Though they don't have a nervous system, plants have been shown to be capable of storing memories in their DNA. Scientists are still figuring out how exactly this process works, but they know it occurs because previous research has established that plants not only form memories, but that they can actually pass their memories on to their offspring.
Memory formation like this is especially important for plants, as it allows them to adapt to changing patterns in their environment. For instance, if a plant experiences drought conditions, it can limit its growth in the following years. Of course, that requires remembering a year later what the conditions were like previously.
But if memories could be made and not forgotten, then this could lead to problems of its own when conditions shift back to the norm. In this instance, the plant will need to be able to forget about droughts when weather patterns are more favorable for growth. Similarly, if a plant seed is born with memory of drought, but is blown to a different locale where drought is less common before sprouting, then it will want to forget this inherited memory.
A small team of researchers with Australian National University, Canberra, were able to pinpoint how this process in fact works. They found that for a plant to create a memory, it creates a protein that can affect its DNA. Since DNA is transcribed into RNA before being translated into proteins, any disruption to this process can therefore prevent "recollection," or the formation of the memory. Researchers found that the mechanism at play here is called RNA decay, a natural process of degradation of the RNA molecule which serves to control RNA levels.
Interestingly, there's also some evidence that a plant's ability to form memories goes beyond this genetic process. For instance, plants have shown the ability to form short-term memories that are not transcribed in DNA or RNA. So far it's unclear how this process might work, but it's a subject for future research.
Needless to say, plants have a lot more going on than researchers previously realized. The research could ultimately lead scientists to rethink the idea that memory is strictly a neuronal event, that in fact memories may form in different ways throughout the bodies of all organisms.