributing to root formation and overall vigor.
The latest study focuses on leafless orchid species, specifically Chiloschista parishii Seidenf. and Microcoelia moreauae L. Jonss. Researchers collected root samples from greenhouse plants and examined their microbial communities through DNA sequencing, enabling a detailed look at who lives where and when on the roots.
As observed in leafy orchids, cyanobacteria dominated the root ecosystems of the leafless species, but their species makeups were broader, and their communities formed more substantial biofilms than those found on leafy relatives. The composition of these bacteria shifted in tandem with the development stage of the roots, highlighting a dynamic, stage-dependent symbiotic relationship.
Overall, the study reveals a tight link between leafless orchids and their bacterial partners. Cyanobacteria colonize the aerial roots, supplying nitrogen-containing compounds that fertilize the plants and support their growth in nutrient-poor environments. This mutualistic interaction suggests that orchids rely on their microbiomes not merely for basic nutrition but also for adaptive resilience in variable moisture conditions.
Another well-documented example of such cooperative systems is Azolla, a water fern that hosts cyanobacteria inside leaf hollows, specifically Anabaena. When used as a bio-fertilizer in rice fields, Azolla enriches the crop with additional nitrogen, illustrating how plant-microbe partnerships can play a practical role in sustainable agriculture by reducing synthetic fertilizer inputs and boosting soil fertility.