Prospects of domesticating wild perennials for improved food security

by Lee Tang

While the world's food production has been more or less able to keep pace with population 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. Tang examines the ecological impact of agriculture and one potential solution, through the use of perennial crops.

In order to discuss the problems, we must first address some of the aspects of agriculture that lead to the problems. In short, agriculture, requires the use of 'annual' plants; which germinate, flower, and die within a one year period. Annual grasses like wheat, rice, corn, rye, and barley account for 70 percent of all human calories. 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, often thought to lead to soil erosion, is required yearly. Soil erosion, in turn, leads to loss of fertility of the land and thus decreased crop growth. 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 methane) in the synthesis of ammonia, an important nitrogen source for plants. Basically, the natural resources needed for this mode of agriculture is significant.

Monoculture systems, with their lack of genetic variation, are particularly sensitive to parasitic attacks. 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.

To solve these problems of sustainability and pollution, many scientists have looked at natural 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. 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 from the norm. There are currently only a few perennial crops readily available for use by agricultural industry around the world and these are primarily limited to hay, forage, and pasture crops. Wild perennial plants would have to undergo the process of domestication in order for them 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, including the qualities of synchronous maturity, large seeds, structurally stiff, erect, robust, and resistance to shattering or shedding of plant parts.

There are currently two approaches to breeding perennial crops; direct domestication and wide hybridization. In direct domestication, wild species of plants or animals are habituated through a series of steps of selection of desired 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 under ideal situations.

Wide hybridization is a complementary method to direct domestication and involves interbreeding 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. This obstacle can be overcome by using broader genetic ranges of the crossed species (for a larger gene pool) until fertile progeny are achieved, but is a slow process requiring tremendous effort.

A major roadblock to the conventional selection of traits is that 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. The molecular markers are “indicators” that show whether the gene of interest is present or not. MAS in crop science has cut down the selection time for plant breeding and may play an especially pertinent role in the development of perennial crops.

There has also been an increase in plant genomic research in recent past years due to improved technology. For example, the rice genome has been mapped and sequenced. The sorghum genome sequencing project has also been completed. All this 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”.

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 perennial plant in research for several reasoans. It 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, equating to a higher chances of producing fertile progeny.

Another possible perennial legume plant is the nitrogen-fixing alfalfa. Mainly so far used as a forage crop for animals, its self-sustaining nature makes it a good prospect for development as a perennial cereal crop.

Univ.BritishColumbia