Tree Biology

A seed is a wonderful place to live if you are a small insect. Protected by the seed coat and the fruit or cone, nourished by storage compounds like starch and fats, a developing insect has everything it needs. Many insects oviposit (plant eggs) directly into developing embryos. If an insect makes the wrong choice and oviposits into an unfertilized ovule, the developing insect may be out of luck. In conifers, unfertilized ovules become empty seeds, and the tree puts no resources into empty seeds.

Patrick von Aderkas of the University of Victoria and his colleagues found that some insects can avoid making the wrong choice by making the plant feed a larva even if the ovule was not fertilized. A tiny chalcid wasp, Megastigmus spermotrophus (Hymenoptera: Torymidae), deposits eggs in Douglas-fir ovules before they are fertilized. As the wasp larva develops, resources are directed to the seed just as if it had been fertilized. Instead of depending on the tree to supply resources to a fertilized see, Megastigmus manipulates seed development to its own advantages.

von Aderkas P et al 2005 Seed parasitism redirects ovule development in Douglas fir. Proc. Royal Society B Early Online Publishing

While camping along a creek in central Kentucky last week, I was reminded of the unusual chemistry of buckeyes. The creek had long ago been dammed to make a quiet pool for swimming, and buckeyes were scattered along the shore, their branches with young fruits overhanging the water. Long ago, Native Americans would dam quiet creeks like this one and use buckeyes to catch fish. After damming the creek, they would collect and crush the buckeye fruits and float them on the surface of the creek. Fish, stunned by the poisonous buckeye, would float to the surface. Despite the toxicity of the seeds, Native Americans used them as a food source. As with many nuts, the toxic compounds were leached out in water before pounding the seeds into meal to make flour. Buckeyes are uniquely toxic among temperate trees. Most trees in temperate zones are defended by phenolic compounds such as tannins. Their toxicity is generally low. A few trees, such as yellow-poplar, Liriodendron tulipifera, contain alkaloids and are a bit more toxic. Buckeyes are in an entirely different realm of toxicity. Most parts of a buckeye are lethal to livestock and other mammals, including humans. Even the flower nectar is toxic to honeybees. I have seen piles of dead bees beneath buckeye trees in the spring. I don't know what the native pollinators of buckeyes might be. Honeybees are not native to North America. The toxins in buckeye include the coumarin glycoside aesculin, one or more saponins including aescin and one or more alkaloids. Aesculin has potent pharmacological properties, causing severe gastrointestinal and nervous system symptoms. [Note: this paragraph was edited to correct an error in the description of aesculin]. Buckeyes are elegant trees common along creeks, coves and lower slopes in Kentucky. They are easy to spot with their broad, palmately compound leaves, showy fragrant flowers and large fruits. Buckeyes are the earliest understory trees to leaf out in the spring. There are two eastern buckeyes whose ranges overlap in Kentucky: Ohio buckeye, Aesculus glabra, and yellow buckeye, Aesculus flava. In central Kentucky, both species are found along the Kentucky River and its tributaries, and hybrid trees are common. Why is one particular genus so uniquely toxic? It's impossible to say. In the tropics, where pressure from herbivores is greater than in temperate forests, trees have a great diversity of toxic substances. In temperate forests, this kind of lethal chemistry is rare. Off the top of my head, I can't think of another North American tree genus with such toxicity to a wide variety of animals as we find in buckeyes. Update: Aesculin is the glycoside of 6,7-dihydroxycoumarin (also known as aescin or escin). Aescin is a product of commerce, used in the pharmaceutical industry for a variety of drugs, and obtained commercially from flowering ash, Fraxinus ornis. It appears that the toxicity of buckey is related to the combination of this potent drug along with saponins and alkaloids. Update: Coincidentally, Wayne Hughes at Niches has a nice photo essay up about bottlebrush buckeye, Aesculus parviflora, posted on the same day.

Developing fruit of hybrid buckeye, Aesculus sp.

Leaf of yellow buckeye. Leaves do not contain aesculin but do contain other toxins including saponins.

The latest Tangled Bank is up at Geomblog. Tangled Bank is a blog carnival for science writers. The current offering has lots of great articles and is worth a visit.

Wayne Hughes, at Niches, continues his interesting series on the development of complex flowers with beardstongue, whose inflorescence is technically described as a thyrse.

The date palms of Judea are praised in the Bible and the Koran for their beauty, shade, food and medicinal qualities. Now we may get a chance to taste the famed fruit. A date seed found during archeological excavations of Masada, where Jewish Zealots killed themselves rather than fall to the Romans, has germinated and appears likely to survive.

