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Biology Faculty Publications Biology

Summer 2019

Nitrogen Fixation in Roots of Ceanothus

W. John Hayden University of Richmond, [email protected]

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Part of the Botany Commons, and the Plant Biology Commons

Recommended Citation Hayden, John W. "Nitrogen Fixation in Roots of Ceanothus." Sempervirens: The Quarterly of the Virginia Native Plant Society (Summer 2019): 6-7.

This Article is brought to you for free and open access by the Biology at UR Scholarship Repository. It has been accepted for inclusion in Biology Faculty Publications by an authorized administrator of UR Scholarship Repository. For more information, please contact [email protected]. 6 Sempervirens, Summer 2019 Nitrogen Fixation in Roots of Ceanothus Article by W. John Hayden, Botany Chair oots are usually out of sight and, nitrogen-containing compounds. Rtherefore, out of mind. But as But there is a paradox about any good gardener will tell you, it is nitrogen and life. You, me, the of utmost importance to understand chickens in my backyard, and the those unique plant organs, even if their Pawpaw tree that shades them are essential functions occur hidden from all constantly bathed in atmospheric cursory observation. The red roots of nitrogen gas, yet that form of nitrogen the 2019 VNPS Wildflower of the Year, is completely unavailable to our cells, Ceanothus americanus, are particularly or the cells of my chickens, or my important because they host symbiotic Pawpaw. Every breath I take brings a bacteria that perform the essential quantity of fresh air into my lungs, 78 function of nitrogen fixation. These percent of which is nitrogen gas, yet prominent, knobby, distinctively despite the importance of nitrogen- Figure 1. New Jersey Tea or Redroot pigmented roots are also the inspiration containing compounds for life, all (Ceanothus americanus); the bustle of for the common name Redroot, applied that inhaled nitrogen is exhaled, biological activity evident here is possible to many species in the genus. unused and unmodified. For most life thanks to the nitrogen-fixing activity of Nitrogen is important. Nitrogen- forms, nitrogen gas is inert, despite its actinomycete bacteria living inside this containing molecules are diverse importance in the biochemistry of life. plant’s roots. (Photo by Betty Truax) and absolutely essential for life. It is Critically, there are exceptions no exaggeration to say that proteins to the picture painted above. A few This is the importance of symbiotic make up much of the physical different kinds of bacteria can take biological nitrogen fixation; bacteria structure of cells; proteins are also atmospheric nitrogen gas, rip apart convert relatively inert nitrogen gas into responsible—as enzymes—for the triple bond that holds its two ammonia, the host plant gobbles up mediating the majority of biochemical atoms of nitrogen together, and the fixed nitrogen and converts it into reactions that constitute life at the add enough hydrogen to form two diverse nitrogen-containing organic chemical level. Proteins are built from molecules of ammonia. This process, molecules. As we all know, plants feed chains of amino acids, and every known as biological nitrogen fixation, the rest of the biosphere. Animals get amino acid includes at least one atom consumes considerable amounts of the nitrogen-containing molecules they of nitrogen (a few amino acids have energy (provided by ATP, referenced need by eating plants or other animals two nitrogen atoms). Nitrogen is briefly above). Also, nitrogen fixation that have eaten plants. (The Bible also present in DNA and RNA, so, must be performed in the absence has it right: “All flesh is grass” [Isaiah through these molecules, nitrogen of oxygen, because oxygen disables 40:6].) Finally, as plants and animals contributes substantially to the the nitrogenase enzyme responsible die and decompose, microbes recycle storage, expression, and inheritance for this essential reaction. Unlike the their large nitrogen-containing organic of genetic information. Then there is nitrogen gas from which it is produced, molecules into simpler forms that can ATP, a relatively small molecule that ammonia is much more readily used by be taken up by plants—any kind of functions as a little packet of usable cells. Some plants can use the ammonia plant, not just the special ones that host energy that fuels all sorts of activity directly to synthesize the nitrogen- nitrogen-fixing bacteria. There is a bit at the molecular level. And let’s not containing organic molecules listed more complexity to the nitrogen cycle forget the hugely diverse category of above; of course, the bacterial cells can in its entirety: lightning converts some molecules called alkaloids, synthesized do this too. The important point is atmospheric nitrogen gas into oxidized by