Perennials as Future Grain Crops: Opportunities and Challenges

Abstract

Perennial grain crops could make a valuable addition to sustainable agriculture, potentially even as an alternative to their annual counterparts. The ability of perennials to grow year after year significantly reduces the number of agricultural inputs required, in terms of both planting and weed control, while reduced tillage improves soil health and on-farm biodiversity. Presently, perennial grain crops are not grown at large scale, mainly due to their early stages of domestication and current low yields. Narrowing the yield gap between perennial and annual grain crops will depend on characterizing differences in their life cycles, resource allocation, and reproductive strategies and understanding the trade-offs between annualism, perennialism, and yield. The genetic and biochemical pathways controlling plant growth, physiology, and senescence should be analyzed in perennial crop plants. This information could then be used to facilitate tailored genetic improvement of selected perennial grain crops to improve agronomic traits and enhance yield, while maintaining the benefits associated with perennialism.

Keywords: breeding; domestication; genome editing; grain crops; perennial agriculture; perennial grains; perennialism; species-wide hybridization.

Figure 1

A perennial grain crop would offer a sustainable alternative to present day annual crops. (A) Annual crops live only for a single season with annual cultivation dependent on machines for tilling and sowing. In addition to promoting soil erosion, tillage breaks open soil aggregates, exposing the previously protected organic matter to microbes resulting in elevated respiration and losses of CO₂ to the atmosphere. (B) Perennial crops need tilling and sowing only in the first year and thereafter are viable for several seasons. Some perennial grain crops require vernalization, and are typically autumn sown with grain harvested in the following years. Perennial grain crops develop an extensive root system that stores carbon underground, and also grows during cool periods of the year. Depending on the cropping history and management, both annual and perennial crops can contribute to soil carbon sequestration, with the contribution of perennial grain crops significantly greater due to the reduction in tilling and greater allocation of photosynthates to root systems over time. Such differences are illustrated using barley H. vulgare (annual) and H. bulbosum (perennial) as examples.

Figure 2

Grain yield of intermediate wheatgrass (T. intermedium) varies over time, which existing genomic selection practices need to be designed to overcome. (A) Grain yield of intermediate wheatgrass is typically greatest in the first 2 years of propagation, with stem reserves and photosynthates allocated toward the grain (orange arrows). As plantings establish, intermediate wheatgrass outcompetes weeds (interspecific competition), with plants increasingly competing among themselves (intraspecific competition). Over time, the greater resources are allocated toward clonal growth and rhizomatous spread (pink arrows), in place of grain production (orange arrows), alongside deeper rooting (blue arrows), facilitating overwintering and survival. (B) The declining grain yields of intermediate wheatgrass over time are associated with differential resource allocation. However, phenotyping conducted in 1–2 years fails to capture this. (C) Genomic selection (GS) is routinely applied in intermediate wheatgrass breeding. GS relies on the creation of a training set representative of the genotypic and phenotypic variation found within the wider breeding program. Using an iterative approach, phenotypic and genotypic data are integrated to identify trait genetic associations to predict future plant performance (arrows indicate stages, blue = 1, red = 2). (D) In absence of multi-year phenotyping (dashed line; * in B) existing GS pipelines cannot select for yield stability. Additional phenotyping of the same plant stands in 3–4 years (represented by increasingly green rectangles) would help address this, for which there may be little consensus between the earlier, and these newer, selections (different colored squares).

Figure 3

The genetics of perennialism and de novo domestication of perennial grain crops. (A) Interspecific crosses between perennial relatives and their annual counterparts have been attempted to facilitate QTL analysis and genetic mapping of traits associated with perennialism. (B) Perennialism is a complex trait, associated with distinct physiological characteristics and responses to the environmental cues, stresses, diseases, and pests. (C) Accelerated domestication of perennial grasses through gene editing of domestication related genes, as identified in their annual relatives, could be used in de novo domestication and generation of high-yielding perennial grain crops, illustrated for Sorghum sp. The QTL plot reported is for the illustrative purposes only and emphasizes the polygenic nature of perennialism.