Radiocarbon dating confirms the archeological evidence: the seed is about 2,000 years old. This is the oldest seed ever germinated. Lotus seeds of about 1200 years' age have been germinated in China, but none as old as this.

The seed was obtained by Dr. Sarah Sallon, a physician and scholar of medicinal plants of the Middle East. It was germinated by Dr. Elaine Soloway of the Arava Institute for Environmental Studies.

At the time of the fall of Masada, when Israel ceased to exist for 1900 years, vast plantations of date palms were found throughout the region. Dates were an important food and medicine. The biblical land of "milk and honey" refers to date honey. The Koran describes the date as a symbol of goodness, and the date palm is associated with heaven. By the time of the Crusades, all the date palms had been destroyed. Modern Israel, for which dates are important symbolically and economically, grows date trees imported from California and originating elsewhere in the Middle East. So, the growth of this seedling could potentially resurrect a highly important plant, both economically and culturally.

There is an important unknown, that will not be revealed for at least 20 years: is the tree a male or female?  Dates are dioecious, bearing male flowers on one tree and female on another. If this tree is a male, it will only be a historical curiosity. If it is a female, there should be adequate pollen from males of other date palms to allow the tree to bear fruit.

If all goes well, we could once again taste the fruits of the land of milk and honey, thanks to the Zealots of Masada and the curiosity of a group of scientists.

Mycorrhizae, the intimate mixture of plant roots and fungal tissue, benefit the host plant in many ways, including increased phosphorus and water availability. The role of mycorrhizae in nitrogen nutrition has been a subject of debate. Now, USDA scientists report in Nature (article not yet posted) that arbuscular mycorrhizal fungi take up inorganic nitrogen from soil, store it as the amino arginine and then transfer it to the host plant. It now appears that mycorrhizae benefit the host plant by increasing nitrogen availability. If this is borne out by further studies, and extended to the ectomycorrhizae, the other major mycorrhizal type, it has important lessons for plant management. Crop producers may find that they can boost productivity by reducing nitrogen fertilization. It is known that nitrogen fertilizer reduces the frequency of mycorrhizae and favors formation of non-mycorrhizal roots. By promoting the development of mycorrhizae, productivity could be boosted without added nitrogen. Since nitrogen is one of the most expensive, and most polluting, of agrichemicals, a reduction in its use could have many benefits. This research also shows that fungi play a more important role in the world's nitrogen cycle than previously realized. This should prompt additional research into revising the nitrogen budgets of farms and forests to reflect the role of fungi.

Arbuscular mycorrhiza of corn. The bright green object is the mycorrhiza (the combination of root and fungus). The round objects are spores and the filaments are fungal hyphae. Photo courtesy of USDA ARS.

Horticulturists have selected and bred fantastic ornamental plants with showy, often dramatic flowers. Along the way to today's modern flowers, something has been lost. In the last 50 years, as breeders have selected for better or new colors, larger flowers, longer stems and longer shelf life, they seem to have inadvertently selected against smell. Many modern flowers lack odor or have minor smells compared to their wild ancestors or older cultivars. Now breeders are beginning to pay more attention to smell and to select modern cultivars for odor as well as appearance. Nursery catalogs are beginning to list smell along with color and other descriptors. The desire for strong fragrance is also behind the resurgence in traditional and heirloom varieties.

Mycorrhizae, the mixture of plant tissue and fungal tissue to form roots, are critical to the success of plants on land. Mycorrhizae increase the soil volume occupied by a plant and enable plants to obtain nitrogen and phosphorous that otherwise would not be available. Mycorrhizal fungi also helps plant roots resist invasion by disease-causing fungi and increase water uptake. Trees are utterly dependent on the mycorrhizal habit and cannot survive without their fungal partners. The earliest fossil roots are mycorrhizae, and today nearly all wild plants and many domesticated plants mycorrhizal.

How do plants persuade a fungus to colonize their roots? Plant scientists have long known that many mycorrhizal fungi can be cultured in the lab only in the presence of extracts of their host plant roots. What is the essential ingredient that attracts fungi and allows them to grow in intimate contact with plant cells?

In a painstaking series of experiments reported in today's Nature, Kohki Akiyama, Ken-ichi Matsuzaki and Hideo Hayashi of Osaka Prefecture University and the Japan Science and Technology Agency have answered that question, at least for one plant species. Lotus root extracts promote branching of hyphae in germinating Gigaspora margarita fungi. This branching is characteristic of the formation of one type of mycorrhiza, the vesicular-arbuscular (VAM or AM) mycorrhiza. By carefully fractionating chemical groups in Lotus roots , Akiyama and colleagues identified the stimulating compounds as a type of sesquiterpenes known as a strigalactone.

