perennial crops

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Developing Perennial Grain Crops from the Ground Up

This is the fourteenth in a series of posts reviewing the 17 articles found in the October 2014 Special Issue of American Journal of Botany, Speaking of Food: Connecting Basic and Applied Science.

Useful Insights from Evolutionary Biology for Developing Perennial Grain Crops by Lee R. DeHaan and David L. Van Tassel

The environmental impacts of modern agriculture are diverse and extensive. Our growing population needs to be fed; however, practices that have long-term negative effects on soil, water, and air quality are unsustainable. It is imperative that we find better alternatives. Developing perennial grain crops is one way that plant breeders are working to address this issue.

Moving from annual to perennial grain crops could potentially “increase water quality, reduce soil erosion, increase soil carbon, and improve habitat for wildlife.” It may also help “address the looming challenges of land degradation, food security, energy supply, and climate change.” Sounds like a major win if we can do it, right? And maybe we will, but first we must domesticate perennial grain varieties that perform on a similar level with annual ones. Most plant breeding today involves “improvement of previously domesticated species;” however, new perennial grain crops must be developed “de novo” (i.e. from wild species) in a matter of “decades rather than centuries to millennia.”

The roots of perennial grasses are considerably more extensive than annual grasses. (photo taken from an article about perennial grain crops at nationalgeographic.com)

The roots of perennial grasses are considerably more extensive than annual grasses, which helps reduce erosion and limits the need for fertilizer applications. (photo taken from an article about perennial grain crops at nationalgeographic.com)

Little has been published concerning “strategies for the wholesale remodeling of plants,” and so the authors reviewed findings in other fields, such as evolutionary biology and population genetics, in order to devise strategies for developing perennial grain crops. In this article, the authors summarize the published research they reviewed and describe how it relates to breeding perennial grains. It is a dense and lengthy article, so rather than offering a thorough review, I will briefly describe some of the main areas explored by the authors and then summarize their conclusions.

  • Trade-offs – This occurs when “resources allocated to one trait are unavailable for other traits.” Can perennial grain crops achieve yields comparable to annual varieties when faced with “trade-offs between seed and perennial organs?” Are such yields only attainable by “sacrificing longevity?” Strategies must be devised to “create herbaceous perennial crops with abundant seed production.”
  • Genetic Loads – This is simply defined as “the presence of deleterious alleles in a population.” In perennials, compared to annuals, “highly recessive deleterious alleles can arise at a rate faster than they can be efficiently eliminated.” Low seed set, among other things, may be a result of genetic load, so breeders of perennial grains must “account for and actively reduce genetic load.”
  • Bottlenecks – This refers to the loss of genetic diversity that occurs when population size is reduced. During a bottleneck, “previously rare deleterious recessive genes” can accumulate; however, some models indicate that “inbreeding and the associated bottlenecks may be useful in accelerating domestication.” If the population is isolated and introduced to a new environment simultaneously, “the newly exposed variation could now be adaptive.” Also, “if additional genetic diversity is required,” crosses can be made with wild populations.
  • Pleiotropy – This means that “a single gene [is] affecting multiple traits.” When domesticating wild species, “it would be useful to predict the prevalence of pleiotropy and whether to expect positive or negative pleiotropy to dominate.”
  • Epistatsis – This occurs when the effect of one gene is dependent on the presence of another gene or genes. This is particularly important if “large-effect genes” (pleiotropy) are dependent on a “particular genetic background to function optimally,” because “removing one critical element will severely impact the whole structure.” Perennial grain crops will have to undergo “many generations of plant breeding” in order to ensure that desired genes are found “within a genetic background where their benefits can be used without negative side effects.”
  • Cryptic Variation – Genetic variation is cryptic when “the inheritance of a particular mutated allele has no effect on phenotype and thus is hidden from natural and artificial selection.” New environments or mutations can release cryptic variation. “Ranking candidate species for their likely domesticability” may be an effective approach to cryptic variation. “The best candidates for domestication” originate from areas where conditions are highly favorable for growth and reproduction as opposed to areas that are “resource-limited,” because they have experienced periods of “selective enrichment” that make them suitable for agriculture settings.
  • Past Domestication – Domestication involves a series of “evolutionary changes that may decrease the fitness of a species in the wild but increase it under human management.” Historically this was “likely driven by unconscious selection pressures,” but currently it is “driven by conscious selection.” Studies of past domestication events reveal “somewhat predictable stages” in the process. Even though “current domestication efforts might not follow historical precedent,…the order in which traits are subjected to strong selection may be important.” Investigation into domestication also suggests that “dramatic changes” in plant morphology can be accomplished by selection for a “small number of major-effect genes,” so breeding programs are advised to “first search for useful major genes and evaluate their effects before moving on to strategies designed to accumulate genes of small effect.”
  • Selection – The authors describe “four major limits to selection.” 1.) Desired traits “may only exist in our imagination.” 2.) “The necessary genetic variation may not exist in the population,” and so waiting for or inducing mutations may be required. 3.) There may be “negative genetic correlations between characters being selected,” which will slow response to selection. This can be addressed by subdividing the population, evaluating the population in a new environment, or crossing with other populations. 4.) Conversely, “insufficient genetic correlation between traits may reduce the response to selection.” This makes “finding superior genotypes challenging,” so the authors suggest breeding plants in a “uniform environment,” and then later the plants can “accumulate genes for tolerance to specific stresses in separate populations.”
Intermediate wheatgrass (Thinopyrum intermedium) "produces much larger seeds in the greenhouse during the winter than ever seen in the field during the summer," an example of phenotypic plasticity. (photo credit: www.eol.org)

