Poaceae: shaper of the modern world

By Haobo Yuan

Poaceae, commonly referred to as grasses, is the fourth largest flowering plant family and contains approximately 12000 species (Christenhusz & Byng, 2016), including domesticated cereal crops, bamboos, and grasses of natural grasslands and cultivated lawns. Due to its morphological and photosynthetic characteristics, the monocotyledonous Poaceae family is well adapted to marginal and extreme habitats and can be found on every continent, including Antarctica (Peterson, 2013). In addition, grasses are the most important plants in food production and supply over half of the world’s dietary energy (Raven & Thomas, 2010), as well as providing ecological services, such as wildlife habitats and carbon storage. Thus, in terms of both economy and ecology, the nearly ubiquitous grass plants have significantly shaped the modern world and our way of living.

Unique features of the Poaceae family have facilitated its global distribution and adaptation to various environments. The leaf blades of many grasses are hardened by silica within the epidermal cells, which forms a defense mechanism against grazing herbivores (McNaughton & Tarrants, 1983). The growing point (apical meristem) of the leaf is located at or near the soil surface, protecting grasses from growth damage by grazers (Briske, 1996). The floral structures of grass plants, sepals and petals are highly reduced and there is no bright colour or strong scent to attract pollinators. Instead, grass anthers produce abundant pollen grains that are smooth and light, which can travel on air currents and be caught by the feathery stigmata with large surface areas. Wind pollination allows the effective reproduction of grasses in the absence of suitable pollinator species in the plain habitats (Clifford, 1961). 

Most importantly, both C3 and C4 carbon dioxide (CO2) assimilation pathways can be found in grasses. While C3 pathway directly fixes CO2 by ribulose 1,5-bisphosphate (RuBP) to form phosphoglycerate (PGA), C4 pathway fixes CO2 initially by phosphoenolpyruvate (PEP) carboxylase to form four carbon molecules (malate or oxaloacetate). C4 grasses, such as maize, sugarcane and sorghum, contain kranz anatomy, which means that their bundle sheaths are encircled by mesophylls to prevent direct contact with oxygen (O2). The C3 grasses are well adapted to temperate climates, whereas C4 grasses are well suited to tropical environments. C4 photosynthesis has allowed grasses to outcompete other plants in warm tropical conditions by increasing the efficiency of photosynthesis, decreasing photorespiration products and reducing water loss with narrower stomatal apertures (Slack & Hatch, 1967; Osborne & Sack, 2012). 

The reduced floral structure and wind-pollination have allowed the Poaceae family to be well suited to marginal and extreme habitats. Additionally, the basal growth of leaf, as well as structural rigidity due to silicification, enables grasses to be more resistant to vertebrate grazers and other biotic damages or abiotic stresses. Many grasses incorporate two major photosynthetic pathways, which leads to the adaptation of Poaceae species to various environments, covering 31-43% of the Earth’s surface in the form of grasslands (Gibson, 2009). All these features above have contributed to the successful worldwide radiation and colonisation of grasses.

The globally widespread grasses can exist in large grass-dominated biomes called grasslands. Grasslands provide forage for domestic livestock, principally mammalian herbivores, and supply both food and habitats for wild grazing animals, such as deer and elephants, and many species of birds. As a result, grasslands act as important repositories of biodiversity. For example, based on species diversity and priority for conservation actions, 35 of 136 terrestrial ecoregions identified by the World Wildlife Fund-US are grasslands. According to Birdlife International, grassland/savanna/scrub is the key habitat in 23 or approximately 11% of the 217 identified endemic bird areas. Another service of grasslands is the storage of carbon. Grasslands fix CO2 from the atmosphere through photosynthesis and contribute significantly to the global carbon cycle by storing high levels of carbon below the ground in the soil. Soil carbon levels are higher in grasslands than in forests, agroecosystems, and other ecosystems. Such a strong ability of carbon storage in soils is due to the extensive, fibrous root system of grasses. High organic content in grasslands allows the maintenance of soil fertility, and the roots can sustain the ecosystem by preventing soil loss. (White et al., 2000). 

