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For example, plastid epidermal cells manufacture the components of the tissue system known as plant cuticle, including its epicuticular wax, from palmitic acid—which itself is synthesized in the chloroplasts of the mesophyll tissue. Plastids function to store different components including starches, fats, and proteins. [9]
Plastids are broken up into different categories based on characteristics such as size, function and physical traits. [2] Chromoplasts help to synthesize and store large amounts of carotenoids. [ 4 ] Chloroplasts are photosynthesizing structures that help to make light energy for the plant. [ 4 ]
Most plastids are photosynthetic, thus leading to color production and energy storage or production. There are many types of plastids in plants alone, but all plastids can be separated based on the number of times they have undergone endosymbiotic events. Currently there are three types of plastids; primary, secondary and tertiary.
The alternative to monophyly is serial endosymbiosis, meaning that the "chromists" acquired their plastids from each other instead of inheriting them from a single common ancestor. Thus the phylogeny of the distinctive plastids, which are agreed to have a common origin in the rhodophytes, is different from the phylogeny of the host cells. [3]
The peridinin dinoflagellates, named after their peridinin plastids, appear to be ancestral for the dinoflagellate lineage. Almost half of all known species have chloroplasts, which are either the original peridinin plastids or new plastids acquired from other lineages of unicellular algae through endosymbiosis.
Photosynthetic organisms with plastids of different origin (such as brown algae) do not belong to the Archaeplastida. The archaeplastidans fall into two main evolutionary lines. The red algae are pigmented with chlorophyll a and phycobiliproteins, like most cyanobacteria, and accumulate starch outside the chloroplasts.
Janouškovec et al. 2015 presents a somewhat different phylogeny, supporting the work of others showing multiple events of plastids losing photosynthesis. More importantly this work provides the first phylogenetic evidence that there have also been multiple events of plastids becoming genome-free.
A diagram showing the different types of plastid. Amyloplasts are thought to play a vital role in gravitropism.Statoliths, a specialized starch-accumulating amyloplast, are denser than cytoplasm, and are able to settle to the bottom of the gravity-sensing cell, called a statocyte. [5]