Two B3 domain transcriptional repressors prevent sugar-inducible expression of seed maturation genes in Arabidopsis seedlings

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Abstract
During development of plant seeds, embryos import nutrients and store massive amounts of reserves. Seed reserves are rapidly degraded and mobilized to support seedling development after germination. HIGH-LEVEL EXPRESSION OF SUGAR-INDUCIBLE GENE 2 (HSI2) of Arabidopsis thaliana is a B3 DNA-binding domain protein that represses the transcription of sugar-inducible reporter gene. Although disruption of HSI2 or HSI2-Like 1 (HSL1) did not affect growth, seeds with disruption of both HSI2 and HSL1 (KK mutant) developed abortive seedlings that stopped growing 7-9 days after imbibition. KK seedlings developed swollen hypocotyls that accumulated seed storage proteins and oil on medium containing sucrose or other metabolizable sugars, and calluses developed from KK seedlings also accumulated seed storage reserves. The expression of seed maturation genes, which include LEAFY COTYLEDON-type master regulators, in KK seedlings depended on the concentration of sucrose, suggesting that sugar controls the expression of seed maturation genes. Our results suggest that HSI2 and HSL1 repress the sugar-inducible expression of the seed maturation program in seedlings and play an essential role in regulating the transition from seed maturation to seedling growth.