AGRICULTURAL REVOLUTION
The environment is a hot topic these days, with most people being aware of problems pertaining to global warming, air pollution, and/or loss of biodiversity. However, there is another problem that has been under the radar for quite some time; that is the challenge of sustainable agriculture. Whilst, the world’s population has been increasing at a fast rate; and food production has been more or less able to keep pace with this growth, many scientists believe that the future consequences of current agricultural actions (soil erosion and pest control) will lead to severe issues of sustainability and pesticide pollution at the ecological level [1]. In this paper, we will look into the ecological impact of agriculture and eamine one potential solution through the use of perennial crops.
Agricultural activities
In order to discuss the problems, we must first address some of the aspects of agriculture that lead to the problems. In short, agriculture, a 10,000 year old technology, requires the use of crops that are known as annual plants. These are plants that germinate, flower, and die within the context of a one year period. Annual grasses like wheat, rice, corn, rye, and barley account for 70 percent of all human calories [1]. Furthermore, for convenience and economic reasons, annual crops are planted in monocultures, which mean that the cultivars are low in genetic diversity. Indeed, annual crops are genetically very similar to each other if not identical due to domestication. These two aforementioned practices within agriculture are largely considered to be fundamental problems.
The implication of using annual plants as cultivars is that plowing is required yearly. Plowing is often thought to lead to soil erosion. Soil erosion, in turn, leads to loss of fertility of the land and thus crop growth would decrease. Compounding this, due to the lack of topsoil from erosion, chemical fertilizers are now a popular choice to assist in plant growth. However, the synthesis of chemical fertilizers requires natural resources. For example, natural gas (mainly CH4) is used in “Haber process” of synthesis of ammonia, which is an important nitrogen source for plants [5]. Basically, the natural resources needed for this mode of agriculture is significant.
Monoculture systems, with their lack of genetic variation, are particularily sensitive to parasitic attacks [1]. In order to minimize crop damage (and thus economic damage), farmers are known to eradicate pests by applying pesticides which in turn pollutes our groundwater, air, and soil. Again, the environmental consequences are significant.
Perennial plants as the solution
To solve these problems of sustainability and pollution, many scientists have looked at natural terrestrial biomes that are self-sustaining. Most of the plants that are observed in these areas are perennial plants. In general, these scientists want to develop a “natural agriculture” that mimics this type of system, which includes a mixture of perennial plants that are self-sustaining and require no outside input.
The most obvious attribute of interest here is simply the perennial characteristic of the plant itself, which means that they live for longer than two years. Consequently, these plants have a more developed root system, which happens to be an important difference between perennial and annual plants. A longer lifespan suggests that less plowing will be required and the long and dense root system of perennial plants is also capable of securing the soil much more tightly than annual plants – these features decrease soil erosion in general. As well, having a more developed root system, perennial plants absorb water and nutrients more efficiently in comparison to annual plants. Perennial plants also store more carbon in the soil and are more robust against abiotic stresses because of this same root system. In terms of sustainability: less soil erosion, better nutrient management, and less usage of fertilizers make this a more efficient process.
And if that wasn’t enough, wild perennial plants also tend to have genes that code for resistance against parasites, since they have been naturally selected to withstand many types of pests over the course of their perennial lifespans. For example, tall wheatgrass (Thinopyrum ponticum) possess immunity against the Barley yellow dwarf virus (BYDV) [2]. It is this type of characteristic in wild perennial plants that is normally coveted for agricultural cultivars. In all, fewer pesticides would be applied because of perennial crops’ natural resistance against diseases and pests.
So why not go ahead with perennial crops?
Primarily, the reason is because it is different and new from the norm. If the agricultural industry were to adopt the use of perennial crops, difficulties will arise because there are simply too few domesticated perennial crops that are in use in the agricultural industry. There are currently only a few perennial crops readily available for use around the world and these are primarily limited to hay, forage, and pasture crops. Therefore, in order for perennial crops to have an impact on agriculture, wild perennial plants would have to undergo the process of domestication in order for these to work in current agricultural settings.
But in order for a crop to become domesticated, it must meet several criteria for harvesting in contemporary agriculture – these include: synchronous maturity, large seeds, structurally stiff, erect, robust, and resistant to shattering or shedding of plant parts.
Currently, there are two approaches in breeding perennial crops: Direct domestication and wide hybridization [1].
Direct domestication is a process which through human influence, wild species of plants or animals are habituated and incorporated into a part of the human society. There are only a few steps involved in this form of domestication, which is really a series of artificial selection of desired traits.
1. In the case of perennial crop development, domestication starts off with identification of perennial species that have high and consistent seed production.
