Chen et al identify AGL15 as a direct target of HSI2-dependent transcriptional repression in Arabidopsis seedlings https://doi.org/10.1105/tpc.17.00655.
By Naichong Chen and Randy Allen
Background: The developmental transition from seeds to seedlings is a critical step in the plant life cycle. After germination, carbohydrates, fats, and proteins that accumulate during seed maturation are used to provide nutrition for early seedling growth. In the model plant Arabidopsis thaliana, a group of master transcription activators (LEC1, ABI3, FUS3, and LEC2, abbreviated LAFL) are known to promote the expression of genes required for seed maturation. However, after germination, LAFL gene expression is turned off to prevent the expression of seed maturation genes and allow for seedling development. Since mutant Arabidopsis seedlings that lack a transcription silencing factor called HSI2 have increased LAFL gene expression, it is apparent that HSI2 is required to silence LAFL genes in seedlings.
Questions: We asked if HSI2 turns off LAFL gene expression by binding to the genes directly or if it acts indirectly by repressing other regulatory genes that activate LAFL gene expression. We also tested which parts of the HSI2 protein are required for its gene silencing function and searched for other protein factors that interact with HSI2.
Findings: Chromatin immunoprecipitation (ChIP) assays showed that HSI2 does not interact with the LAFL genes in Arabidopsis seedlings, indicating that its effect on the expression of these genes is indirect. Yet, HSI2 interacts with AGL15, which encodes a protein that activates LAFL gene expression to promote seed maturation. Accordingly, HSI2 directly silences AGL15 expression in seedlings, leading to the secondary down-regulation of LAFL gene expression. Our results also show that critical DNA- and histone-binding domains of HSI2 are required for its interaction with specific sequence elements near the AGL15 gene. HSI2 recruits other gene silencing factors to the AGL15 gene, including MSI1, a component of the conserved polycomb repressive complex 2, which is responsible for repressing gene expression by modifying histones, a type of DNA-packaging protein.
Next steps: To better understand the regulatory functions of HSI2, our ongoing research is aimed at identifying additional genes that are directly silenced by HSI2 or the closely related protein HSL1. We are also evaluating the complex of transcriptional silencing proteins associated with HSI2 at target genes.
Naichong Chen, Vijaykumar Veerappan, Haggag Abdelmageed, Miyoung Kang, and Randy D. Allen. (2018). HSI2/VAL1 silences AGL15 to regulate the developmental transition from seed maturation to vegetative growth in Arabidopsis. Plant Cell. Published February 2018. DOI: https://doi.org/10.1105/tpc.17.00655.