Land plants' algal roots.

Land plants' algal roots

Scientists know little about how plants evolved from their green algal ancestors. The problem is plain: Primitive plants, lacking hard parts, made poor fossils. But botanists probing living organisms have inserted a new piece into the plant-ancestry puzzle.

In the alga considered the best model organism for a land-plant ancestor, researchers at the University of Wisconsin-Madison have discovered a compound similar to lignin -- an important structural element of wood and of cell walls in all vascular plants.

The finding provides a "chemical missing link" between land plants and the group of green algae that scientists believe gave rise to them about 400 million years ago, says Cornell University plant scientist Karl J. Niklas. Moreover, it suggests that lignin originated in algae and not, as previously thought, in early land plants, says study leader Charles F. Delwiche, who reports his group's results in the July 28 SCIENCE.

The new evidence also suggests that lignin's first function was not structural, since algae need not stand up in the water. Rather, the woody material probably acted initially as an antimicrobial agent, only later taking on a mechanical role, Delwiche says.

In addition, the Madison research team found a striking similarity between the distribution of the lignin-like chemical in the alga Coleochaete and in a species of hornwort, an early group of land plants related to mosses. This similarity indicates a closer-than-expected relationship between green algae and hornworts, Niklas says.

Delwiche's team began to suspect that something like lignin lurked in Coleochaete when they boiled the millimeter-wide organisms in strong acid and discovered, to their surprise, that much of the tissue remained intact. Using chemical and microscopic tests, they went on to confirm the identity of the durable debris.