Comparative analysis of distinct seed dormancy mechanisms within the genus Lepidium (cress) and the evolution of the Brassicaceae DOG1 dormancy genes
Final Report Abstract
Seed dormancy is an important component of plant fitness that causes a delay of germination until the arrival of a favourable growth season. Dormancy is a complex trait that is determined by genetic factors (including the DELAY OF GERMINATION GENE 1) with a substantial environmental influence (with ambient temperature and water availibility as major cues). The roles of the plant hormones abscisic acid (ABA) and gibberellin (GA) in the regulation of dormancy and germination in mediating environmental effects have been investigated since long. The mature seeds of most angiosperm species are endospermic and our work shows that micropylar endosperm (CAP) weakening is an important process preceeding the completion of seed germination by radicle protrusion. The completion of germination by endosperm rupture is requires the interaction of the key seed compartments CAP and RAD (lower hypocotyl/radicle axis, embryo growth zone). Our work with nondormant Lepidium sativum (garden cress, Brassicaceae) seeds demonstrates that deviations from the optimal ambient imbibition temperature (27ºC) inhibits germination by inducing cold or stress stress. Colder and warmer temperature both inhibit CAP weakening, but thr molecular and biomechanical mechanisms differ considerably between the cold and heat stress. Both inhibit CAP weakening, but only in the cold a decrease in CAP stiffness was evident. Our transcriptome analysis demonstrated that this effect was associated with enhanced expression of four genes enconding cell-wall remodeling proteins. Beyond ABA, CAP weakening is also inhibited by jasmonates and these hormones seem to be important regulators of dormancy and germination. Our work provides an integrated view into the underlying molecular mechanisms by which a plant life-cycle transition is controlled in a temperature-dependent manner by alteration of the biomechanical properties of key seed tissues regulating dormancy and germination. Overexpression of Arabidopsis thaliana DOG1 in L. sativum seeds switches them from non-dormant to dormant and overexpression of L. sativum DOG1 in the non-dormant A. thaliana dog1 mutant complements the phenotype. DOG1 determines the temperature window for germination by regulating endosperm CAP weakening gene expression through temperature control of the gibberellin metabolism. The altered expression of cell-wall remodelling protein genes at certain temperatures confers temperature-responsive control of endosperm CAP weakening and thereby determines the optimal seed germination temperature. Homologs of the seed dormancy specific gene DOG1 are widespread in the genus Lepidium. The highly dormant Lepidium papillosum is a polyploid species and possesses multiple, structurally diversified DOG1 genes, some of which are expressed in seeds during maturation prior to dormancy induction. We conducted a thorough phylogenetic analysis of the DOG1 genes to further elucidate their evolution and roles. Preliminary results for Aethionema arabicum (Brassicaceae) were successfully used to aquire to the 'SeedAdapt' project in the ERA-CAPS competition. The topic of the SeedAdapt project is "Dimorphic fruits, seeds and seedlings as adaptation mechanisms to abiotic stress in unpredictable environments" (www.seedadapt.eu). Taken together, the works conducted in this project has provided in-depths insight into novel molecular and biomechanical mechanisms underlying the dormancy and germination responses of seeds to ambient temperature.
Publications
- (2011). A guideline to family-wide comparative state-of-the-art qRT-PCR analysis exemplified with a Brassicaceae cross-species seed germination case study. The Plant Cell 23: 2045-2063
Graeber K, Linkies A, Wood ATA, Leubner-Metzger G
- (2012). Embryo growth, testa permeability, and endosperm weakening are major targets for the environmentally regulated inhibition of Lepidium sativum seed germination by myrigalone A. Journal of Experimental Botany 63: 5337-5350
Voegele A, Graeber K, Oracz K, Tarkowská D, Jacquemoud D, Turecková V, Urbanová D, Strnad M, Leubner-Metzger G
(See online at https://doi.org/10.1093/jxb/ers197) - (2012). Molecular mechanisms of seed dormancy. Review in: Plant, Cell & Environment 35: 1769-1786
Graeber K, Nakabayashi K, Miatton E, Leubner-Metzger G, Soppe WJJ
- (2013). Spatiotemporal seed development analysis provides insight into primary dormancy induction and evolution of the Lepidium DELAY OF GERMINATION1 genes. Plant Physiology 161: 1903-1917
Graeber K, Voegele A, Büttner-Mainik A, Sperber K, Mummenhoff K, Leubner-Metzger G
(See online at https://doi.org/10.1104/pp.112.213298) - (2013). Transcriptome-wide mapping of pea seed ageing reveals a pivotal role for genes related to oxidative stress and programmed cell death. PLoS ONE 8: e78471
Chen H, Osuna D, Colville L, Lorenzo O, Graeber K, Küster H, Leubner-Metzger G, Kranner I
(See online at https://doi.org/10.1371/journal.pone.0078471) - (2014). DELAY OF GERMINATION 1 mediates a conserved coat-dormancy mechanism for the temperature- and gibberellin-dependent control of seed germination. Proceedings of the National Academy of Sciences of the USA 111(34): E3571–E3580
Graeber K, Linkies A, Steinbrecher T, Mummenhoff K, Tarkowská D, Turečková V, Ignatz M, Sperber K, Voegele A, de Jong H, Urbanová T, Strnad T, Leubner-Metzger G
(See online at https://doi.org/10.1073/pnas.1403851111)
