The shape of “baby plants” in the amaranth family show adaptations to extreme habitats

Plants are excellent at strategizing. Some plants produce as many, small seeds as possible whilst others are more conservative and produce fewer but larger seeds. Plants living in extreme habitats experiencing severe drought or salinity have less competition with other plant species than perhaps at a resource-rich meadow or forest. Whilst researchers really like comparing the seed numbers and sizes to understand different strategies, it might not fully capture how a plant’s seed might be adapted to an environment. Some plants in the amaranth family (Amaranthaceae) grow in extreme habitats (e.g., deserts, saltmarshes) but previous studies did not find differences in seed sizes between contrasting habitats. 

Dr Filip Vandelook from the Meise Botanic Garden and colleagues from Royal Botanic Gardes, Kew and Ludwig Maximilian University of Munich measured and dissected thousands of seeds from 87 species from the amaranth family (Amaranthaceae). Vandelook and colleagues identified four distinct embryo types (ring-shaped, curved, horseshoe-shaped and spirally coiled) and found that seed and embryo morphological characters and photosynthesis types (C₃ and C₄) evolved in association with extreme environments. Species that mostly grew in extreme environments germinated quicker and had larger embryos (with more nutritive tissue). 

Vandelook and colleagues collected seeds from Kew’s Millennium Seed Bank from 84 species in the amaranth family from different habitats and climates. From each species, 40-100 seeds were used to measure germination speed, root to cotyledon length ratio under two temperatures (20°C and 25°C). For microscopic sections, the seeds were dissected, cut at the thickness of 5-10 µm and the surface area of the embryo and – if present – the surrounding nutritive tissues (perisperm) was measured by image analysis. Researchers collected information on photosynthesis (C₃ or C₄), habitat (saline or not), coloniser of distrubed habitats (ruderals; yes or no), adult longevity (annual, short or long lived perennial) and maximum plant height for each species. Using phylogenetics, the researchers tested if there was an evolutionary signal in embryo types and seed.

There was a phylogenetic signal in embryo types, seed mass and embryo to seed size ratio suggesting that these characters evolved in association with habitat salinity, photosynthesis type and ruderality. There were four distinct types of embryos shapes: ring-shaped (annular), curved, horseshoe-shaped and spirally coiled. Most embryos had an annular shape – which could very well be the ancestral shape in the amaranth family – whilst the horseshoe-shaped was only found in three species. 

“Seeds from C₄ plants, adapted to growing in hot and dry environments, germinated faster with decreasing amounts of perisperm, while a weak opposite pattern was observed for C₃ plants,” Vandelook and colleagues wrote. 

“Similarly, for plants growing in saline habitats, seeds germinate faster when the proportion of the embryo was higher, while the opposite relationship was observed in plants from non-saline habitats.”

Whilst only three species had spirally coiled embryos with little or no nutritive tissue, the researchers suggested that those seeds can germinate very quickly as they uncoil rapidly. Larger embryos containing nutritive tissues could give a better chance of establishing and quickly germinating for the stress-adopted, C₄ species. 

Amaranth is an important grain crop as its  (pseudo-)seeds contain more protein than wheat, rice or maize, and can even be enjoyed as popcorn. This study has shown that dissecting seeds and looking at embryo morphology can reveal unique adaptations within the amaranth family.