Do I know you? Plants grow stronger, more selfish roots when around strangers.© 2006 Louis-M. Landry

Telling apart relatives from strangers is crucial in many animal species, helping them to share precious resources or avoid inbreeding. Now it seems that plants can perform the same trick.

Plants have already been shown to compete with others — of their own kind or of another species — when sharing space. For example, they sometimes choose to invest more energy in sprouting roots when they have nearby competition for water and nutrients.

Now, Susan Dudley and Amanda File of McMaster University in Ontario, Canada, have shown that plants grown alongside unrelated neighbours are more competitive than those growing with their siblings — ploughing more energy into growing roots when their neighbours don't share their genetic stock.

Plants 'know' more about their environment than they are often given credit for: they can sense the presence of neighbouring plants through changes in water or nutrients available to them or through chemical cues in the soil, and can adjust their own growth accordingly. "That plants have a secret social life is something well known to plant ecologists," says Dudley.

But the ability to recognize kin has not been demonstrated before.

To get their results, Dudley and File grew batches of a beach-dwelling plant, the Great Lakes sea rocket (Cakile edentula), in pots of four, either with specimens from the same maternal family or from several different families.

Those growing with strangers had a greater mass of roots after two months of growing than those sharing pots with siblings. The work appears online in Biology Letters¹.

Family ties

How the plants work out who's who is still a mystery. Dudley suggests that a protein or chemical signal specific to each plant's family might be secreted and detected by other roots in the nearby area.

It is possible that plants use something similar to the immune system in animals to sense others around them, says Ariel Novoplansky, a plant biologist at Ben-Gurion University of the Negev, Israel. In animals, kin recognition is often accomplished using specific immune proteins that disclose an individual's genetic make-up. If such specific molecules are being emitted from plant roots, and if the roots are close enough to each other, this might allow them to make similar distinctions.

But Novoplansky is quick to add that this is pure guess-work. He runs into the same problem in his own work, on how plants tell the difference between themselves and others. "At this point I cannot imagine a mechanism by which this could happen," he says. "Nothing of this sort has been found before."

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• References

  1. Dudley S. & File A. Biol. Lett., doi:10.1098/rsbl.2007.0232 (2007).