Plants became green by swallowing a bacterium 1.5 billion years ago
Every chloroplast descends from a cyanobacterium that took up permanent residence inside a host cell around 1.5 billion years ago. Astonishingly, it happened again far more recently: an amoeba called Paulinella acquired its own photosynthetic partner between 140 and 90 million years ago. Some animals even steal plastids from the algae they eat.
Plastids are membrane-bound organelles in plants, algae and certain other eukaryotes, understood to be cyanobacteria living inside cells. The founding event in the group including land plants, red and green algae and glaucophytes probably involved a cyanobacterium related to the genus Gloeomargarita. Later rounds of secondary and tertiary endosymbiosis spread plastids to many other lineages, and the habit of keeping ingested plastids is called kleptoplasty. The apicomplexan parasites even retain a non-photosynthetic plastid, the apicoplast. Andreas Schimper was the first to name and clearly define them.
In a plant, every plastid starts as a proplastid in the growing tissue of the meristem, then differentiates according to need, and can even switch forms later. Chloroplasts photosynthesise, capturing carbon and releasing oxygen. Chromoplasts produce and hold pigments. Gerontoplasts oversee the dismantling of the photosynthetic machinery as leaves age. Colourless leucoplasts give rise to specialists: amyloplasts store starch and also sense gravity, helping roots grow downward; proteinoplasts handle protein; tannosomes make tannins. Plastids also build fatty acids and terpenes, and the waxy cuticle on leaves is assembled from palmitic acid made in chloroplasts.
Each plastid carries many copies of its own genome, the plastome: 1,000 or more per plastid in young, rapidly dividing cells, falling to around 100 or fewer in mature cells with many plastids. The DNA gathers in clusters called nucleoids, attached to the inner envelope but not wrapped in a membrane. Long assumed to be circular like a bacterial chromosome, it may in fact be linear.
The plastome encodes transfer and ribosomal RNAs plus some photosynthesis proteins, yet this is only a small share of what a plastid needs. The nucleus supplies the vast majority, and the two sets of genes are regulated together. In organisms that have lost photosynthesis, plastids remain valuable factories for molecules such as isoprenoids.
Source: Plastid
