In plants, the integument is a protective layer of tissue that surrounds the ovule, the structure that eventually becomes a seed after fertilization. Most flowering plants have two integuments (an inner and an outer), which together shield the developing reproductive cells inside, form the opening that allows pollen tubes to enter, and ultimately mature into the seed coat. While the term “integument” in animals refers to skin, in botany it specifically describes the outer coverings of the ovule.
Where the Integument Sits in the Ovule
A plant ovule has a layered architecture. At the center is the nucellus, a mass of tissue that houses the embryo sac where fertilization takes place. Wrapping around the nucellus are the integuments, forming concentric sheaths. The inner integument sits directly against the nucellus like a fitted sleeve, while the outer integument encloses everything from the outside. The whole structure connects to the parent plant through a small stalk called the funiculus.
The integuments do not close completely around the nucellus. They leave a tiny gap at the tip of the ovule called the micropyle. This opening is the entry point for the pollen tube during fertilization. In most flowering plants, both integuments contribute to forming the micropyle, though in some early-diverging species only the inner integument shapes it.
How Many Integuments Plants Have
The number of integuments varies across the plant kingdom. Most flowering plants (angiosperms) are bitegmic, meaning they have two integuments. This appears to be the ancestral condition: the earliest angiosperms likely had two. However, unitegmic ovules (with a single integument) have evolved independently in several different plant lineages, suggesting that losing one integument has happened multiple times throughout evolution. Gymnosperms, the group that includes conifers and cycads, typically have a single integument.
Three Key Functions
The integuments serve three primary roles. First, they physically protect the internal tissues of the ovule, and later the developing seed, from mechanical damage and environmental stress. Second, they define the micropyle, the narrow channel that guides the pollen tube to the embryo sac. Third, once the ovule matures into a seed, the integuments become the seed coat, which regulates hydration, controls dormancy, and influences how and when the seed germinates.
The micropyle’s role goes beyond simply being a passive opening. The inner integument produces chemical signals that help direct pollen tube growth. One such signal is a compound called GABA, which at low levels stimulates pollen germination and tube elongation but at higher concentrations inhibits further pollen tube entry. This dual effect helps ensure that only one pollen tube fertilizes the ovule, preventing a problematic condition called polytubey.
The Innermost Layer: The Endothelium
In some species, including Arabidopsis and tobacco, the innermost cell layer of the inner integument differentiates into a specialized tissue called the endothelium (sometimes called the integumentary tapetum). This layer acts as a nutritional go-between, transporting nutrients from the parent plant into the embryo sac. It also protects the embryo sac to support proper development of both the endosperm and embryo. Its controlled breakdown later in development is critical for normal embryo patterning.
How the Integument Becomes the Seed Coat
After fertilization, the integuments undergo dramatic changes. All the integument cell layers enter a rapid phase of growth and differentiation. In Arabidopsis, which is one of the best-studied examples, the immature ovule has five integument layers: two in the outer integument and three in the inner. As the seed matures, these layers transform in distinct ways. The outermost layer becomes the seed coat epidermis. Beneath it, cells form a dense palisade layer. The inner layers compress and collapse as the seed dries out, eventually forming a thin but structurally important barrier.
One of the most visible changes is the production of pigments. The integument cells accumulate compounds called proanthocyanidins (condensed tannins) and derivatives of the flavonol quercetin. These begin as colorless molecules, detectable as early as the two-cell embryo stage. Around 10 days after pollination, as the seed starts to dry, oxidative browning kicks in and the seed coat turns brown. This pigmentation is not just cosmetic. Mutant seeds that lack these tannins have thinner, less protective seed coats and show altered dormancy and longevity. The brown compounds essentially waterproof the seed and help regulate gas exchange, both of which influence how long a seed can remain viable in the soil.
By the time the seed is fully mature, the cells of the seed coat are dead. They completed their developmental program and were systematically dismantled during late seed maturation. What remains is a tough, chemically fortified shell that can protect the living embryo inside for months, years, or in some cases decades.
Genes That Control Integument Development
Two genes in Arabidopsis illustrate how tightly integument formation is regulated. The gene called INNER NO OUTER (INO) controls outer integument development. When INO is disrupted, the outer integument primordium forms on the wrong side of the ovule and then stops growing entirely. The gene AINTEGUMENTA (ANT) is even more fundamental: mutations in ANT affect the initiation of both integuments, resulting in ovules that are essentially naked.
These genetic studies have also informed a broader debate about where integuments came from in the first place. The most widely accepted idea, the telome theory, proposes that integuments evolved through the gradual fusion of sterile branches around an ancient megasporangium (the spore-producing structure). An alternative view, based on developmental genetics, suggests that integuments are not modified versions of pre-existing structures at all but rather novel lateral organs that arose from the meristematic tissue of the nucellus itself.
Integument Development Before Fertilization
Integument growth begins well before pollination. During ovule development, the inner integument primordium appears first, emerging as a ring of tissue around the nucellus. The outer integument follows shortly after. In Arabidopsis, both integuments start as two-cell-layer structures. As the ovule matures through its final pre-fertilization stage, the inner integument gains a third cell layer through a round of cell division in its innermost layer. This gives the ovule its full five-layer integument architecture before the pollen tube even arrives.
This pre-fertilization growth is entirely under maternal genetic control. The integument layers are maternal tissue, genetically identical to the mother plant. It is only after fertilization that the integuments receive signals from the developing endosperm that trigger the next wave of growth and differentiation, eventually transforming them into the seed coat.