Are there any plants which do not photosynthesize at all? –Swaagahs Saikih, Singapore
Most plants are autotrophic, meaning they produce their own food from basic inorganic substances around them – water, atmospheric carbon dioxide, and mineral nutrients from the soil. Animals, fungi, and most other organisms on Earth are heterotrophic – we consume organic carbon, unable to process the raw material ourselves. Plants form the basis for worldwide food webs, turning inorganic carbon into sugars that are consumed by almost everything else.
Typically, plants “fix” carbon from the air as CO2, mix it with water, and convert it to glucose through photosynthesis. Powered by energy from sunlight, this reaction takes place in specialized structures called chloroplasts — organelles contained in the cells of plants. Each chloroplast acts like a small solar factory, bringing in raw materials and putting out usable carbon and waste products. Although the raw materials may seem easy to find, building and maintaining these chloroplasts, and growing specialized structures to capture sunlight (leaves) can be an expensive processes.
Because photosynthesis can be costly, plants around the world have developed an alternative to collecting and converting their own carbon – they steal it from other organisms. This strategy is known as parasitism, and has independently evolved at least 12 times during plants’ reign on the Earth. Parasitic plants have developed their own specialized structures — penetrating projections in their roots called haustoria that tap into hosts and steal carbon, water, and micronutrients. This process has allowed them to supplement or forego photosynthesis, and occasionally to lose the leaves no longer needed to house their carbon processing facilities.
The degree to which parasitic plants can fix their own carbon varies. Some have the ability to survive without a host and are called “hemiparasites.” These plants retain their photosynthetic abilities and supplement their own carbon production with that from a host when available. Those that have delved so fully into parasitism that they have become wholly dependent on their hosts for survival are known as “holoparasites.” However, not all holoparasites are entirely non-photosynthetic.
In the San Francisco Bay Area, dodder (genus Cuscuta) is a local representative of the parasitic plant lifestyle. This unusual plant appears as yellow coils wound around its host like a sprayed can of silly string. Its leaves are reduced to diminutive scales on the stem, and its root system shrivels and disconnects once the plant is attached to a host. It is an obligate holoparasite, unable to survive and reproduce on its own. Yet some species of dodder have retained the ability to photosynthesize, as evidenced by sparsely distributed chloroplasts in their stem cells. Maintaining the power to acquire its own carbon may give this plant greater flexibility, allowing it to survive when times are tough.
As research tools have changed, so has our understanding of the ecology of parasitic plants. In years past, we used powerful microscopes to look for chloroplasts. Now we can use genetic analysis tools to understand if plants have the ability produce photosynthetic structures. To find truly non-photosynthetic plants, we can look for clues deep within the chloroplast DNA. As plants evolve further into holoparasitism, selective pressure on the chloroplasts decreases. This allows mutations to build up, and as a result, parasitic plant chloroplasts have rapidly evolving genes that are very different compared to their photosynthetic relatives. New genetic information from holoparasitic plants around the world shows a common trait – a dramatically reduced code for photosynthesis.
Thismia tentaculata, a recently discovered parasitic plant from the lowland forests of coastal Vietnam and Hong Kong, is a prime example of shortened genetic code. Named after the tentacle-like petals that burst from its tiny flower, the T. tentaculata chloroplast genome has been reduced by an order of magnitude over its non-parasitic relative in the genus Tacca. This reduction includes a complete loss of the photosynthetic genes, meaning this plant has committed to acquiring all of its carbon from its host.
An even more dramatic example is Rafflesia lagascae, a holoparasite native to the Phillippine Islands. Plants in the genus Rafflesia are well known for both their parasitic nature and their knack for producing the largest flowers in the world – up to a meter in diameter. In a 2014 study, scientists were unable to locate either chloroplasts or the genetic codes to create them in R. lagascae. If these results are replicated, they would indicate Rafflesia may be the first known plants to have completely lost their ability to both photosynthesize and to produce any of the microscopic structures related to photosynthesis.
So yes, we now know there are plants that are completely non-photosynthetic, and many more are on a similar evolutionary path. With between 4,000 – 4,500 parasitic plants in the world having a parasitic lifestyle, we have many remaining places to look.
Trent Pearce is an interpretive naturalist with the California Center for Natural History and the East Bay Regional Park District. Currently residing in Berkeley, California, he enjoys documenting the Bay Area’s flora, fauna, and fungi on www.iNaturalist.org, and regularly explores the trails of the East Bay, camera in hand.
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