The world agricultural production primarily relies on three major crops from the Poaceae family, including rice (Oryza sativa), wheat (Triticum aestivum), maize (Zea mays). Cereal grasses were one of the first plants domesticated by humans about 13000 to 11000 years ago in the Fertile Crescent (Evert & Eichhorn, 2013). The cultivation of cereals is the basis of most early civilisations as it marked the shift from a hunter-gatherer lifestyle to agriculture and animal husbandry; a transition that stimulated social and economic development. Most grass crops produce edible grains, of which the endosperm is rich in carbohydrates and the embryo contains proteins and fats. Grains constitute the major source of carbohydrates and proteins for humans. Up to 42.5 per cent of the world’s food calorie supply is provided by rice, wheat and maize and their contribution to the global protein supply is about 37 per cent, close to that of fish and livestock products (Food and Agricultural Organization of the United Nations, 2016). Sugarcane (Saccharum officinarum) is the main crop to produce sugar, accounting for 86% of the global sugar yield (OECD/FAO, 2018). Apart from dietary use, sugar can be further utilized for the manufacture of alcoholic beverages or biofuels to power automobiles. 

While cereals and sugarcanes form the primary food source, bamboos (subfamily Bambusoideae) have been widely used in Asian countries as building materials for construction and as fibers in papermaking industries. By providing livestock feed, grasses can indirectly contribute to human dietary energy supply through meat production. For instance, ruminant animals, like cattle and sheep, depend on grazing pastures for nutrition. Grasses are also artificially planted in lawns for ornamental use and in sports fields, such as football and golf. Grasslands are nature-based recreational attractions that can generate ecotourism revenues, especially in developing countries of Sub-Saharan Africa which have extensive areas of grasslands (White et al., 2000). 

As one of the largest and most widespread plant families, Poaceae has shaped our lives by creating great economic values in terms of food production, industrial manufacture, artificial lawns and ecotourism. Poaceae has also shaped the ecology of the modern world by creating habitats for diverse species, participating in the carbon cycle, maintaining soil fertility and preventing soil loss. However, there is an increasing concern as grassland areas have declined dramatically since the 1830s until 2000. Human activities, such as urbanisation and conversion of grasslands to agricultural lands, have changed these ecosystems, causing fragmentation and habitat loss (White et al., 2000). In the long term, such drastic changes may hinder global biodiversity, the sustainability of ecosystems and human economic activities. 

References:

Christenhusz, M. J. M. & Byng, J. W. (2016) The number of known plants species in the world and its annual increase. Phytotaxa. 261 (3), 201–217. doi:10.11646%2Fphytotaxa.261.3.1

Evert, R. F. & Eichhorn, S. E. (2013) Raven biology of plants. Eighth edition. New York, W. H. Freeman and Company.

Food and Agricultural Organization of the United Nations. (2016) Save and Grow in practice: maize, rice, wheat. Rome, Food and Agriculture Organization of the United Nations.  Available from: http://www.fao.org/3/a-i4009e.pdf

Gibson, D. J. (2009) Grasses and grassland ecology. New York, Oxford University Press. Available from: https://userweb.weihenstephan.de/lattanzi/Lit/Grasses%20and%20grassland%20ecology.pdf 

OECD/FAO (2018) OECD-FAO Agricultural Outlook 2018-2027. Paris, OECD/ Rome, Food and Agriculture Organization of the United Nations. doi:10.1787/agr_outlook-2018-en

Peterson, P. M. (2013) Poaceae (Gramineae). eLS. doi:10.1002/9780470015902.a0003689.pub2

Raven, J. & Thomas, H. (2010) Grasses. Current Biology. 20 (19), 837-839. doi:10.1016/j.cub.2010.08.031

White, R. P., Murray, S., Rohweder, M., Prince, S. D. & Thompson, K. M. (2000) Grassland ecosystems. Washington, DC, World Resources Institute. Available from: http://pdf.wri.org/page_grasslands.pdf

McNaughton, S. J. & Tarrants, J. L. (1983) Grass leaf silicification: Natural selection for an inducible defense against herbivores. Proceedings of the National Academy of Sciences. 80 (3), 790-791. doi:10.1073/pnas.80.3.790

Clifford, H. T. (1961) Floral evolution in the family Gramineae.  Evolution. 15, 455-460. doi:10.1111/j.1558-5646.1961.tb03175.x

Briske, D. D. (1996) Strategies of plant survival in grazed systems: a functional interpretation. The ecology and management of grazing systems. Wallingford, CAB International. Available from: http://agrilifecdn.tamu.edu/briske/files/2013/01/Briske-StrategiesPlantSurvival-1996_9.pdf 

Slack, C. R. & Hatch, M. D. (1967) Comparative studies on the activity of carboxylases and other enzymes in relation to the new pathway of photosynthetic carbon dioxide fixation in tropical grasses. Biochem J. 103 (3), 660-665. doi:10.1042/bj1030660

Osborne, C. P. & Sack, L. (2012) Evolution of C4 plants: a new hypothesis for an interaction of CO2 and water relations mediated by plant hydraulics. Phil. Trans. R. Soc. B. 367: 583–600. doi:10.1098/rstb.2011.0261

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