Sugar signalling during germination and early seedling establishment in Arabidopsis

Sugars have pronounced effects on many plant processes like gene expression, germination and early seedling development. Several screens for sugar insensitive mutants were performed to identify genes involved in sugar response pathways using the model plant Arabidopsis. These include sun, gin and sis screens explained earlier in this chapter. Both common mutants and dissimilar mutants were identified in the different screens. Interestingly, ABA and ABI4 seem to play an essential function in all these sugar response assays. To gain further insight in sugar signalling we studied sugar response pathways encompassing germination and early seedling establishment. We focussed our research on the sugar-regulated expression of PC in dark-grown seedlings, the glucose-induced delay of germination and the sugar-induced early seedling developmental block (gin and sis response). In Chapter 2 (and partly this Chapter) we describe that sugars delay Arabidopsis seed germination. Already low concentrations of glucose are able to delay seed germination and this delay is independent from osmotic signalling. Analysing mutants with a gin phenotype like ctr1, abi4 and abi5 shows that these mutants respond like wt to glucose with respect of the delay of germination. This indicates that the delay of germination and the gin response are two different processes. Since additional studies that show that stratification of seeds on sugar-free media suppresses the glucose-induced delay of germination but not the glucose-induced early seedling development arrest supports this conclusion. In Chapter 3 we propose a function for ABI3 (and ABI2) in sugar responses as well. We found that ABI3, ABI4 and ABI5 levels rise in seedlings that are challenged with glucose. abi3 mutants are insensitive for a range of sugar assays and abi3 mutants show a severely reduced induction of ABI4 and ABI5 after sugar treatment. These results suggests that ABA and sugar signalling (gin pathway) are more alike than assumed before. It has been hypothesized that elevated sugar concentrations induce ABA signalling by increasing ABA levels. Despite the large overlap in signalling components, analysis of a glucose and ABA insensitive mutant over expressing ABI4 (abi5-1/35S::ABI4) revealed that glucose and ABA are two distinct signals. In Chapter 4 we investigated to sun response pathway in more detail. Thus far, only one sun mutant, sun6/abi4, has been identified. We tested other sugar and ABA signalling mutants for their sun response. The other way around, sun mutants were tested for their gin response as well. Remarkably, ABA deficient mutants, which are strong gin mutants, do not show a significant sun phenotype. Thus, ABA is important for the gin response pathway but not in the sun pathway that regulates PC levels in response to sucrose. In Chapter 5 the results of the preceding chapters are summarized and discussed. The similarities and differences between the sugar response pathways studied are presented