A plant masquerading as a fungus
Deep in the mountain forests of Taiwan, a strange organism emerges from the leaf litter. With its fleshy, club-shaped body, it looks for all the world like a mushroom. This is Balanophora fungosa, a flowering plant that has abandoned almost every trait of a plant. It has no leaves, no chlorophyll, and no roots of its own to draw water from the soil. It lives as a holoparasite, deriving all of its water and nutrients by attaching to the roots of host trees. Common hosts for the parasite include trees in the families Fagaceae (oaks and beeches), Ericaceae (rhododendrons), and Leguminosae.
The name Balanophora comes from the Greek for "acorn-bearing," referring to the shape of its inflorescences. For most of its life, the plant exists as a hard, irregular underground tuber fused to the roots of its host. The visible part is the flowering structure, which erupts from the ground. These structures are covered in thousands of tiny flowers, some of the smallest in the plant kingdom. Depending on the subspecies, a single plant may have both male and female flowers (monoecious) or be exclusively one sex (dioecious). About 20 male flowers, covered in white pollen, typically ring the base of a globe-shaped head that contains thousands of female flowers. The plant emits an odor described as mouse-like, which attracts pollinators including flies, ants, moths, and even rats.
An extremely reduced genome
The parasitic lifestyle of Balanophora fungosa has led to an extreme case of reductive evolution known in plants. Because it outsources all energy production to its hosts, the genetic machinery for photosynthesis became unnecessary. Over millions of years, it has shed these now-useless genes. This process has resulted in one of the smallest and most bizarre plastid genomes of any plant. Plastids are organelles that, in photosynthetic plants, include the chloroplasts where photosynthesis occurs.
The plastid genome of B. fungosa is around 15 kilobases (kb) in size, roughly ten times smaller than that of a typical photosynthetic plant. Its A+T content—the proportion of adenine and thymine bases in its DNA—is nearly 90%, making it one of the most AT-rich genomes ever recorded. The genome has been stripped down to only about 19 genes, mostly for ribosomal proteins. All genes for photosynthesis are gone.
Despite this massive reduction, the plastid is still active. Over 700 proteins are imported into the organelle from the cytoplasm, suggesting it retains non-photosynthetic functions like synthesizing amino acids and fatty acids. This radical genetic streamlining extends to its genetic code itself. In its plastid genome, the codon "TAG," which is a "stop" signal in most organisms, has been reassigned to code for the amino acid tryptophan—a rare change in the universal genetic code for land plants.