2. The progeny of these species are grown and further screened for other traits that would help with their effectiveness as a grain crop, such as synchronous flowering and maturity.
3. Then, there is a repeating of the process of selection and progeny growth until there is a uniform plant population that is expressing the desired phenotypic traits.
The main disadvantage of this method is that it is an extremely time-consuming process. It has been suggested that in order to improve the production yield of intermediate wheatgrass (the perennial counterpart to wheat) to comparable yields, the domestication process would require approximately 48 years assuming ideal situations.
The second method to perennial crop development is through wide hybridization, which is a complementary method to the direct domestication [1]. The main concept of wide hybridization is to interbreed different species or genera with each other (hence the name “wide”). Of the world’s 13 most heavily grown oilseed and grain crops, 10 of them are capable of being hybridized with perennial relatives. Specifically, the process would consist of crossing species of annual with perennial relatives, with the goal being the development of progeny that contain characteristics of both parental types. Annual crops can deliver genes that help shape domestication as well as enable high grain yield. Perennial crops would supply the perennial characteristics to the progeny. Ideally, the progeny would have desirable qualities of both annual and perennial crops at the genetic and phenotypic level.
However, it is worth noting that when annual crops are crossed with their perennial relatives, most often differences in chromosome numbers and lack of chromosomal homology between the two species will lead to sterile progeny [1]. This type of obstacles can be solved by using different genetic variants of the same species (for a larger gene pool) and crossing it with the other species (also using various genetic variants) until fertile progeny is achieved, but is a slow process and requires tremendous effort.
Molecular Biology at work
Since, as mentioned before, the development of perennial crops requires the ability to select for traits, a major roadblock is that to do this conventionally, one would have to wait for a certain developmental stage to visualize the trait of interest being expressed. However, with the development of molecular marker-assisted selection (MAS), the presence of traits can be determined by analyzing the DNA from young leaves, without the need for this waiting. MAS works by an understanding of the linkage of the gene that governs the phenotypic trait of interest with a genetically closely positioned “molecular marker” or site of heterozygosity [4]. In essence, presence of these molecular markers are “indicators” that show whether there is the presence of the gene of interest or not. Overall, the advent of MAS in crop science has cut down the selection time for plant breeding as a whole and may play an especially pertinent role in the development of perennial crops.
Other techniques such as genomic in situ hybridization [1] or GISH, have also been utilize to help characterize fertile hybrids. When hybrids are found to be fertile (which is a rare event), it is important to determine the genomic origin of their chromosomes for further studies. GISH is a technique that distinguishes between the sets of chromosomes from the two parents.
There has also been an increase in plant genomic research in recent past years due to improved technology. For example, at this point in time, the rice genome has been mapped and sequenced. The sorghum genome sequencing project has also been completed. These types of large scale genetic research projects can help in studying the complex nature of perenniality, as thousands of genes can be studied at one time instead of the traditional approach of “one gene at a time”. Furthermore, with the knowledge of genome sequencing at hand, it may help identify genotypes of species that can promote homologous chromosomal pairing and produce fertile progeny.
Prospects for perennial crops
Despite the difficulties and obstacles of perennial crop development, there is ongoing research in this arena. Indeed, already several wild perennial plants have been the focus of research as potential parents to engage in wide hybridization.
The tall wheatgrass (Thinopyrum ponticum) has been a popular plant in research because of several attributes [2]. Apart from being a perennial, the tall wheatgrass is also ecologically adapted to drought, cold, salinity, and is resistant to many diseases and pests. More importantly, reports have shown that several genotypes of tall wheatgrass promote chromosomal pairing during meiosis of its hybrids [2], equateing to a higher chances of producing fertile progeny.
Another possible perennial legume plant is the alfalfa [1]. An interesting quality of alfalfa (and all legumes) is that its root nodules contains bacteria that can directly fix nitrogen from the air. Thus, they are self-sustaining and without the need of fertilizers. Currently, there are no projects involved in breeding alfalfa as a grain crop, although alfalfa has been used in various conditions for domesticated forage (food for animals). Due to this nuance, alfalfa is a likely prospect for future development.
References
1. Cox et al. (2006). Prospects for Developing Perennial Grain Crops. BioScience, Vol. 56, No. 8, pp. 649-659
2. Jauhar, Prem P. (2006). Modern Biotechnology as an Integral Supplement to Conventional Plant Breeding: The Prospects and Challenges. Crop Science, Vol. 46: 1841-1859
3. Tanksley et al. (1992). High Density Molecular Linkage Maps of the Tomato and Potato Genomes. Genetics, 132: 1141-1 160
4. Griffiths. et al (2005). Introduction to Genetic Analysis. P. 128
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