Intermediate wheatgrass (Thinopyrum intermedium) “produces much larger seeds in the greenhouse during the winter than ever seen in the field during the summer,” an example of phenotypic plasticity. (photo credit: www.eol.org)

The authors determined that the best candidates for perennial grain breeding programs are plant populations that have high diversity between and within individual plants, plastic phenotypes (i.e. adaptable to changes in the environment), and “an evolutionary history that includes adaptation to high resource environments.” They also suggest that breeders “focus more on the required functions [like nonshattering fruits] than on morphological traits” because it will increase the feasibility of evaluating “very large experimental populations.” The ideal experimental set-up would consist of very large populations of widely spaced plants that are subdivided in order to perform evaluations from various angles. Lastly, the authors encourage breeders to embrace new plant forms and breeding strategies and be open to the possibility that perennial grain crops may not “look like modern annual grains.”

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Improving Perennial Crops with Genomics

This is the twelfth in a series of posts reviewing the 17 articles found in the October 2014 Special Issue of American Journal of Botany, Speaking of Food: Connecting Basic and Applied Science.

Genomics: A Potential Panacea for the Perennial Problem by Kendra A. McClure, Jason Sawler, Kyle M. Gardner, Daniel Money, and Sean Myles

Compared to annuals, a small but significant portion of our food comes from perennial crop plants. “Approximately one eighth of the world’s total food-producing surface area is dedicated to perennials,” and while that may seem relatively small, there is a good chance that some of your favorite things to eat or drink are perennial crops (apples, bananas, coffee, citrus, sugar cane, coconut, avocados, olives, grapes, cherries, almonds…just to name a few). However, making improvements to and introducing new cultivars of perennial crops is considerably more challenging compared to annual crops simply due to the nature of perennials. This puts perennial crops at greater risk to threats like pests and diseases, climate change, soil degradation, and water and land shortages. Advances in genomics, “the collection and use of DNA sequence information,” could change this.

Because breeding efforts to improve perennial crops is so challenging, “only a small number of elite varieties become popular, and the amount of genetic diversity represented by commercially successful cultivars is therefore often low.” This suggests that there is incredible potential for improvement in these crops, as long as major hurdles can be overcome. Following is a list of some of those hurdles:

