The blossoms of maturity
A newly discovered signaling pathway ensures that plants remember to flower - even without positive signals from the environment.
Plants normally flower in response to seasonal changes, such as those associated with the end of winter or beginning of spring. Scientists at the Max Planck Institute for Developmental Biology have now identified a signaling pathway that allows plants to blossom even without positive signals from the environment. The concentration of a small RNA snippet in plants cells operates like an hour glass: a decline in its level awakes the plant from its vegetative dormancy and allows it to enter the reproductive mode. (Cell, August 21, 2009)
Why do some plants blossom even when days are short and gray? Scientists at the Max Planck Institute for Developmental Biology have found the answer to this question: An endogenous mechanism allows them to flower in the absence of external influences such as long days. A small piece of RNA, a so-called microRNA, has a central role in this process, as a decline of its concentration in the shoot apex triggers flowering.
MicroRNAs are very short RNA snippets that have emerged in recent years as essential regulators of gene function in both plants and animals. By binding to complementary motifs in a messenger RNA, they inhibit its translation into protein. This process thus blunts the activity of the corresponding gene.
In Tübingen, developmental biologists have discovered that the common wallcress, Arabidopsis, uses this regulatory mechanism to switch from vegetative to reproductive development. A group of related regulators, the SPL proteins, play an important role in promoting the onset of flowering. In young plants, production of SPL proteins is prevented by high levels of microRNA156. Jia-Wei Wang and colleagues demonstrate that independent of external cues, the concentration of the microRNA declines over time, like sand running through an hourglass. When the microRNA concentration falls below a certain level, enough SPL proteins are produced to activate the flowering process even in the absence of other regulators that measure day length or external temperature. This in turns allows a sufficiently old plant to flower, even in an unfavorable environment.
Interestingly, the SPLs do double duty, since they have supporting roles when plants flower in response to long days. Furthermore, both the SPLs and other regulators eventually converge on a similar set of targets crucial for flowering. "Flowering is crucial for the long-term survival of plants. The redundancy of environment-dependent and -independent mechanisms ensures that plants do not wait forever until flowering. Better flower once, then never", explains Detlef Weigel, director at the Max Planck Institute for Developmental Biology.