Differential expression of the brassinosteroid receptor-encoding BRI1 gene in Arabidopsis thaliana

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Brassinosteroids (BRs), which were recognized as plant hormones only toward the end of the 1990s, are essential regulators of several important functions during plant development. This hormone group of polyhydroxylated steroids can control seed germination, seedling photomorphogenesis, the elongation and division of cells during tissue differentiation, the initiation and formation of reproductive organs, as well as resistance to various environmental stress factors. Intense studies of several leading laboratories uncovered the pathways and regulatory mechanisms of BR biosynthesis, and also clarified in detail the signaling route that leads from hormone perception of the to specific responses at the level of gene expression. Due to these results, by now BRs became one of the best characterized groups among plant hormones. Functional analyses of BR insensitive Arabidopsis mutants revealed that BRs are perceived by BRI1 BRASSINOSTEROID I NSENSITIVE 1), a leucine-rich repeat receptorlike kinase. This cytoplasmic membrane-localized receptor interacts with the hormone ligand and initiates the intracellular signalization that leads to the hormone esponse. BR binding by the extracellular domain of the receptor results in confo rmational changes that activate the intracellular kinase domain. This kinase activity then generates phosphorylation/dephosphorylation-based signaling steps, which ultimately cause the activation and nuclear accumulation of the transcription factors that mediate BR-responsive gene expression. Based on initial studies it was proposed that plants do not regulate the spatial or temporal expression and distribution of BRI1, thus differential hormone responses were attributed mainly to changes in the BR levels. This simple model, however, was challenged by contrasting data of other research teams, which implied that BR sensitivity varies between organs and can be altered by light conditions. As some earlier results of our group led to similar conclusion, we set out to investigate in detail the expression properties of the BRI1 gene in the model plant Arabidopsis thaliana. We chose this single-copy gene from among those that encode BR signaling components because its receptor product interacts with the hormone and initiates intracellular phosphorylation, making BRI1 is a key element in the signalization process. In addition to wild type Arabidopsis (ecotype Col-0), we also used its BR receptor-deficient bri1-101 mutant, as well as our newly generated transgenic lines expressing fusion products with the bacterial β-glucuronidase (GUS) and firefly luciferase (LUC) reporters, which allowed easy monitoring of BRI1 transcription and of receptor accumulation. The main results of our studies can be assessed as follows: (1) Using transgenic plants that harbored promoter-reporte r gene fusions we determined the organ-specific and developmental patterns of BRI1 gene activity. We demonstrated that in the elongation zone of the hypocotyls the upregulati on of transcription results in the accumulation of BRI1 mRNA, and also of its receptor protein product. (2) With time-course measurements we detected and characterized a diurnal pattern in BRI1 promoter activity, identifying light signaling and circadian control as the main determinants of this periodic daily regulation. (3) We constructed transgenic plants in which BRI1 was replaced by a BRI1-LUC fusion that retained its receptor function, allowing the direct in vivo monitoring of receptor distribution. (4) Ectopic expression of the BRI1-LUC transgene resulted in plants with disproportionate organ development. This highlighted the requirement of properly controlled receptor expression for coordinating BR-dependent morphogenic funct ions. Our data provided evidence for the differential spatial and temporal control of BRI1 gene expression, which well coincides with the localization and timing of BR-requiring physiological processes. We found that, in addition to the o rgan- and developmental stagespecific regulation, BRI1 promoter activity also depends on light conditions. As a further layer of expressional modulation, literature data also indicate hormonal adjustment by auxin and BRs. This compl ex, multi-level control of BRI1 is quite similar to that seen in the case of those genes that code for the key enzymes of BR biosynthesis, indicating the possibility of fine coordination between hormone levels and susceptibility. Ac cordingly, our data suggest that differential expression and distribution of the BRI1 receptor can be an important means of fine-tuning BR sensitivity and, as a result, ensuring proper plant development.

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