For the first time, researchers have found the mechanism that prevents viruses from entering the growing tips of plants, where new cells form. These so-called meristems sit deep within the plant and are protected from insects that might carry viruses on their mouthparts. But there is another reason why viruses so rarely enter the meristem—a molecular barrier consisting of a family of proteins called mitogen-activated protein kinases, or MPKs, as the researchers show in the journal Science.

"You can think of the meristem as the 'Holy of Holies' in a plant," says Detlef Weigel, Director at the Max Planck Institute for Developmental Biology in Tübingen and leader of the research team that made the discovery. "It's the place where all new cells and organs form. That's why protecting it is so vital for the plant."

Viruses can enter the plant through stomata—tiny openings in leaves that allow the plant to exchange air with its environment—or through wounds in the plant. Once inside, they move from cell to cell through plasmodesmata, tiny channels that interconnect them. However, those channels aren't present between meristem cells and the rest of the plant, so the virus can't physically get to the meristem. But as the meristem grows, it constantly generates new cells, and in the process the meristematic cells at its edges begin to differentiate—i.e., to give rise to the various cell types that make up the tissues of the plant. And those differentiating cells do form plasmodesmata to their neighboring non-meristematic cells. It is that differentiation zone between the meristem proper and the rest of the plant that viruses would have to cross if they are going to infect the meristem.

In their experiments, the researchers bypassed the physical barrier surrounding the meristem and artificially forced the expression of viral movement proteins in meristem stem cells. Surprisingly, however, the viruses did not spread into the meristem cells. "If we had only looked at the presence or absence of the viral movement proteins, we would have concluded that the viruses must have another way into the meristem," Weigel says. "But we saw no sign of the virus in the meristem."

Further analyses showed that the proteins responsible for blocking the spread of the viruses were MPKs. When the researchers genetically disrupted the MPK genes, the viruses could enter the meristem. These proteins, the researchers found, signal the plant's immune system to produce defenses that restrict the virus to the differentiating cell layer adjacent to the meristem. If the MPK proteins aren't there to send the signal, the defenses against the virus aren't produced and the virus can spread into the meristem.

The finding reveals a sophisticated level of virus defense that is built into the plant from its earliest stages of growth. Moreover, the work has revealed that the protective MPKs are part of a larger system that controls a switch between meristem maintenance and differentiation.

"By studying the MPK function in protecting meristems, we have also gained new insight into the regulation of plant growth and development," says Weigel.