The root tip of Arabidopsis thaliana plants with a non-functional SMB protein covered with the beneficial fungus Serendipita indica (green). Plant and fungi cells are shown in magenta. Image credit: Ernesto Llamas (CC BY 4.0).
As plant roots grow deeper into the soil, they encounter various fungi and bacteria. Some of these microbes attempt to infect the roots, but certain interactions can be mutually beneficial, promoting microbe survival while protecting plants from harmful infections. However, microbes rarely invade the root tip, which is protected by a special type of tissue called the root cap.
As roots grow, root cap cells are constantly removed and replaced. In the model plant Arabidopsis thaliana, root cap turnover occurs via a combination of cellular shedding and developmental programmed cell death (dPCD). Proteins known as SMB and BFN1 regulate this process by triggering cell death and ensuring dead cells are removed.
To investigate whether the rate of dPCD and the degree of post-mortem corpse clearance affect how fungi accumulate in plant roots, Charura et al. studied Arabidopsis plants with a non-functional SMB protein. Staining techniques revealed an accumulation of dead cells remaining in the root cap, as well as increased growth of the fungus Serendipita indica in the root tip. These changes also disrupted the growth-promoting effects typically initiated by the fungus. Taken together, the findings suggest that under normal conditions, SMB drives the continuous clearance of cells through dPCD, which limits fungal growth in the root tip that could otherwise harm the plant.
Charura et al. next looked at how S. indica infection affects the expression of genes that drive dPCD. This revealed reduced expression of the gene for BFN1 in Arabidopsis plants infected with the fungus. Staining the roots of plants containing a non-functional form of BFN1 also revealed increased dead cell remnants and greater fungal growth further up the root, suggesting that S. indica may exploit host cell clearance pathways to colonize the roots.
In conclusion, the findings show that the rate of dPCD in plant roots is key to limiting fungal invasion. The decreased BFN1 gene expression observed with S. indica infection suggests that fungi may manipulate BFN1 to help them form more beneficial partnerships. Understanding the interplay between root cap turnover and fungal invasion could lead to more sustainable agricultural practices and may help researchers to improve plant nutrition and tolerance without relying on chemical fertilizers or pesticides.
