Perennializing Staple Grain Crops: A Literature Review of Success and Challenges

Abstract 

This review analyzes the successes, challenges, and applications of the perennialization of staple grain crops such as sorghum, corn, rice, and wheat. Successes and advances in perennialization have been achieved in sorghum and rice using interspecific hybridization, with promising results in increasing soil health and crop yield. Research on the perennialization of corn is conflicting as past research has not reached a consensus nor has a fertile perennial corn been bred, yet there has been success in identifying a specific gene controlling the regrowth life cycle of this crop. Challenges reviewed mainly pertain to the wheat crop, in which perennialization has proven most difficult to achieve. This impediment is most likely due to the complex genome of wheat, compared to those of sorghum, rice, and corn. 

Introduction 

Climate change and a continuously growing population have increased demands on global food yields. However, meeting the planet’s hunger needs does not have to come at the price of depleting its natural resources. A possible sustainable solution to this issue is the perennialization of all staple grain crops. Many staple grain crops that supplement peoples’ everyday diets have an annual lifestyle. This means that plants like wheat, rice, and corn complete their life cycle in one year, die back after flowering at the end of their growing season, and have to be replanted the following year. In contrast, when provided with the adequate amount of resources, perennial plants are capable of year-long growth and do not need to be replanted after the growing season is over. These plants are able to grow deeper root systems and play more important roles in soil biology, such as increasing soil aeration and adding more organic matter to the soil than annuals [1]. 

Within the past decade, interest has increased in discovering which genes and biological synthetic pathways control the life cycle of staple grain crops, but not without acknowledging its challenges and present obstacles [1-8]. Perennial grains have the potential to introduce sustainable agricultural practices, which are agricultural techniques that protect nonrenewable resources (such as soil and water) [2, 3]. Benefits of sustainable agriculture include growing food for the general population, while also improving soil quality [2, 4], and creating profitability for farm workers and consumers through increased crop yield [5, 6]. The focus is on the perennialization of staple crops because transitioning their annual life cycle is a sustainable way to combat climate change, reduce agricultural inputs (such as fertilizer) [4], and increase global food security [1-7]. The summarization of this research begins with the applications of perennialization by analyzing how this direct research can improve current sustainable agricultural practices in response to climate change [1-7]. The review explores how geneticists and plant breeders have achieved perennialization in sorghum [5] and rice [4] through interspecific hybridization, which is a cross between two species from the same genus [5, 4, 8]. The latter section looks at the challenges of perennializing wheat and corn, to understand why similar methods have been unsuccessful for these crops [6, 7, 8]. The paper concludes with further implications of these studies and a discussion of where future research efforts should be focused [1-7].

Benefits of Perennialization

Perennialization of staple grain crops has many applications that are beneficial for both humans and the environment. For example, rice and sorghum perennial varieties produce larger yields compared to their annual counterparts [4, 5]. This means that growing perennial grain crops, instead of annual, can provide food security benefits for future generations through increased annual yields without needing to increase agricultural inputs such as water and fertilizer. Other studies on cereal grains like sorghum and rye also showcased the potential of perennial crops as a more sustainable method of producing larger crop yields through results such as larger seeds [5] and higher protein content in the plant [1]. In addition to providing a larger yield, perennial rice reduced farmers’ labor and input costs by an average of 50% compared to farmers who grew its annual counterpart [4]. Overall, perennial crops reduce labor costs associated with replanting seeds every year and can produce more food than annual crops. 

Perennial crops also perform important ecological functions. Multiple researchers agree that perennial crops are capable of reducing soil erosion and increasing organic content in the soil [1-4, 7]. This implies that perennial plants promote a healthier underground ecosystem, and therefore yield a healthier, more robust plant aboveground. Studies analyzing the advantages of perennial crops also reported benefits such as increased carbon sequestration [1-4, 7], the capture of soil methane [1], less frequent watering [2-4], and decreased fertilizer inputs [2-4]. Since perennial plants don’t die back each year, they are able to accumulate and store carbon within their plant material and reduce levels of carbon dioxide in the air [1, 3, 4], unlike annual plants, since decomposing plant material releases carbon dioxide back into the air. Continual growth also means that perennial plants can develop a longer and more robust rooting system, which allows them to reach water and nutrients stored deeper in the soil [1-3]. This is a direct benefit for both humans and the environment through decreased costs of water and fertilizer application, reduced depletion of aquifers, and less fertilizer runoff entering bodies of water and disturbing aquatic ecosystems. Perennial plants provide several indisputable benefits to both the human population and the natural environment. 

