A new discovery about the interaction between plant hormones and light signals could have implications for food crop production. Until very recently, it was thought that light signals switch certain hormones on or off and thus control growth and development in plants. But biologists at the Carnegie Institution for Science now have found that, instead, the hormones dictate how sensitive each plant is to light. The hormone in question is called brassinosteroid, found throughout the plant kingdom, and the new study focuses on its effect on a protein called GATA2, which tells developing seedlings whether to grow as in darkness (when they are under the soil) or as in light (once they push above ground). The researchers found that GATA2 switches on many genes that are turned on by light but turned off by brassinosteroid. They then showed that brassinosteroid inhibits the production of GATA2 and that light stabilizes the presence of GATA2 protein in plant cells. First, the team members demonstrated that GATA2 is a major promoter of light-type growth. They showed that in the presence of light, it turns on select plant growth genes. Arabidopsis plants genetically manipulated to overproduce GATA2 demonstrated growth patterns of plants growing in light, even when they were in darkness. When plants were genetically manipulated to be brassinosteroid-deficient, they exhibited the same reaction, which shows that both GATA2 protein and brassinosteroid have antagonistic effects on developing plants. The team then showed that brassinosteroid actually turns on a protein that inhibits GATA2 when the seedling is in the dark, and exposure to light stops this inhibition. This is likely due to the involvement of yet another protein, according to the researchers, but further study is needed. The results, in combination, show how important GATA2 is for signaling light-type growth. It also is a communications junction between internal plant systems that are turned on by light and those that are turned on by brassinosteroids. Both the hormones and light act as antagonistic regulators of GATA2 and thus affect the creation of proteins as stimulated by GATA2. The team’s findings on interactions between brassinosteroid and light in sprouting seedlings have changed the prevailing model for understanding the relationship between light conditions and hormone signals in regulating photosynthesis and growth. Their results were published Dec. 14, 2010, in Developmental Cell.