By Dr. Mohammed Sa’id Berigari, Senior Soil and Environmental Scientist, USA, 20/05/2012.
Which USA Crop Wild Relatives Are Identified
as Most Useful?
Adapting agriculture to climate change is one of the greatest challenges of 21 st century facing humanity. By 2050 world population is expected to exceed 9 billion. We will reach the limits of the world arable land expansion and other resources for agriculture. Is it possible to meet the demands of such a large population for food, shelter, and fiber without huge increases in agricultural products? The answer is definitely no. Therefore, it is an urgent matter to plan now for new varieties of crops that can adapt to climate change, resist biotic stresses such as pests and diseases; and tolerate abiotic stresses such as drought, cold, heat, and other conditions; and to a great extent sustain high yields with desirable qualities.The crop wild relatives (CWR) are wild plant taxa closely related genetically to crop plants. CWR posses a wide range of biodiversity i.e. genetic variation in contrast to the cultivated crops for which domestication has tended to create genetic bottleneck. Moreover, CWR represent an excellent source of genomes for plant breeders to utilize their potentials for developing desirable traits into cultivated crop. And CWR in general have been identified as critical useful groups of plants for agricultural and environmental sustainability and improvement.
However, the survival of some CWR is alarmingly threatened by irreversible agro-environmental practices, destruction and fragmentation of their habitat, and climate change. Therefore, concerted efforts are needed to establish global networks for the in situ conservation and protection of CWR.
A historical approach is appropriate to follow here for the reader to appreciate the contributions of numerous scientists in this field of vital importance to food security of the entire world population now and in the foreseeable future.
N.E. Hansen, a botanist, brought a wild Siberian alfalfa species to the US in the late 1800s with expectations of its potential to survive extreme cold and drought, thus, to benefit farmers of the northern Great Plains, which he called” My American Siberia”. Farmers sowed the unusual, yellow-flowered alfalfa into their farms, while plant breeders used it to breed winter- hardiness into cultivated blue-flowered alfalfa. To Hansen’s surprise the yellow-flowered alfalfa escaped cultivation and went wild, adapting to the Great Plains environment and generating greater people’s interest. After many decades a rancher from South Dakota in 1997 reported that wild populations of yellow-flowered alfalfa were boosting forage production and fattening his cattle. The rancher began exploring the plant’s ability to restore degraded soils of his pasture. Plant breeders also gave the yellow-flowered alfalfa a second chance, using it to develop a new variety of alfalfa adapted to the cold and dry rangeland of the region.
Meanwhile, wild populations of yellow-flowered alfalfa had spread to the Grand River National Grassland in northwestern South Dakota, and their ability to displace other species bewildered scientists what to do. Should they allow this wild species to become a part of the landscape? Or should they eradicate it to protect native communities? For either option the plant seemed hardly to require such actions. “Or does it?” That is the question went around “crop wild relatives”: species that are genetically related to domestic crops, forages, ornamentals, medicinal herbs, and other beneficial plants but are not domesticated. Some are common and weedy like the yellow-flowered alfalfa, while others like the walnut relative, Juglans hindsii, are endangered globally.
Stephanie Green, a plant geneticist, claims that the lack of attention has put CWR in a precarious position. According to a recent estimate one fifth of all wild plants are now threatened with extinction, that is before the potential influence of climate change is included in that estimate. Despite world initiatives to conserve biodiversity, according to Greene, “it is recognized that crop wild relatives have been left behind.”
She is currently leading an effort to tally the CWR in USA, identify which are of greatest importance to global and American agriculture, and to initiate a nationwide strategy for protecting them both in gene banks as ex situ and in their natural habitat as in situ. However, conserving CWR is the first step forward. The real objective is to obtain their diverse genetic materials and allow plant breeders to adapt crops to tolerate increased drought, intensified disease pressure, and climate change.
Greene started that task while CWR were and still are threatened by climate change just like all wild species , they are also the same plants that could help us adapt our food systems to the new conditions.
Greene after reviewing scientific literature and other resources listed CWR growing in USA and collected information about them including their conditions in the wild and the crops they used to improve. She also assessed how closely related each wild relative was to its respective crop and obtaining help in this area from John Wiersema, a plant taxonomist with USDA. Wiersema is the manager of taxonomic information for National Plant Germplasm System’s database of crop genome, known as GRIN. Along similar lines Wiersema and coworkers began a project in 2008 of identifying methodically CWR from the entire world and classifying them taxonomically. The idea was that the closest relatives should be more suitable and the easiest for plant breeders to use.
Wiersema said” that is true in traditional breeding and perhaps even true with advanced genetic techniques.” And he also stated “the most related plants will offer the best chance to transfer favorable genes into crops.”
Greene started to put together an inventory and when she finished, her inventory contained over 3,000 species, subspecies and varieties of the CWR in the US and she emphasized the need for the next step in Maxted’s strategy which is setting up priorities for conservation of CWR. Colin Khoury, a former student of Maxted came along to work with Greene on CWR of the US. He started on prioritization plan from Colombia, South America where he currently works for the International Center for tropical agriculture (CIAT).
