In a review article entitled “Seed Dormancy and Germination” published in the February issue of Current Opinion in Plant Biology, Maarten Koornneef, Leonie Bentsink and Henk Hilhorst discuss current research on the regulation of seed dormancy and germination in angiosperms. The authors focus on the importance of plant hormones in controlling dormancy and germination. They also describe the use of modern molecular techniques such as QTL (quantitative trait loci) mapping and transcriptome and proteome analyses for identification of crucial regulatory genes.
The authors begin by discussing the application of QTL mapping to identification of genes involved in seed dormancy or germination. Genetic variation for these traits exists in both wild and cultivated populations. This is advantageous, as the identity and relative contribution of individual QTLs can be determined by genetic analysis. Recombinant inbred lines and lines derived from crosses between wild type and cultivated strains have been used to map QTLs for seed dormancy in a number of species, including Arabidopsis. However, the molecular identity of seed dormancy QTL has not yet been reported.
The authors then describe the roles of hormones in regulation of seed dormancy and germination, focusing on data derived from analysis of hormone synthesis or response mutants. Abscisic acid (ABA) plays a critical role in seed dormancy, acting as a positive regulator of dormancy. For example, rice mutants affected in enzymes involved in ABA biosynthesis show a viviparous phenotype in which seeds sprout while still associated with maternal plants. In tobacco, overexpression of zeaxanthin epoxidase, an enzyme involved in ABA biosynthesis, caused increased seed dormancy. Mutants with reduced expression of this gene, in contrast, have reduced seed dormancy.
Other hormones in addition to ABA also regulate seed dormancy or germination. In contrast to ABA, gibberellic acid (GA) is required for seed germination. Various ethylene response mutants show increased sensitivity to ABA, indicating that ethylene may inhibit ABA action and thus suppress seed dormancy. Recent studies also indicate that sugar signaling and ethylene signaling may interact strongly during seed germination and early growth. Thus, multiple regulatory pathways may influence these processes.
Other mutants that have been isolated due to seed maturation defects may have general developmental arrest. Because of differences in timing of premature seed maturation, the authors suggest that LEC1 and FUS1regulate germination in a manner distinct from that of other genes involved in ABA-controlled seed dormancy, such as ABI3. Recent studies using the brassinosteroid mutants det2 and bri have also shown that brassinosteroids facilitate germination, but are not an absolutely required.
Two mutants have been described that may regulate germination potential by additional mechanisms. The sleepy1 (sly1) mutant of Arabidopsis, was identified as a suppressor of abi1-1, and thus appears similar to the GA auxotrophs. However, application of GA to sly1 does not restore germination capability, and thus the affected protein may be involved in perception of GA. Another mutant, comatose (cts), does not respond to the application of gibberellin, although the effect appears to be seed-specific as adult plant morphology is not characteristic of GA deficiency. This suggests that the enzyme encoded by this gene may be involved in GA signaling in the seed.
Other mutants have been described that affect the seed coat or other maternal tissues. The authors describe mutants that are affected in the structural integrity of the seed coat (testa), which normally constrains radical emergence and thus germination. Another mutant, DAG1 (Dof affecting germination), is affected in a DNA transcription factor and shows reduced seed dormancy. The trait is inherited maternally, and expression of the wild type allele is limited to vascular cells arising from maternal tissue.
The authors describe particular genes involved in seed dormancy and germination that have been identified not from forward genetic screens, but rather on the basis of their expression patterns. Expression profiling approaches may either utilize unbiased searches for seed-specific genes or biased analyses of seed regulatory gene candidates. The authors describe two genes recently discovered using the latter method: one encoding a 3-beta-hydroxylase controlling gibberellin biosynthesis in a seed-specific manner, and the other encoding a dormancy-specific NADP+ phosphatase. The NADP+ phosphatase is expressed more highly in dormant seeds than non-dormant seeds.
Gene expression profiling has shown that certain genes are expressed during both late embryo development and germination. In an effort to identify genes encoding enzymes regulating these processes, mRNA profiles of immature siliques of abi3 fus3 and wild type plants have been compared. A number of genes encoding metabolic enzymes, ribosomal proteins, and regulatory proteins have been correlated to seed germination status.
Two genes products involved in control of germination have been identified in tomato and tobacco. In these species, enzymatic weakening of the endosperm cap facilitate emergence of the radicle. The authors describe two genes encoding enzymes involved in this process: expansin and endo-beta-mannanase, both of which are specifically expressed in the endosperm cap of tomato.
Promoter trapping has also been used to identify genes with seed-specific expression patterns. One such gene, AtEPR, encodes an extensin-like protein. This gene is expressed in the endosperm during seed germination and has been shown to be under control of GAs.
The authors describe the application of microarray technology to identify genes involved in seed dormancy and germination. Proteomic approaches have also sought to identify seed-specific proteins. In one proteomic study, out of 1300 proteins identified from seeds, 74 were found to change in abundance during either the imbibition- or the radicle protrusion phase. Some novel proteins not previously thought to be involved in these processes were also identified.
In conclusion, the authors point out that seed dormancy and germination are processes that involve a great number of genes and that are influenced by environmental and developmental factors. Both maternal and embryonic factors are critical during these processes. Genetic analysis has indicated the important role of ABA in seed dormancy and the requirement of GAs for germination. The authors express the hope that additional genes will be identified using molecular methods such as QTL mapping, genomics and proteomics. The functional analysis of novel genes, some of which may be involved in regulatory mechanisms independent of ABA and GA control, will enable an understanding of the entire processes of seed dormancy and seed germination.