In you or me, the path from fertilized egg to mature embryo is relatively smooth and predictable. By contrast, plant embryonic development is fitted with a safety catch: seeds can remain dormant until a combination of intrinsic and extrinsic factors triggers them to germinate. Environmental cues, which might be temperature or moisture, are well documented, as are the intrinsic signals, such as the plant hormone gibberellin (GA), with which they interact in the seed. But until now, a direct molecular connection between the two had not been found. Reporting in Genes and Development, Lee and colleagues have found that the Arabidopsis RGL2 gene — which encodes a putative transcription factor — acts at the interface between imbibition (water uptake by the seed) and signalling by GA.

The germination of Arabidopsis seeds begins when, under favourable moisture conditions, GA reaches a critical level. How GA regulates germination is not understood, but genetic analysis indicates that neither GA nor the proteins that mediate GA signalling directly link moisture to germination. Reasoning that the crucial integrator probably resembles these proteins, Lee et al. focused on several uncharacterized genes that encode functional domains related to those in some GA signalling molecules. Genetic epistasis analyses with the gene RGL2 — using null alleles that were recovered from a transposon integration database — were particularly striking. Mutations in RGL2 completely restored germination to plants that were genetically or chemically deficient for GA signalling. This implies that RGL2 is a negative regulator of GA responses — an effect that the authors found was specific to germination.

But how do moisture, GA signalling and RGL2 interact? A clue to this came from looking at RGL2 expression: in the embryonic shoot of wild-type Arabidopsis, levels of RGL2 mRNA rise on imbibition, then fall as the embryonic shoot breaks through the seed coat. RGL2's expression is therefore more or less directly induced by moisture. When GA is not around, RGL2 transcript levels stay high — although, significantly, they fall if GA is administered. By fitting this, and other information, into a model, Lee et al. propose that RGL2 levels rise on imbibition, blocking germination until levels of GA are high enough to bring RGL2 levels back down.

This work describes the first direct molecular link between an environmental and an endogenous cue in seed germination. More genes and more studies will no doubt complete the picture, but this remains a successful example of how gene functions can be assigned in this 'post-genomic' organism.