Seed germination is a precisely controlled process, involving multiple regulatory pathways. Cytokinin (CK) has been proposed to negatively regulate light-dependent seed germination in A. thaliana. Thus, the overall aim of the current study was to provide an in-depth analysis of the role of CK in seed germination induced by red (R) and far-red (FR) light. In this work, the seed germination phenotype of various CK mutant- and transgenic plants in low and very low fluence R and FR light conditions was studied. Germination rates of CK mutants and transgenic lines were significantly higher than wild-type germination rates, underpinning the negative impact of CK on seed germination in low fluence R light as well as in very low fluence FR light. This study further identified increased germination rates compared to the wild type in seeds (i) impaired in CK biosynthesis, (ii) with an increased CK catabolism, (iii) with a reduced signal perception at the level of histidine kinase receptors (AHKs) and (iv) seeds impaired in CK signal transduction via histidine phosphotransfer proteins (AHPs). Consequently, the signaling components in the respective mutants may well contribute to the repressive effects of CK on germination. A putative role of signaling components downstream of AHPs, such as A- and B-type response regulators (ARRs) in very low fluence germination need further clarification. The photoreceptor phytochrome A (phyA) is an essential part of the regulatory pathway controlling the onset of germination in very low fluence FR light. Germination assays analyzing CK biosynthesis- and CK receptor mutants lacking functional phyA led to the conclusion that phyA is essential for the induction of germination in very low fluence FR light both in the wild type and in CK mutants. Quantification of phyA protein levels did not confirm a repressive effect of CK on phyA abundance in seeds. To dissect the hormonal pathways which may influence germination of CK mutant seeds in very low fluence FR light conditions, the contribution of abscisic acid (ABA) and gibberellins (GA) was analyzed into more detail. Hormone measurements indicated neither elevated GA level nor decreased ABA level in CK deficient seeds, suggesting a CK-independent regulation of bioactive GA- and ABA levels in imbibed seeds. Interestingly, CK negatively influenced GA sensitivity, which may be an additional mechanism for CK to suppress germination. However, CK had no measurable effect on ABA sensitivity. A multitude of maternal effects are known to shape the germination response of the offspring. The present thesis revealed, that a lower CK status in maternal seed tissues led to increased germination rates of the respective seeds in FR light. However, a reduction of the CK status exclusively in the testa or the endosperm was not sufficient to increase germination rates significantly in FR light compared to wild-type seeds. These results exemplify the prominent role of CK as a negative regulator of germination in seed tissues with a higher maternal genome dosage. In the present thesis, also the gene regulatory network underlying the negative effects of CK on the germination processes in FR light was studied. Although the seeds' CK status had only a minor effect on transcriptomic changes during imbibition, a major reprogramming of the transcriptome during FR light-induced germination dependent on the seeds' CK status was evident. Overrepresented GO categories revealed that lipid-associated, seed maturation-associated and cell wall organization-associated transcripts were differentially regulated in ahk2 ahk3 seeds in response to FR light. These results indicate that the aforementioned pathways might be relevant for the negative impact of CK on seed germination in very low fluence light. Additionally, environmental factors such as the light environment of parental plants during seed development affect the germination phenotype of their offspring. This thesis demonstrated, that growth of parental plants in shade light conditions (enriched in FR light) did not affect the germination response of their F1 offspring in FR light. However, when parental plants were grown for two subsequent generations in shaded conditions, germination rates of F2 seeds were increased in very low fluence FR light conditions. These effects were independent of the seeds' CK status. Since CK is a prominent regulator of seed size, in the last part of this work the connection between CK, seed size and seed age in FR light-induced germination was investigated. The current study found smaller seeds to germinate better when germination was induced by FR light, again this effect was similar in wild-type seeds and seeds with a reduced CK signal transduction. Regarding seed age, the repressive effect of CK on germination in FR light was retained in aged seeds during long-term storage. Overall, the presented results improve the understanding how seed germination in non-optimal low and very low fluence light conditions is regulated by CK. CK exerts a negative influence on germination, which is not dependent on altered GA or ABA hormone levels, but seems to involve seed tissues with a higher maternal genome dosage.