Khoury’s method takes into account a number of factors. Ensuring food security is his major objective, thus US wild relatives of the 70 most important food crops of the world make up the bulk of his prioritization list. And he added to the list wild relatives of what he named” iconic U.S. Crops” plants of value primarily to American agriculture, including sugar maple, pecan, wild rice, and Echinacea. Khoury reduced the list further where he identified the very closest relatives of crop plants or” primary genetic relatives” and some of the CWR that are rare or endangered species. After intensive reviews by curators, plant breeders, and others the list now contains about 300 taxa that seem to have the greatest potential to contribute to crop improvement.
Which crops in the US have valuable wild relatives? According To Greene Sunflower is a major agronomic crop plant that has originated in North America. Her list includes the fruit crops: cranberry, blueberry, and currant; the nut crops: pecans, hazelnut, and walnut; and the stone fruits: almond, peach, and cherry. And also, lettuce, onion, bean, squash, sugarcane, and grape have rich native genomes in the US, all of which surprised Greene.
“ The general consensus is that CWR taxa are usually found in the Mediterranean Basin or in the Fertile Crescent in the Middle East and that North America is kind of depauperate ” according to Greene. However, North America has a good list of CWR. That is why Maxted argues for setting global priorities for CWR protection but also establishing national conservation strategies too. The most important CWR are found in the Vavilov Centers and in terms of food security they should be focused internationally. However, each country has some CWR and is the source of biodiversity community for which each country should show the ecosystem service value in their native plants.
According to Wiersema people also assume only native plants, such as sunflower in the US are the critical CWR. However, the US inventory contains dozens of non-native CWR taxa that developed valuable traits as they became adapted or naturalized to North American environment. Wiersema was surprised to find in the US that some genetic resources are weedy, non-native plants that breeders considered interesting. This suggests as Wiersema stated that there’s still an opportunity for evolution of naturalized species even though they have not been in their current habitat for a long time and that scientists who look for crop diversity only in places like the Vavilov Centers may overlook some important genetic resources elsewhere.
Looking for and Identifying the Gabs- then Filling Them
With the key US crop wild relatives being identified, Khoury plans to use a method developed by Maxted called “gap analysis” to determine the exact conservation action needed for each plant on the list. During the first stage Khoury uses database information and Geographic Information Systems (GIS) mapping models to predict the full geographic range of each CWR taxon across the US. Within those ranges, he will determine the places where each CWR has been collected previously, if any, and the locations where they are currently protected: in a national park or on the US Forest Service land.
According to Khoury the idea behind this effort is to find “interesting gaps” in collection and conservation, whether they are taxonomic, geographic, or environmental. Large taxonomic gaps in gene bank collection are known to exist. For example out of 450,000 plant varieties, or accessions, currently stored in by the National Plant Germplasm System, fewer than 3% are wild plants collected in the US. But obtaining one of each missing CWR taxon also won’t be enough, said Wiersema.
“We want not just to conserve these wild relatives, but also to conserve the diversity of CWR, which is something that‘s probably not paid attention to in conservation generally” said Wiersema. It is vital to preserve CWR with very limited distributions, such as Cucurbita okeechobeensis, a threatened squash relative that grows only along the shore of Florida’s Lake Okeechobee. It is critical to conserve weedier CWR, but also those more widely distributed CWR across the full range of the environment where they occur-especially in places where they may have developed useful traits such as tolerance to abiotic parameters of drought, heat, or salinity; or resistance to biotic agents such as pests, bacteria, fungi, or viruses.
“Our goal is to preserve all that genetic variability because for crop breeding purposes we are likely going to need it” said Wiersema, “
When Khoury locates the critical gaps, the National Plant Germplasm System will try to fill them by collecting CWR seeds in US and other genetic materials for their backup in gene bank or ex situ collections. Meanwhile, both Maxted and Khoury are engaged in a similar task at the international level, spearheaded by the food security foundation which is known as the Global Crop Diversity Trust (GCDT).
Gene bank collections are indispensible, but they also represent mere” snapshots in time” of adapting and evolving plants continuously. This is why Wiersema, Khoury, and others advocate a second complementary approach of protecting CWR plants in situ.
The advantage of keeping the CWR in situ is to allow species to continue evolving and helps protect their native habitat as well as associate species. According to Maxted in country after country in situ protection has proven harder than one would expect. People who want to conserve CWR can’t just set up their conservation areas. They must work with managers of national, state or provisional parks; national forests; and other lands to ensure that existing conservation strategies are expanded to include CWR. And this will require cooperation and understanding between agricultural scientists and plant conservation specialists. But getting those two groups to communicate with each other is very difficult as Maxted pointed out because of their different aims.
Maxted’s CWR management plan is currently moving ahead with UK conservation authority and still has a real chance of establishing the first reserve for CWR genetic diversity in Europe. According to Greene, the National Plant Germplasm System and US Forest Service signed a memorandum of understanding, outlining how the two parties can cooperate to protect CWR on national forest lands.
There is still much more to do. The human population is growing rapidly and habitat for CWR is shrinking due to urbanization and agriculture expansion. And plant distribution is shifting in response to higher global temperatures. It takes 10 years or more for a novel source of germplasm to get to farmers as an improved plant variety-leaving breeders valuable time now to adapt crops to climate change.
Khoury pointed out that the window of opportunity is narrowing for securing these genetic resources so that they can be safe but also be used. Therefore, it is really time to move forward and conserve these indispensible resources.
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