Seed development: The hunt for the missing receptor ABAtes
 |
Exclusive to Signaling Gateway
 |
|
The Mg-chelatase H subunit (CHLH) is the elusive abscisic acid receptor (ABAR) required for seed development and stomatal aperture regulation.
Absciscic acid (ABA) is a key phytohormone required for seed development and other developmental processes in plants. A study published in Nature last year revealed the RNA-binding protein FCA as the abscisic acid receptor required for regulating flowering time, however the receptor required for seed development and stomatal aperture regulation remained elusive. Now, Shen et al. have gone further by revealing the Mg-chelatase H subunit as the missing Abscisic acid receptor (ABAR) in this setting.
 | |
The authors previously described an ABA binding protein from broad bean that was implicated in stomatal signaling. They showed this protein to be the putative H subunit (CHLH) of the magnesium protoporphyrin-IX (Proto) chelatase (Mg-chelatase). In the latest study, the authors demonstrate that purified ABAR from Arabidopsis binds ABA in a highly stereospecific manner. Co-immunoprecipitation assays revealed binding activity proportional to the starting amount of ABAR protein. In order to assess the role that this receptor plays in plant development the authors make use of antisense and overexpression lines. RNAi knockdown of ABAR results in ABA-insensitive phenotypes in seed germination and inhibition of stomatal opening, while plants overexpressing ABAR displayed ABA-hypersensitive phenotypes such as increased resistance to leaf dehydration. The authors further describe a transferred DNA (T-DNA) insertion mutant in the ABAR gene that gives a seed phenotype similar to other ABA-signaling component seed maturation phenotypes.
Mg-chelatase is composed of three subunits and catalyzes a reaction crucial to chlorophyll synthesis. CHLH is a Proto-binding protein and has a previously described role in plastid-nucleus retrograde signaling. Treatment with ABA resulted in stimulated ABAR expression and Mg-chelatase activity. ABAR RNAi knockdown reduced the mRNA level of genes that respond positively to ABA. Analysis of mutants defective in chlorophyll metabolism or plastid signaling show that ABAR is a positive regulator of ABA signaling in a pathway that is distinct from that required for chlorophyll metabolism and Mg-Proto-mediated plastid signaling.
The authors show that CHLH is expressed not only in green but also in non-green tissue, including roots, implicating functionality for ABAR at the whole plant level. Indeed ABAR RNAi downregulation reduced levels of ABA responsive regulators in leaves and seeds. While ABAR is shown to be a receptor for ABA signaling, other components of this transduction pathway remain to be determined. ABAR is evolutionarily conserved and hints at putative roles in other organisms.
Clare Garvey
Signaling Gateway
References
- Yuan-Yue Shen et al. The Mg-chelatase H subunit is an abscisic acid receptor. Nature 443, 823-826 (19 October 2006) | Article | PubMed |
 more stories
| 
