Biology students are taught that the concept of vaccination came from Edward Jenner's discovery that milkmaids and dairymen infected with the mild cowpox virus were protected against smallpox. It is less widely appreciated that plants can also be protected from a severe virus by prior infection with a mild strain of a closely related virus. This cross protection in plants was recognized as early as the 1920s, but its mechanism has been a mystery - plants do not possess an antibody-based immune system analogous to that found in animals. This was probably the first observation of a plant's intrinsic defence mechanism against viruses which, 75 years later, is just beginning to be understood.

In the past decade, there has been considerable research into transgene-mediated virus resistance, co-suppression, virus-induced gene silencing (VIGS), antisense suppression and transcriptional gene silencing (TGS) in plants. There has also been intense research into RNA interference in Drosophila and nematodes, and quelling in fungi. These seemingly disparate endeavours have produced pieces of a jigsaw puzzle which, when put together, begin to reveal the existence and characteristics of a natural defence system in plants against viruses and transposable DNA elements.

Many of the details and ramifications have yet to be determined, but the current picture is that of a wonderfully elegant system that can generically recognize invading viruses and transposable elements (TEs) and marshal the plant's defences against them.