Success with Perennialization 

Staple grains such as sorghum [5] and rice [4] have seen success in the perennialization of their annual counterparts through interspecific hybridization and backcrossing. Interspecific hybridization is a common technique used by geneticists in which two species of the same genus are crossed [5, 8]. Backcrossing is a cross between the newly hybridized plant and one of the parent plants [5]. A team led by Stan Cox achieved the perennialization of sorghum, which is used mainly for biofuel production and livestock feed, by breeding two wild type perennial species within the Sorghum genus [5] and then backcrossing with annual domesticated Sorghum. Wild type species are those found in nature that have not been bred to have specific traits used for agricultural purposes, while domesticated plants have undergone breeding that makes the plant easier to harvest and use for human purposes. This experiment crossed perennial wildtype species of grasses, either S. halepense or S. propinquum, with the annual domesticated species of sorghum, S. bicolor. This experiment was performed between 2002–2009 (with research continuing since 2009 [5]) in Kansas, in which perennial sorghum showed rhizomatic growth, improved grain yields, and heavier seed weights [5]. This methodology was successful because the annual and wild type sorghum genome is rather simple due to its diploid and tetraploid nature [5]. Sorghum plants have a maximum of 3 sets of chromosomes, which is a smaller genome compared to other grain crops, such as wheat, which can be hexaploid [8], meaning 6 sets of chromosomes. It should be noted that life cycle traits influencing perennialization are not always controlled by a single gene or chromosome, and different genes can trigger varying characteristics that affect the development of the plant [8]. Therefore, if a plant has a larger genome, there are many more genes and development pathways that can influence perennialization. The smaller genome in sorghum made it easier for geneticists to localize desirable traits, such as perennialization, within the crop. Although sorghum may not be as fundamentally important to people’s diets as corn or rice, it can give some insight as to how perennialization can be achieved in other closely related species, such as wheat or barley. 

A second crop that has a successful perennial counterpart is rice. Zhang et al. also used intraspecific hybridization (a cross between two individuals of the same species) to breed rice for the perennialization life cycle trait [4]. The study began with the hybrid between annual domesticated Asian rice, Oryza sativa ssp. indica, and the undomesticated African perennial, O. longistaminata, which was first developed in 1996 due to its desirable traits displaying strong rhizomes (horizontal underground stems that indicate traits of perenniality), partial pollen fertility (pollen that is mostly fertile), and self-compatibility (the plant’s ability to inbreed)  [4]. Using pedigree selection, researchers continued to breed this plant with itself until a progeny seed, grown in 2007, displayed high pollen fertility and seed-setting rate (the number of grains in a panicle, which is the grains’ flower head), and moderately strong rhizome production [4]. Beginning in 2018, this hybrid rice was grown and tested over a period of 4 years and 8 harvests for characteristics such as pollen fertility, plant height, tiller number, grain number per plant, seed-setting rate, panicle length, and grain size (measured using the grain length, width, and weight) [4]. This discovery was not without the extensive effort of screening over 7,200 individual plants to identify which individuals had inherited the best combination of traits [4]. Perennial rice was distributed to farmers in varying regions of China in 2018 and 2020, who saved 58.1% of labor costs and 49.2% of input (fertilizer and water) costs per growing season [4]. It was also reported that perennial rice was grown on 15,333 hectares of land by 44,752 smallholder farmers in southern China in 2021 [4].

Despite obstacles, researchers were able to develop successful perennial counterparts to sorghum and rice crops that have shown progress in sustainable agriculture development [4-5]. Since sorghum and rice are both largely diploid, the genome and DNA material are much smaller than those found in wheat. Using inter/intraspecific hybridization to achieve perennialization is therefore more successful in diploid and tetraploid plants, as there is less variability in the recombination of chromosomes and less genetic material impacting gene development. However, given the fact that many of these staple grains share homologous genes (a gene that came from one common ancestor and is found in two different species [8]), there are still several applications of perennial sorghum and rice that can help current researchers understand the challenges met in perennializing grain crops such as corn and wheat.

Challenges Facing Perennialization 

It is especially important to find a perennial counterpart for wheat, as it is consumed by over 2.5 billion people in 89 different countries [9]. Consumption of wheat also surpasses the consumption of maize and rice as a source of protein for low- and middle-income nations [9]. However, interspecific hybridization may not be the immediate solution for perennializing corn and wheat. Interspecific hybridization is not as easily attainable in corn and wheat because of their large, complex genomes. 

Research conducted with corn perennialization has found partial success for a perennial counterpart but has present challenges that are addressed by the study conducted by Ma et. al. [6]. This study mitigates the discrepancies in past research on the perennialization of corn by testing specifically for the plant’s ability to restart a new lifecycle [6]. The experimental setup involved breeding a wild-type perennial relative of corn, with 6 different cultivars (varieties) of domesticated annual corn [6]. Since the hybrid plants showed a predominantly perennial lifestyle, scientists were able to determine that the alleles from the wild-type corn were dominant. This made it clearer to identify which chromosomes were responsible for perennializing the hybrid crop using DNA isolation and PCR-based marker assay testing [6]. Though a certain degree of success was achieved through interspecific hybridization, it should be taken into consideration that there are still certain limitations within the understanding of how to perennialize corn, such as the role of environmental factors and how they influence gene expression.  Ma et. al. reported that environmental factors greatly influence perenniality in corn compared to other staple grains. Certain corn species may be able to grow as a perennial in one part of the world, but as an annual in others [6]. There are still much more complex genomic pathways to research in order to understand exactly how the life cycle of corn works [6]. Yet despite these limitations, the results of this study have large implications that perenniality can be bred in corn. There have also been some advances in the perennialization of wheat, but not like the discoveries made in corn. 

