Interaction between sugar and abscisic acid signalling during early seedling development in Arabidopsis

Bas J. W. Dekkers 0 1 Jolanda A. M. J. Schuurmans 0 1 Sjef C. M. Smeekens 0 1 0 B. J. W. Dekkers (&) De Ruiter Seeds, Leeuwenhoekweg 52, 2660 BB Bergschenhoek, The Netherlands 1 B. J. W. Dekkers J. A. M. J. Schuurmans S. C. M. Smeekens Department of Molecular Plant Physiology, Utrecht University , Padualaan 8, 3584 CH Utrecht, The Netherlands Sugars regulate important processes and affect the expression of many genes in plants. Characterization of Arabidopsis (Arabidopsis thaliana) mutants with altered sugar sensitivity revealed the function of abscisic acid (ABA) signalling in sugar responses. However, the exact interaction between sugar signalling and ABA is obscure. Therefore ABA deficient plants with constitutive ABI4 expression (aba2-1/35S::ABI4) were generated. Enhanced ABI4 expression did not rescue the glucose insensitive (gin) phenotype of aba2 seedlings indicating that other ABA regulated factors are essential as well. Interestingly, both glucose and ABA treatment of Arabidopsis seeds trigger a post-germination seedling developmental arrest. The glucose-arrested seedlings had a drought tolerant phenotype and showed glucose-induced expression of ABSCISIC ACID INSENSITIVE3 (ABI3), ABI5 and LATE EMBRYOGENESIS ABUNDANT (LEA) genes reminiscent of ABA signalling during early seedling development. ABI3 is a key regulator of the ABA-induced arrest and it is shown here that ABI3 functions in glucose signalling as well. Multiple abi3 alleles have a glucose insensitive (gin) phenotype comparable to that of other known gin mutants. Importantly, glucose-regulated gene expression is disturbed in the abi3 background. Moreover, abi3 was insensitive to sugars during germination and showed sugar insensitive (sis) and sucrose uncoupled (sun) phenotypes. Mutant analysis further identified the ABA response pathway genes ENHANCED RESPONSE TO ABA1 (ERA1) and ABI2 as intermediates in glucose signalling. Hence, three previously unidentified sugar signalling genes have been identified, showing that ABA and glucose signalling overlap to a larger extend than originally thought. - Photosynthesis provides plants with sugars that play a central role in the plant life cycle as energy sources, storage molecules, structural components or intermediates for the synthesis of other organic molecules. Next to these metabolic functions, sugars act as signalling molecules with hormone-like properties. Both hexoses and disaccharides are able to induce signalling via different pathways (Jang and Sheen 1997; Loreti et al. 2001; Rolland et al. 2006). In plants glucose has been shown to affect many processes, including germination, early seedling growth, flowering and senescence (Gibson 2000, 2005; Smeekens 2000; Rolland et al. 2006). Moreover, glucose feeding of Arabidopsis seedlings affected the expression of many genes as shown by micro array studies (Price et al. 2004; Villadsen and Smith 2004; Li et al. 2006). Sugar-induced signal transduction has been shown to control gene expression via diverse mechanisms that include transcription, translation, and modification of mRNA and protein stability (Rolland et al. 2006). Genetic analysis showed that sugar signalling in plants is closely associated with plant hormone biosynthesis and signalling, in particular with that of abscisic acid (ABA, for review see Finkelstein and Gibson 2001; Gazzarini and McCourt 2001; Leon and Sheen 2003; Rook et al. 2006; Dekkers and Smeekens 2007). Four screens for sugar response mutants i.e. sucrose uncoupled (sun), impaired sucrose induction (isi), glucose insensitive (gin) and sugar insensitive (sis) identified ABA deficient mutants (i.e. aba2/isi4/gin1/sis4 and aba3/gin5) and ABA insensitive4 (abi4/sun6/isi3/gin6/sis5) as sugar insensitive (ArenasHuertero et al. 2000; Huijser et al. 2000; Laby et al. 2000; Rook et al. 2001). ABI4 encodes an AP2 domain containing transcription factor that binds a CE1-like element present in many ABA and sugar regulated promoters (Finkelstein et al. 1998; Niu et al. 2002; Acevedo-Hernandez et al. 2005). These observations linked sugar regulation to ABA signalling. However, there are many more examples of co-regulation of sugar and ABA in plants. The expression of many genes is co-regulated by sugar and ABA. A study of Li et al. (2006) showed that 14% of the ABA upregulated genes are induced in response to glucose as well, in total nearly 100 genes. These included genes in stress responses, carbohydrate and nitrogen metabolism and signal transduction. Another group of nearly 40 genes was repressed by both ABA and glucose. In addition the authors identified a group of genes that showed a synergistic upregulation when treated with both ABA and glucose, including the ApL3 (ADP pyrophosphorylase large subunit) gene, which is involved in starch biosynthesis. An earlier study showed that ABA was not able to induce the ApL3 gene but that ABA in combination with sugar boosted transcription levels in comparison to sugar treatment alo (...truncated)