The colonization of land by plants 450 Mya marked a major transition on Earth and was one of the critical events that led to the emergence of extant terrestrial ecosystems.
Chief among the challenges the terrestrial environment presented for these early algal colonizers was acquiring nutrients, but the exact mechanisms that enabled these challenges to be overcome are not well understood.
Until now …
In a new paper in PNAS, Delaux et al. (2015) propose
the algal ancestor of land plants was preadapted for interaction with beneficial fungi [that improved a plant’s ability to capture nutrients] and employed these gene networks to colonize land successfully.
Delaux et al. (2015) exploited newly described phylogenetic relationships in order to explore the key innovations found in bryophytes (the earliest diverging land plants) that are not found in the “advanced charophytes” (the closest algal relatives to land plants).
One of the key innovations that distinguishes the bryophytes from the charophytes is the ability to form symbiotic relationships with fungi. Recent genetic and transcriptomic work in model legumes, such as Medicago, has enabled the characterization of the genes that control the steps leading to these functional symbioses.
[Delaux et al. (2015)] hypothesized that tracing the origin of these symbiotic genes would allow [them] to investigate the link between the appearance of AM symbiosis and land colonization.
The authors performed a phylogenetic analysis of 259 transcriptomes and 10 green algal and basal land plant genomes. They found the
symbiotic signaling pathway predated the first land plants. In contrast, downstream genes required for root colonization and their specific expression pattern probably appeared subsequent to the colonization of land.
Figure 4. Stepwise acquisition of symbiotic genes in the green lineage. The appearance of new domains in advanced charophytes led to the emergence of new protein families and to the evolution of the symbiotic signaling module via the neofunctionalization of existing proteins (DMI1 and CCaMK). Most of the downstream symbiotic genes evolved in basal land plants via gene duplication in these newly evolved gene families (i.e., STR) or by combining existing domains in new proteins (i.e., VAPYRIN). Finally, genes originating from lineage-specific duplications were recruited independently to fulfill their symbiotic functions (PT and H+-ATPase). Delaux et al. 2015
Delaux, P-M et al. (2015) Algal ancestor of land plants was preadapted for symbiosis. PNAS 112 (43): 13390–13395.