Getting to the bottom of potato tuberization
How do plant leaves talk to roots and vice versa? Not a trivial question - it's a matter of how a plant coordinates what's happening above ground to what's happening underground. Plant leaves sense the environment and, having perceived appropriate conditions, transmit signals underground to roots, stolons and rhizomes.
Iowa State Horticulture professor David Hannapel has found in his studies that the signal is an RNA, a messenger RNA. That would have been unheard of ten years ago. Messenger RNA was thought to stay within a cell slavishly carrying information from the nucleus to cytoplasm.
Using the potato as a model, Hannapel has shown that shorter days cue leaves to export RNA down the vascular system of the plant, signaling stolons to tuberize. The results of this study are described in The Plant Cell, "Dynamics of a Mobile RNA of Potato Involved in a Long-Distance Signaling Pathway," December 22, 2006.
Hannapel's study showed that a particular RNA that encodes a protein called BEL 5 is signaled to move by short days. This was demonstrated by growing plants under both long-day and short-day conditions and measuring the accumulation of BEL 5 throughout the plant. Under short days, higher levels of BEL 5 RNA were found at base than at the top of the plant, and concentrations increased through the stem to the stolon tip indicating that the BEL 5 RNA had traveled. The concentration levels were the reverse for plants grown under long day conditions.
Experiments also showed that untranslated ends of the BEL 5 RNAs contain the travel information. What Hannapel calls "chaperone" proteins bind to untranslated regions of the RNA and carry it to other regions of the plant.
"When we looked at plants with just the coding sequence, the BEL 5 RNA wouldn't move in response to short days. But when we looked at them with the untranslated regions it did move in response to short days," Hannapel said. He added that the photoperiod seems to regulate the process.
Increased movements of BEL 5 RNA toward the bottom of the plant were correlated with enhanced tuber production - overexpression of BEL 5 consistently resulted in shorter growing periods and higher yields. A higher yielding potato could be a future application of this work and would be particularly useful in developing countries. In the meantime, Hannapel said, this model system will help scientists to better understand the process of tuberization and the role of mobile RNAs in development. Hannapel's research has been supported by the Plant Sciences Institute and the National Science Foundation.
