Two bacteria can be grouped as the same kind, yet carry different genes and make the same plant grow differently. That hidden variation may matter for healthier, more sustainable crops. Plant-associated microbes can influence plant health, making them a possible route toward more sustainable and environmentally friendly farming practices.

But collections of these microbes often represent each broad bacterial group with just one strain, overlooking differences within that group and sampling only a limited number of habitats. This study examined genetic differences among common, harmless bacteria living on leaves of Arabidopsis thaliana in south-west France.

The results found genetic differences among strains, differences in how quickly they grew, and differences in how plants responded to them. The central problem is like judging every apple in a basket by tasting one apple.

Most collections used one representative strain for each broad bacterial group and sampled very few places, so they could miss differences that change plant responses. Earlier work had already found extensive genetic variation among harmful bacteria associated with Arabidopsis thaliana, especially variation in virulence across several bacterial species.

The focus was the bacterial community living on leaves, because leaves offer many small habitats and nutrients, and some leaf bacteria can help plants grow or protect them from disease. The study gathered several strains from the most common and widespread harmless leaf-bacteria groups found across natural Arabidopsis thaliana populations.

The strains were then compared for their genetic differences, their growth, and their effects on different plant types at the seed and seedling stages. The search began with more than seven thousand bacterial colonies collected from natural Arabidopsis thaliana populations.

The researchers then identified which colonies belonged to the bacterial groups of interest. Among the identified colonies, the groups appeared at very different frequencies, ranging from just five samples for OTU2 to 365 for OTU6. The bacteria most often found living on these leaves are not all alike: they span several distinct family relationships and produce visibly different colony forms.

That diversity matters because the researchers are not studying one uniform resident, but a varied community whose members can be tested separately. The differences were not limited to small genetic changes. Strong variation in gene content appeared among strains within the same broad bacterial group.

In one example, most gene groups were shared by six strains, but some were found only in one or two strains, meaning closely grouped strains still carried distinct genetic capabilities. The six strains carry a remarkably shared genetic toolkit: four thousand three hundred ten gene groups appear in all of them.

Yet each strain also has its own additions, showing that closely related bacteria can still differ in the traits they may bring to their plant host. The bacterial groups grew much more differently when nutrients were plentiful than when they were scarce.

Even within the same group, related strains could follow noticeably different growth patterns, showing that both group identity and fine-scale genetic differences shape growth. The plant responses varied too. Significant genetic variation among the eight Arabidopsis thaliana accessions appeared in response to twelve of the twenty-two bacterial strains.

The pattern depended on when the bacteria were added: some differences appeared at the seed stage, some at the seedling stage, and some at both stages. For the remaining bacterial strains, no significant differences among the plant accessions were detected.

Overall, plant genetic variation in response to bacteria depended on the plant's developmental stage. The response depended more on the individual strain than on its broader bacterial group, so two closely grouped strains could affect plants differently.

The genetic differences among bacterial strains matched differences in plant responses, and those responses depended on both the plant type and the plant's developmental stage. One possible explanation is that plant types and stages provide different nutrients, creating different living conditions for particular strains.

Differences in bacterial growth support that possibility. For anyone using bacteria with plants, these results suggest that the individual strain and the plant's developmental stage may matter, not just the broader bacterial group. Leaf bacteria that look alike by broad classification can differ genetically and affect plants differently.

Understanding those differences could make future plant-friendly microbes more predictable and useful.