Plant biology

Smoke Signals

Sci. Signal.  13 Mar 2012:
Vol. 5, Issue 215, pp. ec76
DOI: 10.1126/scisignal.2003031

Strigolactones and karrikins are structurally similar butenolides that elicit tissue- and stage-specific growth responses in plants. Whereas strigolactones are endogenously produced hormones that inhibit shoot branching and stimulate branching of mycorrhizal fungi, karrikins are produced by burning plants and aid in revegetation after wildfires by promoting seed germination and improving the responsiveness of seedlings to light. The F-box protein MAX2 is a component of the SCF ubiquitin ligase complex and is required for responses to both classes of butenolides, but how plants distinguish between these compounds is not yet clear. Waters et al. report that two members of the DWARF14 family of α/β hydrolases are differentially required for strigolactone and karrikin responses in Arabidopsis thaliana. Mutational analysis coupled with treatment of seeds and seedlings with karrikins or the synthetic strigolactone GR24 revealed that AtD14 (Arabidopsis thaliana DWARF14) mediated only strigolactone responses and that the related hydrolase KAI1 (karrikin-insensitive 1) mediated responses to both classes of compounds. Atd14 and kai2 mutants shared distinct subsets of phenotypes with max2 mutants, and genetic interaction experiments suggested that both acted upstream of MAX2 at different stages of development. Because KAI2 does not exhibit enzymatic activity in vitro, the authors propose that these hydrolases could act as ligand-binding proteins that convey signal-specific information to MAX2 rather than as enzymes that generate active signals from the compounds. kai2 mutants had impaired responses to light, a phenotype that was not seen in Atd14 or strigolactone biosynthesis mutants, and plants that are not found in fire-prone environments retain karrikin responsiveness, suggesting that, although smoke is the only reported source of karrikins, there may be endogenous karrikins that regulate germination and seedling development.

M. T. Waters, D. C. Nelson, A. Scaffidi, G. R. Flematti, Y. K. Sun, K. W. Dixon, S. M. Smith, Specialisation within the DWARF14 protein family confers distinct responses to karrikins and strigolactones in Arabidopsis. Development 139, 1285–1295 (2012). [Abstract] [Full Text]