many different plants, largely to that once the relatively inert nitrogen forms that plants can assimilate, deter herbivory, but also having impact gas has been converted to ammonia, and there are other types of bacteria on people as poisons or, beneficially, the chemistry of moving that form of that take oxidized forms of nitrogen as medications. It is impossible nitrogen into proteins, DNA, RNA, and convert it back to relatively inert to imagine life on Earth without ATP, etc., is relatively easy. (See Ceanothus, next page) Sempervirens, Summer 2019 7 Botanical art exhibit reflects journey across America The Botanical Art Society of the in houses in Washington, Potomac, National Capital Region cordially and Alexandria. Green Spring is an invites members of the Virginia Native ideal venue to display pictures of Plant Society to the opening reception the kinds of plants that she might of “All in a Garden Green” at Green have encountered on her American Spring Gardens Park on Sunday, adventure. Subjects will include plants September 8, from 1-3 p.m. native to the eastern U.S., as well as The exhibit is inspired by English those that were cultivated on early writer Frances Trollope who, in 1828, American estates such as Green Spring. Illustrations such as this Flame Azalea by began a four-year journey around the The exhibit will be held at the Elena Maza Borkland will be on display at United States. In her book, Domestic Horticultural Center (4603 Green Green Spring Gardens until late October as Manners of the Americans (1832), Spring Rd., Alexandria, VA 22312) and part of the “All in a Garden Green” exhibit she noted with delight the plants she sponsored by the Botanical Art Society of runs from August 27 to October 27. the National Capital Region. encountered. Trollope spent most of 1830 —Elena Maza Borkland Ceanothus continued atmospheric nitrogen gas. The point is its leaves that serve the same Compared with intensively studied that nitrogen fixation and subsequent purpose. And free-living (i.e., legume crops, there is little published recycling of fixed nitrogen accounts nonsymbiotic) cyanobacteria also material on nitrogen fixation in for the majority of the ways in which perform nitrogen fixation. Ceanothus; the few studies that have nitrogen sustains life on this good Earth. The roster of bacteria that been published focus on species from The most well known examples of can perform symbiotic nitrogen the western United States, where symbiosis-based biological nitrogen fixation includes one additional Ceanothus is diverse and, in at least fixation involveRhizobium and closely group known as actinomycetes. some plant communities, abundant. related bacteria that inhabit the root These are minute, filamentous, Our local Redroot, Ceanothus nodules of legumes. In essence, the bacteria that, decades ago, were americanus, shares with its western legume plant feeds the bacterium thought to be fungi, hence the relatives a tendency to inhabit nutrient- energy-rich molecules derived from “-mycetes” suffix in their name. poor, arid, rocky soils—the sort of photosynthesis from which the When present symbiotically, environment in which having a built-in Rhizobium bacteria generate ATP that actinomycete bacteria inhabit nitrogen fertilizer factory would fuels the production of ammonia from plant roots, which alters provide an obvious adaptive advantage. nitrogen gas; a portion of the ammonia their morphology; generally, Next time you encounter a produced is released to the plant for actinomycete-inhabited roots are specimen of our 2019 Wildflower of its diverse biochemical needs. Vast enlarged and knobby, compared the Year and admire its handsome amounts of nitrogen enter the biosphere with uninfected roots. Host plants foliage, its intricate small flowers, by means of legumes and their nitrogen- of nitrogen-fixing actinomycetes and the diverse pollinators fixing bacterial symbionts. are diverse, including Alders attracted thereto (Figure 1), take a But the legume–Rhizobium (Alnus.), Sweetfern (Comptonia), moment to acknowledge also the partnership is not the only Wax Myrtle (Morella, Myrica), hardworking actinomycete cells, nitrogen-fixing symbiotic system. She-oaks (Casuarina), Oleasters sight unseen, collaborating with Cycads have special roots that (Elaeagnus), and Redroots Ceanothus root cells in the task of grow near the soil surface, and, (Ceanothus). converting atmospheric nitrogen internally, these roots harbor In Ceanothus, the symbiotic into the nitrogen-containing organic cyanobacteria (blue-green algae) actinomycete has been identified as molecules essential to all the life that perform nitrogen fixation. Frankia ceanothi, colonies of which forms that surround us ... and that, The peculiar floating aquatic fern develop inside cortex cells of its indeed, are essential to us too. v Azolla hosts cyanobacteria inside nodulated (enlarged, knobby) roots.