Interestingly, these same compounds had previously been identified as germination promoters for seeds of Orobanche, Striga and other hemiparasitic plants. These plants couple to host plants via the fungal partner of the mycorrhizal relationship.

This is an important finding. It will allow scientists to focus on the factors that stimulate or inhibit formation of mycorrhizae, and possibly to select plants with improved promotion growth due to better mycorrhizal relations.

This has been a remarkable year in plant biology so far, with major strides in many areas, such as the discovery of the way auxin works. Identification of the stimulating factor in mycorrhizal formation is very important. Both of these discoveries open up new avenues for research in areas of plant science that were stagnated by a lack of new routes to explore.

Our earlier post on gigantic corpse lilies blooming in San Francisco and Wisconsin led to the following comment from Darrell Schulte at UW-Madison. Click on the link to see the Big Bucky corpse lily in bloom. Watch for a while - only when you see people in the scene will you appreciate how huge the flower is, and you can listen to people talking about the stink. Thanks, Darrell!

FOR IMMEDIATE RELEASE

6/6/05

CONTACT: Brian Rust (608) 263-9484, rust@doit.wisc.edu TITAN TV: LIVE SMELLY PLANT NOW ON A DESKTOP NEAR YOU! MADISON - The University of Wisconsin-Madison's giant stinky flower is now just a click away. As of Monday (June 6), live views of the titan arum known as Big Bucky are available on the Internet. The large flower is expected to transition into malodorous bloom sometime this week, and it may draw large crowds seeking to experience the rare plant. But for those unable to make the trek to the UW-Madison campus and the Birge Hall greenhouse where the plant resides, a live video broadcast can be accessed from the Internet at: http://webstreamer2.doit.wisc.edu/titan_arum/ The play-by-play broadcast, which features the striking plant from three camera angles, including a bird's-eye view, will continue until the blooming event is over. Additional information about the plant, as well as news and viewing hours and opportunities, can be found on the Web at http://www.news.wisc.edu/titanarum2005/. Special features, such as interviews with the plant's caretakers, are planned. Produced by the UW-Madison Division of Information Technology, the titan arum webcast is streaming at two different rates to accommodate the range of broadband users, and downloadable clips and "highlights" and interviews for dial-up users. In 2001, a cruder Web cam protocol was used and generated an enormous volume of interest worldwide. The current broadcast is a live video stream, as opposed to the photo sequences used by Web cams. The titan arum is native to the equatorial rain forests of Indonesia. It is a relatively rare plant and is known for its capacity to generate an overpowering smell of carrion when it blooms. The smell attracts pollinators such as flies, beetles and bees. In captivity, there have been only an estimated 65 blooms since 1889, when the first cultured titan arum blossomed at the Royal Botanical Gardens in Kew, England. ### - Terry Devitt, (608) 262-8282, trdevitt@wisc.edu

An inflorescence is a cluster of flowers on one stem. Botanists have a lot of names for different kinds of inflorescences, like spike, corymb, raceme. These names are hard to keep straight, but they are important for plant identification and understanding reproduction. Dr. Wayne Hughes, at Niches, has the most lucid, simple explanation I have seen of how different inflorescences form. By following the expression of each floral meristem, Wayne focuses on the important distinctions among inflorescence types. Thanks, Wayne for another great article.

The USDA has one of the world's largest collections of Citrus at its National Clonal Germplasm Repository for Citrus and Dates in Riverside, California. The collection is an important ex situ repository for preserving Citrus genotypes (meaning a site outside the natural range of the genus). Citrus originates in Southeast Asia, China and India. Many Citrus populations are threatened by industrial and housing development, so an ex situ germplasm collection is especially important. An interesting article in the ARS Agricultural Research magazine describes the Citrus conservation work of Robert Krueger and his colleagues at Riverside. One problem with the ARS germplasm collection is that recordkeeping was often spotty. Of the 900 accessions in the collection, many were of unknown or poorly documented origin. Using molecular techniques, Krueger found that most of the genetic diversity in the collection was in about 50 trees. This is not surprising because most commercial Citrus is of hybrid origin, so a small number of plants can encompass much of the genetic diversity. This combination of a traditional germplasm collection and molecular analysis to sort out the collection is very powerful in helping conserve the genetic diversity of this important genus. The article is well worth reading.

Fruits of lemon, Citrus limon, in a commercial orchard in Ventura County, California