Root privileges.
Searocket siblings whose roots touched grew less than did plants living among strangers.

Nicholas Chu

Rooting for the Home Team

Humans tend to treat relatives better than strangers--"blood is thicker than water," as the axiom goes--and now it appears that plants play by the same rules. A new study provides the first evidence that flora can recognize its family and that plants treat kin better than outsiders.

Plants sense their neighbors and respond competitively: Some grow more leaves, some grow additional flowers, and some bloom earlier--all in a jostle to create more offspring. Still, undergraduate student Amanda Pile at McMaster University in Hamilton, Canada, wondered whether plants might be more easygoing if the plant next door is related.

She and McMaster plant evolutionary ecologist Susan Dudley looked at Great Lakes sea rocket (Cakile edentula var. lacustris), an annual plant that self-fertilizes to produce a batch of nearly identical siblings. They stuffed pots with four plants each--all either related or unrelated--so that their roots touched. They then grew the plants for 8 weeks, until the sea rocket started to flower, and then uprooted them to see how fully the roots, stems, leaves, and buds had developed. Plants potted next to their own ilk allocated less of their mass to root development than did those dwelling among strangers, the researchers report online this week in Biology Letters.

This study is the first to show that plants distinguish between relatives and strangers, Dudley says, and that plants can respond altruistically by growing a smaller root system when they sense family nearby. In a community where siblings share so many genes, "their success is your success," Dudley explains. "If they can agree to be nice to each other, then everybody does better."

The mechanism the sea rocket uses to discriminate remains unknown. And the behavior is "not altruism so much as reduced antagonism," says evolutionary geneticist John Kelly of the University of Kansas, Lawrence. Ray Callaway, a plant community ecologist at the University of Montana in Missoula, adds that the polite familial relations may sour if the soil conditions worsen and it's every plant for itself. Furthermore, says Hans de Kroon, a plant ecologist at the University of Nijmegen, the Netherlands, the study is long-term enough to tell whether there's a net benefit to cooperation.

Dudley agrees and is looking into field studies to see whether cooperation boosts overall fitness. But everyone seems to concur that such kin recognition and generous response to family members wrecks the assumption that individual plants always exploit their resources to the fullest possible extent.

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