Cold Spring Harbor, N.Y. -- A key item in the developmental agenda of a plant leaf is the establishment of an axis that makes a leaf's top half distinct from its bottom half. This asymmetry is crucial for the leaf's function: it ensures that the leaf develops a flattened blade that is optimized for photosynthesis, with a top surface specialized for light harvesting and a bottom surface containing tiny pores that serve as locales for gas exchange.
For years, plant biologists have known that this top/bottom axis analogous to the front/back or "dorso-ventral" axis in animals is established by a signal derived from the meristem, the stem cell-rich growing tip of the plant from which new leaves arise. Other signals that traffic between the upper and lower sides of the leaf are thought to stably maintain this polar axis. Just as a GPS signal tells drivers where they are, these signals give cells positional information about where they are located within the leaf, causing them to acquire their correct identities by switching specific genes "on" or "off."
Associate Professor Marja Timmermans, Ph.D., and her team of scientists at Cold Spring Harbor Laboratory (CSHL) are the first group to uncover the identity of one such positional signal. In a study that appears in the March 1st issue of Genes and Development, they describe a family of mobile small RNAs that patterns the top/bottom axis in leaves. These small RNAs, they discovered, are generated on the upper surface of young leaves but traffic from this source to form a concentration gradient across each leaf. This graded distribution pattern creates discrete regions of gene activity so that cells in each half of a leaf develop a distinct "top" or "bottom" identity.
"We've known that small RNAs produced upon viral infection can move from cell to cell," explains Timmermans. "But this is the first time anyone has shown that small RNAs that are native to the organism are similarly m
|Contact: Hema Bashyam|
Cold Spring Harbor Laboratory