When angiosperms, or flowering plants, first appeared more than 160 million years ago, they rapidly spread and diversified. Throughout the Cretaceous period, they outcompeted previously dominant ferns and conifers. The unknown cause of their success has led to much speculation over the last two hundred years. Coevolution with pollinators and herbivores has been the dominant theory, although some scientists have suspected changes to angiosperm leaf anatomy played an important role. But what changes took place?
New research by Kevin Simonin at San Francisco State University and Adam Roddy at Yale University has found that the genomes of angiosperms decreased in size, which made plant cells more compact. Plant growth is partly limited by the amount of carbon dioxide that can be absorbed through leaf pores, called stomata. Because smaller cells allow for greater density of stomata, they facilitate growth by allowing greater absorption of carbon dioxide. Greater stomatal density also increases the amount of water lost during transport, which plants can counter by increasing leaf vein density, thereby increasing the efficiency of water delivery from the roots.
Using genome size data for 393 plant species, the researchers conducted a meta-analysis, comparing genome size with cell size, stomatal density, vein density, and stomatal conductance, a measure of energy productivity. They showed that not only across all angiosperms, but also with ferns and conifers, plants with smaller genomes had smaller cells, higher stomatal densities, higher vein densities, and greater stomatal conductance, suggesting that smaller cells facilitate growth across several plant types.
Tracking these changes over geologic time, the researchers showed that even at the start of the Cretaceous, angiosperms had smaller genomes than ferns or conifers, and that over the next 50 million years their genomes evolved to be even smaller while the density of their stomata and veins increased. A model created by the researchers showed that regardless of the type of plant, such physical changes would result in higher energy productivity, resulting in faster growth.
Future research may investigate why these changes only evolved in angiosperms and how they may have interacted with other angiosperm traits, such as their uniquely complex vascular anatomy. (PLOS Biology)