Molecular messengers carry vital instructions from leaves to far-flung stems, say US researchers. Long-distance traffic in grafted tomato stems switches leaves from pointy to round, the team found. This insight into how plant parts talk may help farmers to propagate fruit trees and defend plants against disease1.

Sprouting shoots need to know what's happening elsewhere in the plant and equip new leaves for their surrounds. A debate has been blossoming about whether roving messenger RNAs (mRNAs) - molecules that code for proteins - carry the news.

Familiar pointy tomato leaves were replaced by rounded foliage when young shoots were grafted onto a round-leaved 'Mouse ears' strain, Neelima Sinha and her colleagues at the University of California at Davis have now shown. Mouse ears plants make a mutated version of a leaf-patterning protein.

Carried into the grafted tips, mRNAs coding for the protein override normal leaf shape, the team reports, by detecting the accumulating protein in the budding shoot tip.

Along with plant hormones, products of metabolism and proteins, RNA transport could enable plants to coordinate growth, suggests Sinha. "Plants may have evolved a system for transporting RNAs in order to communicate," she says.

Lacking a circulatory system for carrying signals, plants have a one-way - upwards - transport infrastructure for nutrients and communication. Small channels called plasmodesmata link neighbouring cells. Through these, proteins, sugars and metabolites pass from cell to cell until they empty into the plant super-highway: vessels called phloem, which carry material long distances from leaf to growing stem.

The existence of long-distance signals in plants was first suggested by the finding that leaves exposed to short day lengths triggered flowering in the plants to which they were grafted. The discovery of plant phloem packed with RNAs fuelled speculation that RNA trafficking is a transport mechanism.

Simply finding them isn't enough, explains Karl Oparka, who studies phloem transport at the Scottish Crop Research Institute in Dundee, UK. "The key was to show they were doing something."

The messages could bear breaking news about the environment - such as day length, nutrient supply and temperature - from mature leaves to budding ones, he agrees. "Older tissues are telling new ones that the situation has changed," he says.

But the tomato RNAs in the latest work are abnormal ones, points out David Jackson, who works on RNA transport at Cold Spring Harbor Laboratory in New York. Whether the same applies for other RNAs has still to be proved. But he concedes: "It's quite likely."

Viruses exploit plants' transport system to spread infection: "They hitch a ride," says Bill Lucas of the University of California at Davis, whose work provided earlier evidence for RNA transport. When one leaf is infected, the whole plant often launches a defence response. Working out the messages that plants send when under attack could aid disease prevention, he hopes.

Incompatible RNA signals might also explain why some plant grafts don't take, thinks Lucas. Cuttings of fruit and nut trees that are tricky to raise from seed are often grafted onto alternative good-quality 'root stock'. "A lot of money is wasted on incompatible grafts," he says.