Life Lines: A tale of two plants and what their genomes tell...

Life Lines: A tale of two plants and what their genomes tell us

The flowering plant Amborella trichopoda is the oldest ancestral form of the angiosperms .

By Elof Axel Carlson

Elof Axel Carlson

Flowering plants are familiar to us as bouquets and garden plantings that delight us as they emerge in spring and summer. They are collectively part of the angiosperms, which also include familiar trees with generous-sized leaves that are shed in the fall.

They first appear in the fossil record about 130 million years ago. For those not familiar with how old life on Earth is estimated to be by biologists, that is about 60 million years before the dinosaurs went extinct.

Ferns, mosses and conifer trees (like gingkoes) existed long before the angiosperms. If the angiosperms are arranged in a sequence from oldest to most recent types, the oldest ancestral form of the angiosperms alive today is found in the Pacific Ocean on New Caledonia, an island northeast of Australia and northwest of New Zealand. That flowering plant is known as Amborella trichopoda.

A lot has been learned about the biology and history of Amborella. Its pollen, or ovule, has 13 chromosomes (and thus its leaf, stem and root cells have 26 chromosomes each). The Amborella ancestor gave rise to 250,000 species of flowering plants. About 75 percent of them have seeds with two fleshy modified leaves called cotyledons.

If you eat a fresh green pea from a pod and look at it before you pop it into your mouth, it has two halves, which is why you call it split pea soup when you cook a bag of dried peas.

The flower of the Amborella trichopoda

The DNA of Amborella has been worked out. It has 870 million base pairs. These are organized as 25,347 genes. Shortly before Amborella arose, it had experienced a doubling of its chromosome number. No major changes have occurred in its chromosomes since that event. Its nuclear genes have few inserted repetitive sequences. But, curiously, its mitochondrial DNA has many horizontally transferred genes from algae, mosses and lichens.

The ancestral genome of the angiosperms can be inferred because the major branches of the angiosperms share that core set of genes. This will allow botanists and chemists studying plant evolution to work out the functions of these shared genes as well as the distinctive genes that gave rise to the six major branches of flowering plants.

Quite different is the loblolly pine. It is a gymnosperm rather than angiosperm. They have a much longer history on Earth than the angiosperms. The conifers are the most familiar of the gymnosperms whose seeds are “naked” and enclosed in cones. Imagine the pine cones used in foods and compare them to the peas and beans in your soups.

The loblolly pine can live up to 300 years.

The loblolly pine, or Pinus taeda, is a common pine tree found from Florida to Texas and as far north as New Jersey. The trees can live 300 years and they are a major source of industrial lumber and paper pulp. The name loblolly is from an English idiom for food boiled in pots producing soups, broths or porridges. It has the largest known genome of any living organism, 23.2 billion base pairs (about seven times more than human cells and about 22 times that of Amborella. Unlike Amborella, 82 percent of its DNA is repetitive (formerly called junk DNA) caused by infectious insertions of tiny sequences of DNA. It has 50,172 genes in its pollen, or ovule, genome and they are located in 12 chromosomes per gamete.

One of my six students who got their doctorates with me at UCLA, Ronald Sederoff, pioneered the molecular biology of woody plants using the loblolly pine. He devised a technique to insert genes into woody plants, enabling his laboratory to study how wood is formed and how genes could be studied without waiting many years to study their genetics.

I was very pleased to learn that he was the recipient of the Wallenberg Prize, which is given by the king of Sweden for a contribution to plant biology, a field that is usually overlooked in the Nobel physiology and medicine prize. He attended the ceremony in Stockholm last October.

Elof Axel Carlson is a distinguished teaching professor emeritus in the Department of Biochemistry and Cell Biology at Stony Brook University.