Extra blue-light receptors helped ferns thrive in early angiosperm forests

Though ferns pre-date angiosperms by more than 200 million years, the rise of closed angiosperm forest canopies during the Cretaceous led to an explosive diversification in leptosporangiate ferns. These newly evolved species thrived on the shaded, blue-light-enriched forest floors. This success has led to leptosporangiate ferns being the dominant modern fern group, making up the vast majority of all living fern species. Yet a clear physiological explanation for the ferns’ efficient use of blue light has not yet been found.

In a new article published in New Phytologist, lead author Shengguan Cai and colleagues used molecular and physiological evidence to investigate links between leptosporangiate ferns’ increased stomatal sensitivity to blue light and their increase in the geological record. The researchers focussed on blue light photoreceptors in the largest subclade of leptosporangiate ferns, the Polypodiales, as well as in several older eusporangiate fern lineages and a representative angiosperm.

The authors found that ferns in the Polypodiales have a significantly more rapid stomatal response rate when exposed to blue light than do either of the other groups tested. This sensitivity, which allows the ferns to quickly capitalize on transient sun flecks, is tied in particular to the cryptochrome (CRY) signalling pathway, the only blue-light receptor family that differed significantly between leptosporangiate and eusporangiate ferns.

Molecular dating showed that the Polypodiales underwent four separate duplications of their CRY genes, with the two last events occurring as the angiosperms were emerging as a dominant group. This left the clade with six CRY gene copies – three times more than other closely-related clades – and a stomatal response time comparable to what’s seen in the grasses, which are known to have an exceptionally rapid response.

“Many plant species, and most ferns, cannot outcompete tall trees for access to light and have developed strategies for shade tolerance to cope with dim light and to optimize light capture,” write the authors. “The Cretaceous decline in diversity and range of many eusporangiate fern species followed by the diversification of leptosporangiate ferns points to a fundamental shift in functional traits that distinguishes leptosporangiate from other fern lineages.”