Plants are the most diverse group of organisms on the planet – genomically speaking. Paris japonica, the Japanese canopy plant, has the largest genome of any organism analysed to date. At 149,000 million base pairs (mega bp or Mbp) of DNA, it’s about 50 times bigger than the human genome (1). At the other end of the scale, the flowering plant with the smallest genome is Genlisea tuberosa, a tiny carnivorous plant found in Brazil, coming in at 61 Mbp.
Plants also have an extraordinarily large range of ‘ploidy’ – the number of complete sets of chromosomes in their cells. Humans and many animals are diploid, with two copies of each chromosome, one from each parent. Plant species may have anything from two to 96 copies of each chromosome. One 96-ploid species of fern has over 1,400 chromosomes per cell - the highest chromosome number known to science.
The often huge amounts of DNA inside plant cells affects how they function. It may also influence their ability to adapt and evolve, especially in periods of rapid environmental change. Understanding how plants evolve and survive in the face of climate change is crucial for the future, especially considering 90 per cent of humanity’s energy intake comes from just 15 species of plants. Knowledge of plant genomes will help with agriculture and biotechnology; wild relatives of domesticated species may harbour traits that will help them adapt to, for example, global heating, nutrient loss, or aridification.
To date, just over 900 of the estimated 450,000 plant species on Earth have had a genome sequenced (2). Part of the reason is that their wide variety, and often huge amounts of DNA, make decoding plant genomes a complex task.
We spoke to researchers in the Darwin Tree of Life project who are aiming to sequence the genomes of all complex life in Britain and Ireland, including plants. The project is about to publish its first plant genome – the common oak. It has a small genome of about 800 Mbp – a third of the size of the human genome: if you stretched out all the DNA from a single cell of an oak tree, it would stretch to just 66cm. Our own genome would reach 2m, and Paris japonica would be 100m long.