  • Time – Most perennial crops have “extended juvenile phases,” meaning they won’t produce fruit for as much as ten years, considerably delaying evaluation of the final product.
  • Space – Perennial crops, especially trees, are large compared to annual crops, so the area required for evaluation is extensive.
  • Infrastructure – “Many perennials require trellis systems, extensive land preparation, and substantial costs for specialized equipment and skilled horticultural labor.”
  • Complex Evaluations – Automated assessments are “either unavailable or poorly developed,” so evaluations that include “size, shape, color, firmness, texture, aroma, sugars, tannins, and acidity” require “tasting panels” to ensure that the final product “satisfies consumer demands.” This process is expensive, and it differs depending on whether the crop will be consumed fresh or processed.
  • Vegetative Propagation – “Many perennials suffer from severe inbreeding depression when selfed,” so cultivars are maintained through vegetative propagation. This is a plus, because it means that the fruits of perennial crops are reliably uniform, so growers and consumers know what to expect year after year. However, this also means that while pests and pathogens evolve, the crops do not, making them more susceptible to such threats. Additionally, the “long histories” of certain cultivars “discourages [growers] from undergoing the risk of trying recently developed cultivars.”
  • Consumer Preferences – “Consumers often exhibit an irrational reverence for ancient or heirloom varieties,” despite the fact that the development of new varieties can result in crops that are higher yielding, resistant to pests and diseases, tastier, more nutritious, more suitable for storage, and require fewer chemical inputs. This obsession with traditional varieties leaves a “tremendous amount of untapped genetic potential for the improvement of perennial crops.”
"Modern avocado breeding still depends heavily on open-pollination because of the difficulty associated with making controlled crosses." (photo credit: wikimedia commons)

“Modern avocado breeding still depends heavily on open-pollination because of the difficulty associated with making controlled crosses.” (photo credit: wikimedia commons)

Apart from issues of social and cultural preference, the challenge of breeding perennial crops comes down to time and money. Advances in genomics can help offset both of these things. Using DNA-based predictions, a plant’s phenotype can be determined at the seed or seedling stage. Genomics techniques can also be “used to reduce the generation time thereby enabling combinations of desirable traits to be combined on a timescale that is more similar to annual crops.” Below are summaries of specific areas discussed in the paper for using genomics in perennial crop breeding programs:

  • Reduction of Generation Time – This can be done using transgenic technology in ways that do not result in transgenic (GMO) cultivars. One method uses virus-induced gene silencing, in which a host plant is infected with “a virus that is genetically modified to carry a host gene;” the host plant then “attacks itself and uses its own endogenous system to silence the expression of one of its own genes.” Early flowering in apples has been induced after seedlings were inoculated with apple latent spherical virus that expresses a flowering gene derived from Arabidopsis thaliana.
  • Genetic Modification – Advances in genomics have brought us transgenic technology, and several commercial crops have been genetically modified using this technology. Most of them are annuals, but one perennial in particular, SunUp papaya, has been a major success. Its resistance to ringspot virus rescued the papaya industry from a devastating pathogen that “almost completely destroyed the industry in Hawaii.” Consumer disapproval, however, poses a major obstacle to commercial production of genetically modified organisms, and unless this changes, “their widespread use is unlikely.”
  • Marker-Assisted Selection – This is the “primary use of genomics in breeding.” The time between initial plant crosses and the introduction of a new cultivar can be dramatically shortened when genetic markers are used to determine the phenotypes of adult plants at the seedling stage. This technology is also useful when crossing domesticated plants with wild relatives, since genetic markers can be used to determine when desired traits are present in the offspring.
  • Ancestry Selection – After crosses with wild relatives, offspring may “perform poorly because wild germplasm often harbors numerous traits that negatively affect performance.” To overcome this, the offspring is crossed with cultivated plants until undesirable traits are eliminated. This is called backcrossing. Using marker-assisted selection, breeders can “select a small number of offspring in each generation that carry both the desired trait from the wild and the most cultivated ancestry.”
  • Genomic Selection – The success of marker-assisted selection is greatest when used for traits that are controlled by one or a few genes. However, many traits involve a complex set of genes. Genomic selection is a new technique that “uses dense, genome-wide marker data to predict phenotypes and screen offspring.” It is “especially useful for predicting complex traits controlled by many small-effect genes.” Genomic selection is in its infancy, so there are kinks to work out, but it is a promising technology for perennial crop breeding efforts.

The use of genomics will not replace every aspect of traditional perennial crop breeding and “should be viewed as a potential supplement…rather than a substitute.” Geneticists and plant breeders are encouraged to work together to develop and implement these technologies in a concerted effort to improve the crop plants that help feed the world.

"Despite the remarkable phenotypic and genotypic diversity in bananas," the Cavendish banana is responsible for the "vast majority" of banana production. (photo credit: wikimedia commons)

“Despite the remarkable phenotypic and genotypic diversity in bananas,” the Cavendish banana is responsible for the “vast majority” of banana production. (photo credit: wikimedia commons)