Plants Have Memories
June 09, 2010 — Have you ever noticed how plants have an uncanny ability to know, without eyes or brains, when the time has come to bloom? Even when spring comes early or late in some years, they sense the right time, and out come the flowers. This is even more remarkable when you consider that the natural environment is a noisy place. The temperature is rising and falling every day and night. Storms come and go. Early warm spells might trick the plant into thinking spring has arrived just before more snow, with disastrous results for the plant. How does the plant tease out the right signal from all this noise, and remember the overall seasonal trend? And then, when blooming time has come, how does it tell the rest of the plant to go into action? Some Japanese scientists have helped get a partial glimpse into the amazing memory system of flowering plants. It’s all done with controls on gene expression.
They published their results this week in PNAS.1 Using a systems biology approach, they observed the favored lab plant, water cress Arabidopsis thaliana and some of its relatives – but this time not in the laboratory but out in the wild. This was a rare field experiment where scientists could observe day and night cycles and seasons having their natural effects on gene expression. They watched several gene levels known to relate to flowering. “We expected that the gene regulation of FLC orthologs may serve as the mechanism to extract seasonal cues from natural environments, because they are regulated by histone modification, which is often involved in stable cellular memory,” they said. Histone modifications are like small molecular “tags” that are put on the proteins onto which DNA is wound. These affect where promoters seek and find genes to transcribe, or repressors bind to genes to prevent expression.
The tags allow a kind of “memory” that can ride out the noisy highs and lows of short-term variations. Once a certain threshold is reached, the gene can activate. In this particular case, the scientists found that most of the expression was responsive to temperature for the prior six weeks, but not over periods longer or shorter – indicating a memory for that particular range. “The accuracy of our model in predicting the gene expression pattern under contrasting temperature regimes in the transplant experiments indicates that such modeling incorporating the molecular bases of flowering time regulation will contribute to predicting plant responses to future climate changes,” they said. One of the master regulators they studied controls many “downstream” genes that affect flowering. It acts as a repressor on their activities. Like a general overseeing a major operation, it commands the other genes, which are prevented from acting till given the signal.
The authors recognized at the end of the paper that this is just one piece of a larger puzzle about plant regulation. For instance, it appears that some plants have a “chilling requirement” of a certain time period before they will sprout and bloom. Some perennials need the temperature-dependent gene controlling flowering to switch on at the right time, but then to switch back off as the temperature rises further after flowering, so that they will go back to vegetative growth till the next year. There must be multiple interacting factors in a complex network of gene expression patterns at play that botanists are only beginning to fathom. The use of systems biology approaches and observations in natural settings are helping to elucidate the mechanisms involved with a more comprehensive view than was possible before. The authors had nothing to say about evolution.
1. Aikawa et al, “Robust control of the seasonal expression of the Arabidopsis FLC gene in a fluctuating environment,” Proceedings of the National Academy of Sciences USA, published online before print June 7, 2010, doi: 10.1073/pnas.0914293107.
Wonders of nature are all about us, even under our feet, if we will only take a moment to look and learn. They are best understood by considering their design. The intricacy and complexity of plants is truly mind boggling. How did these “robust” systems, that respond magnificently to noisy environments, come to be? If it were true that they only make sense in the light of evolution, this team would surely have made evolution the centerpiece of their paper. But notice: they did not even mention it once. Instead, the language was about patterns, codes, regulation, mechanisms and robustness. That’s design talk. Look at this paper. There wasn’t any religion, and there wasn’t any useless Darwin just-so storytelling, either. Everyone can read, appreciate, and enjoy it for its elucidation of incredible design in common natural phenomena we would otherwise take for granted. This understanding might lead to improvement in crops and other benefits for our lives. Let’s move ahead with observable, testable, understandable, empirical, inspiring, design-based science like this.