How Do Pollen Grains Recognize the Right Stigma?
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How do pollen grains recognize the right stigma? A pollen grain that lands on a stigma has to be judged before it can grow. In many flowering plants the stigma can accept pollen from a different plant of the same kind and reject pollen from the same plant or from closely related individuals. The recognition step happens on the stigma surface, and the best-understood version of it is found in the Brassicaceae, the cabbage family. This explainer describes the mechanism that two review articles report, explains how the signal moves between pollen and stigma, and marks where the sources stop.
Quick answer
- Recognition happens at the stigma surface, when a pollen grain touches a stigma papillar cell.
- In the Brassicaceae, two highly variable S-locus proteins act as a receptor and its ligand, one on the stigma and one on the pollen.
- When the pair signals “self,” the pollen is stopped early, and the stigma can reject pollen from the same plant or closely related plants.
- Other plant families use different molecular systems, so no single mechanism covers every flower.
Why a stigma needs to tell pollen apart
A stigma is more than a landing pad. Pollen-pistil interactions have become a model system for studying how cells pass information to one another. Plants have evolved several ways to reject some pollen, and the general name for the family of these mechanisms is self-incompatibility. The review that describes this field notes that the pistil recognizes and actively rejects self-pollen or pollen from genetically closely related individuals.
The process is not a single event. After a pollen grain arrives, several steps can take place in sequence. Pollen has to stick to the stigma and take up water. The pollen tube then has to grow and find its way through the pistil. In some families the rejection is aimed at pollen germination or at pollen tube entry into the pistil, and the reviews describe the timing differently for each family. One of the review articles puts the general idea this way: during this molecular courtship process, various recognition processes take place, and the outcome depends on which molecules meet.
The same review notes that molecular studies over the last two decades have shown that the term self-incompatibility does not describe one mechanism. The S loci of different plant families are not the same genetic region, so the answer to “how do pollen grains recognize the right stigma” depends on which family is being discussed. The rest of this explainer follows the best-described case first, then shows how other families differ.
The Brassicaceae system: a signal carried by the pollen
In the Brassicaceae, the signal starts with the pollen grain. The pollen carries a molecule that the stigma detects, and the detection leads to a response inside the stigma cell. The recognition of self-related pollen takes place when a pollen grain touches an epidermal papillar cell on the stigma surface. That is the point at which the stigma decides whether the pollen will be accepted.
The molecular parts were identified in Brassica species. Studies in Brassica, and later in Arabidopsis lyrata, showed that two S-locus-encoded proteins work as a receptor and a ligand. Both proteins are highly variable, and the review describes the two proteins as functioning as receptor-ligand pairs that determine specificity in the stigma epidermis and in the pollen.
The stigma-side receptor is a kinase. In the Brassicaceae, self-incompatibility is regulated by a pistil-expressed S-receptor kinase, called SRK. It interacts with its matching pollen ligand, the S-cysteine-rich protein (SCR). The review describes this interaction as the step in which the stigma recognizes and rejects self pollen.
The timing is one of the clearer points in the review. It reports that self pollen is inhibited at the stigma surface within minutes of pollen-stigma contact, which places the rejection at the first step of the interaction rather than later in the pistil.
Other families use different signals
The Brassicaceae is not the only family with a well-studied mechanism, and the sources make clear that the systems differ. For a broader overview of how pollen reaches the ovule after landing, see how pollen reaches the ovule. In the Papaveraceae, the poppy family, the direction of the signal is reversed. The review states that in Papaver the signal is produced by stigmatic cells and perceived by pollen, which then triggers a transduction cascade inside the pollen tube. The Papaver system also works through calcium. The second review describes poppy self-incompatibility as mediated by a calcium-based signalling pathway. In that account, the trigger is an interaction between a receptor on the pollen and a ligand produced by the stigma.
The Solanaceae, the nightshade family, has been studied in a different way. The second review says that compatibility has been studied most extensively in species from the Solanaceae. In Nicotiana and Petunia, stigmatic lipids are essential for pollen development and pollen tube guidance on the stigma. Other molecules, including receptor kinases, their ligands, lipid-transfer proteins, and arabinogalactan glycoproteins, affect pollen tube growth and guidance once the tube is inside the pistil.
