Have you ever wondered how plants know which way is up?

It's not just a question children ask adults, but one that vexes agricultural scientists who would like to know how to optimize the growth of crops and for NASA researchers planning distant future missions to the stars where getting plants to grow efficiently is nigh on impossible without the training effects of gravity.

Now, a team from North Carolina State University and the University of Michigan have looked at how signal transduction pathways involved in plant growth are affected by gravity after a stem has been bent by wind and rain. The latest results of this NASA-funded work were presented at the American Association for the Advancement of Science (AAAS) annual meeting in San Francisco.

	Wendy Boss

Botanist Wendy Boss of NCSU pointed out that it is well known that plant stems do respond to gravity, it was something Darwin observed. Moreover, research had previously revealed that the distribution of plant growth regulators throughout the greenery change in response to gravity but, she said, until now no one knew how.

She and her team used oats and maize plants to show that when a plant is re-oriented the relative effect of gravity on its biochemistry changes the distribution of inositol triphosphate, the ubiquitous cell signaling molecule found in plants and even the human brain. They found that if the plants were laid on their sides, there was a rapid increase in concentration of inositol triphosphate at the base of the stems, in the pulvinus cells. The researchers believe this concentration increase, which amounts to a fivefold rise in a few seconds, quickly alerts the plant that it is not standing up straight.

Additionally, the researchers found that the amount of inositol triphosphate fluctuates between the upper and lower halves of the plant's pulvinus. The researchers suspect this is because the pulvinus cells are communicating about whether organelles (amyloplasts) within the cells have fallen to the bottom, pulled there by gravity. Boss says that the continued rise in inositol might help a plant decide whether it has been knocked to the ground, or is blowing in the breeze.

If it has fallen over the plant responds by inducing elongation of those cells nearest the ground, which gradually pushes the stem back to the vertical. Being able to enhance this effect might be useful in raising better crops that get back to their "feet" if they are knocked over by the wind so reduce disease risk and make them easier to harvest. The effect might also be of benefit to cut flower distributors who might be able to prevent their produce from knowing it has been packed horizontally. "If we were able to decrease or eliminate their ability to sense that they were on their side, we would not necessarily increase their shelf-life, but the florist would have straighter stems to work with and not ones that would bend in response to having been shipped horizontally," muses Boss.

And, of course, from NASA's perspective long-term space missions will be impossible without plants as a bioregenerative life support system, that generate oxygen, take up carbon dioxide and purify water. "If you do the calculations, with our current technology, we would have trouble putting enough supplies on a space craft to support humans on a Mars mission," Boss told Reactive Reports.

One intriguing question that remains unanswered is, "Why would plants have a signaling pathway similar to brain cells?" Boss asked.