How Does Wheat Rust Get Through Leaf Stomata?

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How does wheat rust get through leaf stomata? The fungus germinates on the leaf surface, grows along the leaf until it meets a stomatal pore, and then forms a structure over the pore that pushes a thin penetration peg through the gap between two guard cells. Once inside, it grows through the space under the stomata before it enters the leaf tissue. This explainer answers the question directly, then covers each stage: germination, the appressorium, the penetration peg, the substomatal growth, and the role that light and carbon dioxide appear to play.

Quick answer

The spore germinates on the leaf, and the germ tube grows until it encounters a stoma, the pore in the leaf surface.
An appressorium forms over the guard cells, and a penetration peg and substomatal tube pass between the two guard cells to start entry.
Inside, the fungus grows through the substomatal space and then into mesophyll cells, where haustoria form.
Older work linked penetration to light, but the stem rust study reports that penetration itself is light independent in its conditions.

Stage one: germination and the search for a stoma

The rust fungus begins with a spore on the leaf. The stem rust paper describes the first phase as spore germination and appressoria formation, which requires a dark period and moisture. After Puccinia graminis spore germination, the germ tubes grow perpendicular to leaf veins until they encounter stomata.

The stomatal pore is the target because it is the natural gap in the leaf surface. The paper notes that the topology of specific host guard cells plays an important role in stomata identification. The appressorium is then induced over the stomata around 4 to 16 hours after inoculation. The stripe rust paper reports that at 12 hours after inoculation most urediospores had germinated and only several substomatal vesicles were produced.

Stage two: the appressorium and the penetration peg

The appressorium is the structure that does the entering. The paper describes that the appressoria form a penetration peg and substomatal tube between the two guard cells to initiate host penetration. This is the point where the fungus passes from the surface into the space beneath it.

For a long time, researchers thought this step needed light. The stem rust paper explains that the second phase, host entry by the penetration peg originating from the appressoria formed over the guard cells, was thought to require light to induce natural stomata opening. Earlier work concluded that inhibition of colonization by the dark was due to lack of penetration through closed stomata. The paper says that appressoria “remain quiescent on top of stomata until light induced stomatal opening facilitated pathogen penetration.”

The newer study challenged that model. The paper reports that the improved method was used to investigate infection processes of the stem rust fungus during stomata penetration and colonization in barley and wheat, and that the work showed penetration is light independent. Using that method, the authors demonstrated that the fungus penetrates stomata guard cells of barley and wheat, gaining forced entry during the initial dark period.

Stage three: the substomatal space and the haustoria

After penetration, the fungus does not stop at the pore. The stem rust paper describes penetration as shortly followed by substomatal growth, which includes the formation of primary infection hyphae that grow intracellularly until they encounter mesophyll cells.

When the hyphae reach the mesophyll, the next step is to feed. The paper notes that the cell wall of the host is degraded, followed by invagination of the host cell’s plasma membrane, leading to the formation of intracellular haustoria. The haustoria are the pathogen’s feeding structures. The paper describes them as the powerhouses of pathogen colony growth, and it explains that the pathogen hijacks host cell physiology utilizing an effector repertoire that establishes an artificial nutrient sink that leads to pathogen feeding and profuse growth.

The stripe rust study follows the same general sequence in a different rust species. It reports that an urediospore’s germ tube grows, a substomatal vesicle forms, and two primary infection hyphae develop from that vesicle. Haustorial mother cells form at the hyphal tips and then invaginate into wheat mesophyll cells to form haustoria. That stripe rust work is on Puccinia striiformis f. sp. tritici, which is a different fungus from the stem rust studied in the other paper.

The role of light and carbon dioxide

The light question is not settled. The stem rust paper cites an earlier study that tested extended dark periods. It states that the researchers conclude that if a 20 hour extended dark period was provided after inoculation, then only 2 to 5% penetration occurred. The same discussion says that plant stomata opening and stimulation by daylight was not the limiting factor for stem rust penetration, and that carbon dioxide concentration also plays a major role.

The paper offers one explanation for the carbon dioxide effect. It was further speculated that in the dark, due to respiration and the absence of photosynthesis, increased intercellular carbon dioxide levels negatively impacted penetration through stomata. This is a speculation in the source, not a settled result. The paper then concludes that penetration is not always solely dependent on light, and that other factors that manipulate stomata function during the penetration process can affect penetration efficiency.

The growth afterward also depends on conditions. The paper reports that the differences in pathogen growth dynamics in both resistant and susceptible genotypes was due to light induced pathogen growth after penetration into the substomatal space. It also says that light induced plant or pathogen cues trigger pathogen growth in planta after penetration.

Why the stomatal route matters for the fungus

The stripe rust paper describes the substomatal vesicle and the hyphae that grow beneath the surface. It also reports that UV-B radiation inhibited urediospore invasion and hyphal elongation, with decreases in germination rate, hyphal branches, haustorial mother cells and haustoria, and hyphal length.

The stripe rust paper compares several mutant strains. It reports that mutant strains produced more haustorial mother cells and haustoria per infection site than the original strain, which indicated that each of the virulence-mutant strains had stronger infectivity than the original. The comparison is about how far the fungus can grow once inside, not about the stoma itself.

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How we chose

The two sources were picked because each one addresses a different part of the question. One is a stem rust study that focuses on penetration through stomata and the role of light. The other is a stripe rust study that describes the substomatal and haustorial stages in a different rust species. Both are open-access papers on PubMed Central, and both were opened for this article on 2026-10-28. Each claim is tied to a passage in one of those two papers.

What we could not verify

The two papers study different rust species on wheat. The stem rust work is on Puccinia graminis, and the stripe rust work is on Puccinia striiformis f. sp. tritici. Neither source gives one answer that covers every rust species or every wheat variety. The light and carbon dioxide results come from particular experimental conditions.

What the sources do not say

The sources do not give a field rate for any rust infection, and they do not say how to prevent entry on a home crop. They also do not describe how the appressorium forces its way through the guard cells at the level of molecules. Nothing here should be read as advice on managing rust in a garden.

Frequently asked questions

What is the stoma in this process?

It is the pore in the leaf surface, formed between two guard cells. The stem rust paper describes the appressoria as forming over the guard cells, with a penetration peg passing between them to start entry.

Does wheat rust need light to get inside the leaf?

The stem rust paper reports that penetration is light independent under its conditions, and that the fungus gains forced entry during the initial dark period. Light appears to matter more for growth after penetration.

What happens after the fungus enters?

Primary infection hyphae grow intracellularly until they encounter mesophyll cells. Haustoria then form, and the paper describes them as the feeding structures that power colony growth.

Are all rust fungi the same?

No. The two papers study different species, Puccinia graminis for stem rust and Puccinia striiformis f. sp. tritici for stripe rust. The stages are similar in the descriptions, but the conditions and the results differ between the studies.

Can environmental factors stop the fungus?

The stripe rust paper reports that UV-B radiation inhibited urediospore invasion and hyphal elongation. That is a laboratory result for a specific set of strains, and it does not show a general control method.

Sources

Shedding Light on Penetration of Cereal Host Stomata by Wheat Stem Rust Using Improved Methodology. https://pmc.ncbi.nlm.nih.gov/articles/PMC6538696/
Investigation of urediospore morphology, histopathology and epidemiological components on wheat plants infected with UV-B-induced mutant strains of Puccinia striiformis f. sp. tritici. https://pmc.ncbi.nlm.nih.gov/articles/PMC6813489/