The variation in genome size between different plant species is enormous, with a variation of 2300-fold in flowering plants alone. Whilst variation in plant genome size is well recorded, it is debatable how much of this variation arises from selective pressure rather than simply neutral drifting in genome size. Intriguingly, the size of plant genomes is believed to correlate with the size of plant stomata, which are essential for gas exchange in plant tissues. The fossil record shows that plants of the Devonian and Mesozoic periods (roughly 420-360 and 250-60 million years ago respectively) had large stomata, which has been used to suggest that plants of these periods also had large genomes.
Large stomata are generally thought to be less efficient at gas exchange and the majority of the Devonian and Mesozoic periods had high atmospheric CO2 levels. This indicates that plants of the Devonian and Mesozoic periods possibly didn’t need to bother with the numerous small stomata that maximise CO2 uptake, as it was very abundant anyway. It is however currently impossible to know whether historic plants that grew in high CO2 surroundings actually had large genomes, and whether climate variation may have been a driver in producing the diversity of plant genome sizes seen today.
In their recent paper in Annals of Botany, Pavel Veselý and colleagues suggest a compromise. They examine stomata and genome size in geophytic plants (plants with underground storage organs) found in South Africa. Many of these geophytes have prostrate leaves, meaning that their leaves lie flat to the ground and create a humid microclimate underneath them. The authors hypothesise that this humid microclimate could be used as a proxy for past conditions in the Devonian and Mesozoic periods to investigate the effect such conditions may have had on stomata and genome size. They found stomata in prostate-leaved plants to be larger than those in plants with upright leaves growing in the same environments. Moreover, stomatal size in these plants correlated positively overall with plant genome size but this trend was weaker in plants with larger genomes.
This data lends support to the assumption that genome size variation may indeed be driven by climate variation, and a requirement to alter stomatal size and patterns. The authors also observe that when comparing species pairs (i.e. a prostrate species to a related upright species), the increased stomatal size in prostrate species was not necessarily associated with the traditional decrease in stomatal density or decrease in conductance. This suggests that the changes in stomatal size and patterning may not only be driven by a need to improve access to CO2, but also by a need to optimise gas exchange in response to other environmental factors such as humidity. The authors note that this aligns well with other experimental data in Arabidopsis, and point out that the Devonian and Mesozoic periods are believed to have had high humidity ‘greenhouse’ climates. This strengthens the argument that different aspects of stomatal evolution respond to different environmental pressures, and that these may also have been and continue to be a driving force in evolution of plant genome size.
Increases in genome size through whole genome duplication has often been suggested to be major driver to increased developmental complexity during plant evolution. Understanding how these increases may have come about will be key to understanding the diversity of plants present today and their extensive variation in form and function.