Scientists at the CBGP, in a study led by Dr. Luis Oñate-Sánchez, have identified two important proteins (homeodomain transcription factors) that regulate gene expression only in the epidermis and upon perception of the hormone gibberellin. An article published in the Plant Cell describes this mechanism required for seed germination.
Gibberellins (GAs) are key plant hormones important for plant growth and development whose levels are modulated in response to developmental and environmental cues. The plant seed is an excellent system to study GA signalling (molecular events occurring upon GA perception) since GA is indispensable for seed germination. Mechanistically, the ability of the seed to germinate, results from a balance between a physical restriction imposed by the embryo-surrounding tissues and the ability of the embryo to grow and protrude. GAs favor germination by promoting the weakening of seed coat layers and embryo growth. Scientists at the CBGP have demonstrated that GA signalling in the epidermis is required for proper germination and characterized the molecular mechanisms underlying this regulation. The identity of the target genes being regulated suggest that this mechanism is coordinating cell elongation between epidermis and inner layers to allow organized growth of the embryo.
Results from this research have been published in the prestigious journal The Plant Cell and have been coordinated by Dr. Luis Oñate-Sánchez at the GBGP (UPM-INIA). The authors have studied GA signalling by focusing on a lipase gene (LIP1) expressed in the epidermis during seed germination and induced by GA and repressed by DELLAs. DELLAs are nuclear proteins that negatively regulate GA signalling through interaction with other transcription factors (TFs) blocking their ability to bind DNA. When GA is synthesized by the plant, DELLA proteins are targeted for degradation and inhibition imposed by these repressors on their interacting TFs is released, alleviating GA responses.
An L1-box DNA sequence present in the LIP1 promoter, and in the promoters of other epidermis-specific genes, has been identified responsible for their expression in response to GA. Moreover, the L1 sequence is overrepresented in the promoters of genes induced by GA and repressed by DELLAs, indicating that it is a DNA sequence commonly used to respond to the hormone. The L1-box sequence is bound by two homeodomain TFs (ATML1 and PDF2) that are specifically expressed in the epidermis and interact with DELLA proteins. Reduction of ATML1 and PDF2 levels or blocking GA signalling only in the epidermis, cause a reduction in the expression of epidermal genes resulting in delayed seed germination.
Cells in the epidermis can restrict or promote embryo growth. Several of the ATML1/PDF2 epidermal target genes encode enzymes that catalyze the first step in the elongation of very-long-chain fatty acids (VLCFA), compounds that influence organ growth (cell elongation) by restricting cell division in inner tissues. Loosening cell walls is also required for growth, and expansins play a major role in this process. Expansin 8, an important gene for cell wall remodeling, is another ATML1/PDF2 target gene.
In light of these results the authors have proposed the following regulatory model: In the absence of GA, DELLA proteins sequester ATML1/PDF2, thereby blocking GA signaling in the epidermis. The GA produced upon seed imbibition destabilizes the interaction between the DELLA proteins and ATML1/PDF2, leading to activation of L1-box gene expression and downstream events involved in elongation of epidermal cells and coordination of epidermal growth with inner tissues.