The Asteraceae, which includes sunflowers and daisies, adds another variation. Its stigma is described as semi-dry rather than the dry type typical of the Brassicaceae. Studies of Senecio squalidus show that stigma structure and the molecular mechanism of self-incompatibility in the Asteraceae and the Brassicaceae are different. In other words, a similar outcome, rejecting self pollen, can be reached by different machinery.
What happens after the stigma accepts the pollen
Acceptance does not finish the story. Once pollen is accepted, the pollen tube has to grow down the style and reach an ovule. The second review lists pollen adhesion and hydration, pollen tube growth, and navigation through the pistil as separate stages of the pollen-pistil interaction. Each stage involves its own signals.
For the path pollen takes to reach flowers in the first place, see how pollen travels to flowers. The review also points out a competition effect. Ovules are often in limited supply, so compatible pollen tubes have to compete for ovules. The review describes this as a second level of selection, acting on the male gametophyte.
How the mechanism was worked out
The receptor-ligand picture came from molecular work over about two decades. The first step was cloning the S-locus genes whose products are expressed specifically in the stigma epidermis. The second step was identifying the specificity-determining region of the S locus. The review credits this work to several laboratories, and it is the reason the Brassicaceae is the best-documented case.
Genetic structure also complicates the picture. The S haplotypes of Brassica differ in gene order and in the distances between genes, which the review describes as scrambled gene order and variable intergenic distances. This makes the S locus harder to compare between plants and between species.
How we chose
The two sources were chosen because they are peer-reviewed review articles in the open-access PubMed Central archive, and because together they cover two different plant families and two different molecular systems. The Brassicaceae review gives the receptor-ligand detail. The Asteraceae review gives the comparison across families and the stages after acceptance. Each factual sentence in this explainer is tied to a passage in one of the two pages.
What we could not verify
Neither source gives a general rule that applies to every flowering plant, and the explainer does not state one. The sources also do not give timing data for individual crops, so no crop-specific rate of rejection is stated here.
What the sources do not say
The sources do not describe how wind-pollinated grasses or other non-model families recognize pollen, and they do not measure how often a stigma rejects pollen in a field setting. The Brassicaceae review states that the family is currently the only system for which the stigma and pollen recognition components are known, so the receptor-ligand description cannot be extended to other families from these pages. Where a family is not covered, this explainer makes no claim about it.
Frequently asked questions
What is self-incompatibility in plants?
Self-incompatibility is the general term for mechanisms that let a plant reject its own pollen or pollen from closely related plants. The second review describes it as the pistil recognizing and actively rejecting self-pollen or pollen from genetically closely related individuals.
Where does recognition happen on the stigma?
In the Brassicaceae, recognition happens when a pollen grain interacts with an epidermal papillar cell on the stigma surface. The stigma epidermis is where the receptor is expressed.
Is the Brassicaceae the only family with a known receptor and ligand?
The Brassicaceae review says it is currently the only system for which both the stigma and pollen recognition components are known. Other families have different systems, and their components are not described in these two sources.
Can pollen from a different species be rejected?
The second review states that rejection of incompatible pollen can happen at interspecific and intraspecific levels, which means between species as well as within one species. The sources do not give rates for each case.
Why do pollen tubes compete?
The second review describes this as a second level of selection on the male gametophyte. It applies to tubes that have already passed the stigma step, which is why the competition is described as a separate stage.
Sources
- PubMed Central, “Self-Incompatibility in the Brassicaceae: Receptor-Ligand Signaling and Cell-to-Cell Communication,” https://pmc.ncbi.nlm.nih.gov/articles/PMC151257/ (opened 2026-10-28)
- PubMed Central, “Pollen-pistil interactions and self-incompatibility in the Asteraceae: new insights from studies of Senecio squalidus,” https://pmc.ncbi.nlm.nih.gov/articles/PMC3170154/ (opened 2026-10-28)
