Paris japonica (Japanese canopy plant)

The canopy plant is restricted to Japan, where it grows in the central and northern mountains of Honshu Island.

Paris japonica is a perennial plant with a bud-bearing rhizome (underground stem). Adult plants can reach a height of 35-75 cm and have a solitary stem with about 7-10 obovate leaves (12-25 cm wide) in a pseudo-whorl. In Japan P. japonica blooms between May and August, producing a single, stalked, star-like flower composed of up to 10 whitish tepals (2.5-3.5 cm). The fruit, which is a berry or fleshy capsule, becomes dark purple when mature and contains scarlet seeds.

The canopy plant is planted as an ornamental in gardens. Mature fruits are reported to be edible.

Ideally, Paris japonica should be grown in acid to neutral soil that is humus-rich and moisture-retentive, but well-drained. It has a preference for damp and dappled shade environments and usually grows in woodlands. It can be propagated either by rhizome stem division in spring or from ripe seed sown in the autumn. A key factor that must to be taken into account, however, is that seeds may take three or more years to germinate.

Two specimens of Paris japonica grow at Kew, and are exhibited in the Davies Alpine House during their flowering period. One of the specimens was collected from the foothills of Shirouma-dake Nagano-ken, Honshu, and donated to the Royal Botanic Gardens, Kew, by the Japanese Alpine Rock Garden Society.

As part of the ongoing research into genome diversity in angiosperms (flowering plants) in general, and monocots in particular, genome sizes have been measured in a wide range of plants and incorporated into the Plant DNA C-values database. Recent work has particularly focused on monocot families in the Liliales which includes the family Melanthiaceae. The discovery of the enormous genome in Paris japonica was certainly a surprise, especially as it now holds the record for being not only the largest plant genome but also the largest for any eukaryote. This makes P. japonica an interesting source of information which may help us to understand why some plant genomes get so big and how the enormous amounts of DNA are organized and function.

Paris japonica chromosomes (Image: RBG Kew)

Genome size is the total amount of DNA in the nucleus of an organism, and includes both the genes and the non-coding sequences of DNA. There is a staggering diversity of genome sizes. The smallest genome so far reported (0.0023 picogram (pg) of DNA) is found in a parasite (Encephalitozoon intestinalis) of humans and other mammals. The human genome, at 3.0 pg, is 1,300 times larger than this, but this pales into insignificance when compared to the size of the genome in some other animals and plants.

Among animals, some amphibians have enormous genomes, but the largest recorded so far is that of the marbled lungfish (Protopterus aethiopicus) with 132.83 pg. Among plants, the record holder for 34 years was a species of fritillary (Fritillaria assyriaca, Liliaceae). However earlier this year a Dutch group knocked the fritillary off the top spot when they found that a natural hybrid of trillium (Trillium × hagae), related to herb Paris quadrifolia, had a genome just 4% larger than the fritillary (132.50 pg).

This was widely thought to be approaching the maximum size that a genome could reach, until this summer when a team of Kew scientists discovered that the genome of another close relative of herb paris, Paris japonica from Japan, is a staggering 15% bigger than the genome of either the trillium or the lungfish at a whopping 152.23 pg.

Ilia Leitch, Research Scientist at the Jodrell Laboratory at Kew says “We were astounded when we discovered that this small stunning plant had such a large genome - it’s so large that when stretched out it would be taller than the tower of Big Ben.”

“Some people may wonder what the consequences are of such a large genome and whether it really matters if one organism has more DNA than another. The answer to this is a resounding “yes, it does”, and the consequences operate at all levels from the cell up to the whole organism and beyond. In plants, research has demonstrated that those with large genomes are at greater risk of extinction, are less adapted to living in polluted soils and are less able to tolerate extreme environmental conditions – all highly relevant in today’s changing world.”

Kew News – 5 Oct 2010 - Kew scientists discover the largest genome of them all

Kew Press Release – 7 Oct 2010 - Kew scientists discover the largest genome of them all!