Subtle nudges
Most of us know penicillin as an antibiotic. It is made by the soil dwelling Penicillin rubens fungus. Now this fungus has many brothers and sisters, some of which like to nestle themselves between the cells of roots. For the plant this is positive as colonized plants grow more roots. Now a new study by Spanish a Czech researchers shows that Penicillin melinii enhances root growth with subtle nudges.
As a well-developed root system is a benefit for the plant, crops not excepted, the researchers looked if there where any microbes that could help. The only other criterium they had; the microbe also needs to like to grow under laboratory conditions, as that makes it easier to work with it.
The researchers found one that not only gave the model plant Arabidopsis (tale cress) more roots, but also quinoa and tomato plants. These last ones the researchers also gave the isolated microbe when growing in the field, these plants grew also more leaves and tomatoes. Showing that a bigger root system helps with getting a higher yield. Following this the researchers identified the microbe as the fungus Penicillin melinii.
Few clues
Now they knew which microbe made the plant grow extra roots, they wanted to know how. Starting with where in the root does it make its home. This they found was between the cells in the outer layers of the root. But, it did not make it into the veins.
Next up was finding out how this Penicillin helps plants to grow more roots. For this the researchers looked at which genes were more or less active in the roots colonized with the fungus compared to roots without the fungus. Surprisingly few it turned out. Most of those had to deal with how plants deal with intruders. But one other group stood out.
This other group consisted of genes that are involved in regulating lateral root development. These were slightly more active. To test if these were indeed the genes that the fugus needed for the induction of more root growth the researchers looked if the fugus could still promote root growth when plants missed one of these lateral root development genes. For one of them this was indeed the case, in plants without that gene, Penicillin could not promote root growth. For two others, which are part of bigger gene families, Penicillin could still promote root growth in plants missing those. So, the researchers suspect that some related genes might be taking over the function of the missing genes.
No auxin
Now, one of the main regulators of root branching is the plant hormone auxin. However, the researchers did not find any evidence that levels of auxin or any of the other plant hormones they looked at, was different between the fungus colonized roots and those that were not. Still wondering how the fungus manages this without auxin, they looked at the auxin distribution in the roots that were or were not colonized by the fungus. While finding more sites at which the root was preparing to grow a side root, they did not find any evidence that there was more auxin. Suggesting that the fungus indeed promote root branching without extra auxin.
So, Penicillin melilii colonized plants grow extra roots which helps them do better in general. But despite efforts to find out how this fungus does this, the researchers only find a few hints. But maybe the biggest discovery was that Penicillin only needs a subtle nudge to steer the plant to producing more roots. And this shows that a small gesture goes a long way.
Literature
Laura Gutierrez-Manso, et al., Penicillium melinii promotes root growth through subtle host reprogramming across model and crop species, Journal of Experimental Botany, 2026; erag396, https://doi.org/10.1093/jxb/erag396
Plant en Zo 