How algae evolved into land plants

Japanese researchers have analysed the genome of a terrestrial alga, revealing the presence of genes that enable plants to cope with the extremes of life on land.

The transition of plants from water to land was a key evolutionary event, providing the nutrients and oxygen that allow terrestrial creatures to survive. But there has been little evidence of the stages in the evolution of plants that allowed them to adapt from the significantly more stable conditions of aquatic life to conditions on land, which include drought and exposure to high-intensity light and UV.

The ancestors of present land plants are widely believed to be charophytes, a division of green algae that is distributed throughout the world. The charophyte Klebsormidium is a very simple multicellular organism and most species that have adapted to live on land can also live in water.

Researchers from the Tokyo Institute of Technology (Tokyo Tech), Kazusa DNA Research Institute and RIKEN, led by Hiroyuki Ohta from Tokyo Tech, analysed the genome of the terrestrial Klebsormidium flaccidum. They compared the sequences of 31 proteins from K. flaccidum, five land plants, seven charophyte algae and nine other algae.

Differential interference microscope image of Klebsormidium flaccidum strain NIES-2285. K. flaccidum consists of non-branching long filamentous cells. Each cell contains a large chloroplast, which is positioned against the cell wall (parietal chloroplast) and contains a pyrenoid. Arrowhead indicates a pyrenoid surrounded by a few starch grains.

The comparison suggests that many of the genes required to survive on land were already present in the ancestor of K. flaccidum. The presence of some hormones which land plants release in response to environmental changes were identified in K. flaccidum, although many receptors crucial for plant hormone signalling were absent, suggesting that the alga had developed primitive land-plant signalling pathways. The researchers also found seven genes that encode pathways for cyclic electron flow - a process which causes quenching processes and ATP synthesis, helping dissipate excess light energy from high-intensity sunlight and protecting land plants from damage.

The researchers conclude their report by saying, “Our analysis provides evidence that K. flaccidum has the fundamental machinery required for adaptation to survival in terrestrial environments.”