Manipulating splicing variants of SIK1 enhances both biomass yield and salt tolerance
The Hippo pathway established in Drosophila melanogaster is a key regulator of animal organ size. The Arabidopsis SIK1 is a Hippo orthologue promoting mitotic exit for organ size control. We note the alternative splicing in the first intron of SIK1, with two splicing variants SIK1.1 and SIK1.2. Transcripts of both were detected in Arabidopsis, and the level of SIK1.1 is tens of times that of SIK1.2. SIK1.1 and SIK1.2 have the same expression pattern and subcellular localization. SIK1.1 is only 27 amino acids more than SIK1.2, we found these 27 amino acids determine the structure of the ATP-binding pocket of the kinase, and confirmed by kinase assay and ectopic expression in Drosophila. sik1/SIK1.1 lines expressing only SIK1.1 but not SIK1.2 could complement the sik1 mutant and exceed WT. sik1/SIK1.2 was similar to the sik1 mutant. SIK1.2 inhibits the function of SIK1.1 by inhibiting its phosphorylation of its substrate MYC2. sik1/SIK1.1 lines have the advantages of increased biomass, decreased root-to-shoot ratio, shortened growth cycle, increased yield and enhanced salt tolerance. It may be possible to adjust the ratio of active SIK1 kinase to inactive suppressor to produce new varieties of good quality crops.