A study performed by Abbasi et. al. focuses on the introgression (transferring new genetic material from one species to another through hybridization [8]) of a wild type perennial wheatgrass with an annual wheat species, to conduct gene mapping and locate which genes are associated with perennialization [8]. Despite the success of localizing chromosome 4E, which influences the perenniality of wheat, the study also found that it was not the only chromosome affecting the lifecycle development of wheat [8]. Wheat is a polyploid plant that can have up to 6 or 8 sets of chromosomes [8], meaning that the most challenging aspect of understanding perennialization in wheat is the significantly larger amount of genetic material than in sorghum, rice, and corn. Even though wheat shares homoeologous genes pertaining to perenniality with rice, the genome of wheat is too large for perenniality to only be influenced by a single chromosome [8]. This indicates that perenniality would be even more difficult to breed in wheat using interspecific hybridization.

The intricate genomes of plants are most likely the greatest challenge facing the perennialization of crops. Annual crops may also resist perennialization via interspecific hybridization if one of the parents is genetically incompatible, and if there are any recombination errors during meiosis [7]. There is also some concern about the loss of biodiversity by perennializing staple crops and introducing them worldwide – however, that seems to be of lesser concern than increasing food security [7]. The Crews and Cattani study commends the advances made in the field of plant breeding, however, it also recognizes and acknowledges the slow process of traditional and technological breeding practices [7]. They acknowledge that creating new viable crops to supplement the growing global population is not the only feasible solution to address climate change. More studies must be conducted to fully understand the global impact that perennial grain crops would have in combating climate change [7]. Overcoming these challenges and conducting further research are important in being able to develop perennial crops, and understanding how perennial crops can lead to sustainable agricultural solutions to alleviate the effects of climate change.

Conclusion 

After reviewing the successes, challenges, and applications of crop perennialization, there is an overwhelming amount of evidence for why this area of research deserves more awareness. Due to genetic innovations, grain crops such as sorghum [5], corn [6], and rice [4] have been able to achieve advances in the perennialization of its domesticated annual variety. Therefore, efforts should be focused on the continued research of corn to fully realize its perennialization, and on wheat [8] to overcome the challenges surrounding its intricate genome. Considering that wheat composes a large portion of people’s diets around the world [9], this crop would be an excellent candidate for understanding perennialization. Additionally, future research efforts should also be focused on the direct benefits of perennial crops in sustainable agricultural methods using the advances made in the perennialization of rice and sorghum. Effective perennialization of all staple grain crops could mean year-round food security and a means of producing more food for an increasing population that doesn’t decimate and deplete the soil of this Earth.

References

1. Rudoy D, Pakhomov V, Olshevskaya A, Maltseva T, Ugrekhelidze N, Zhuravleva A, Babajanyan A. 2021. Review and analysis of perennial cereal crops at different maturity stages. IOP Conf Ser Earth Environ Sci [Internet]. 937(2):1-13. doi:10.1088/1755-1315/937/2/022111
2. Soto-Gomez D, Perez-Rodriguez P. Sustainable agriculture through perennial grains: wheat, rice, maize, and other species. A review. 2022. Agric Ecosyst Environ [Internet]. 325(325)1-14. doi:10.1016/j.agee.2021.107747
3. Steinwand MA, Young HA, Bragg JN, Tobias CM, Vogel JP. 2013. Brachypodium sylvaticum, a model for perennial grasses: Transformation and inbred line development. PLoS One [Internet]. 8(9):e75180. doi:10.1371/journal.pone.0075180
4. Zhang S, Huang G, Zhang Y, Lv X, Wan K, Liang J, Feng Y, Dao J, Wu S, Zhang L, and others. 2022. Sustained productivity and agronomic potential of perennial rice. Nat Sustain [Internet]. 6:28–38. doi:10.1038/s41893-022-00997-3
5. Cox S, Nabukalu P, Paterson A, Kong W, Nakasagga S. Development of perennial grain sorghum. 2018. Sustainability [Internet]. 10(2):172. doi:10.3390/su10010172
6. Ma A, Qiu Y, Raihan T, Paudel B, Dahal S, Zhuang Y, Galla A, Auger D, Yen Y. 2019. The genetics and genome-wide screening of regrowth loci, a key component of perennialism in Zea diploperennis. G3 (Bethesda) [Internet]. 9(5):1393–1403. doi:10.1534/g3.118.200977
7. Crews TE, Cattani DJ. Strategies, Advances, and Challenges in Breeding Perennial Grain Crops. 2018. Sustainability [Internet]. 10(7):2192. doi: 10.3390/su10072192
8. Abbasi J, Xu J, Dehghani H, Luo M, Deal KR, McGuire PE, Dvorak J. 2020. Introgression of perennial growth habit from lophopyrum elongatum into wheat. Theor Appl Genet [Internet]. 133(9):2545-2554. doi:10.1007/s00122-020-03616-x
9. CGIAR Research Program on Wheat. WHEAT in the World. Accessed August 19, 2023. Available from: https://archive.wheat.org/wheat-in-the-world/#:~:text=First%20and%20foremost%20a%20food,billion%20people%20in%2089%20countries.