2023-07-31 09:54:57
Brassinosteroids (BR) are essential hormones for the growth and development of plants throughout their life cycle. They are involved, alongside other hormones such as auxin and abscisic acid, in cell signaling networks to regulate the adaptation of plants to their environment, in particular by controlling the formation of organs and the growth of tissues. . In particular, BRs play a crucial role in root growth by regulating meristem size and cell elongation. However, the mechanisms governing BR levels in cells and their differentiated distribution in the root remain poorly understood. In this article published in the journal Nature Chemical Biologyscientists have discovered a previously unknown mechanism of BR transport in the model plant Arabidopsis thaliana.
The combination of microscopy and genetic approaches resulted in a major discovery: BRs and their precursors move from cell to cell via intercellular pores called plasmodesmata, which connect the cytoplasm of neighboring cells. This transport via these communication channels that are the plasmodesmata, allows the establishment of a signaling gradient in the root, making it possible to modulate cell signaling differentially according to the position of the cells in the root. In addition, the scientists discovered that BRs are able to regulate the permeability of plasmodesmata to optimize their own mobility. Thus, a feedback mechanism is triggered in response to endogenous BR levels to control the transport rate and thus the biosynthesis of BRs depending on their cellular concentration. This process of regulating the opening and closing of plasmodesmata involves the synthesis or degradation of callose, a polymeric sugar present in the cell wall. Scientists found that increased callosis (inducing closure of plasmodesmata) reduces the amount of BES1 (a transcription factor that regulates the expression of BR-responsive genes, an indicator of signaling level) dephosphorylated and localized in the core. Conversely, the expression of a callose-degrading enzyme (inducing the opening of plasmodesmata) results in the accumulation of dephosphorylated BES1, and therefore an increase in BR signaling. To further study the interaction between callose and BRs, the researchers showed that exposure of the roots to active BRs (brassinolide) induces the deposition of callose in the plasmodesmata and decreases the molecular flux between the cells. On the other hand, the application of a BR inhibitor (brassinazole) reduces the rate of callosis, leading to a significant increase in intercellular diffusion. It is thanks to a collaboration with the France-BioImaging Bordeaux Imaging Center microscopy platform that the consortium of researchers has developed a system for measuring intercellular diffusion using two-photon imaging and a cytosolic fluorescent protein photoactivatable.
In summary, this work uncovered a previously unknown mode of steroid transport in eukaryotes, revealing that intercellular transport via plasmodesmata is essential for BR signaling in plants. These findings reinforce the idea that plasmodesmata are signaling centers that integrate hormonal, environmental and developmental signals to regulate plant development.
© Emmanuelle Bayer
Figure: Brassinosteroids, phytohormones that regulate plant organ growth, and their precursors travel from cell to cell via plasmodesmata, kinds of cytosolic bridges connecting plant cells. This movement facilitates the establishment of a concentration gradient of brassinosteroids thus controlling the division and elongation of the cells of the root.
Learn more:
Wang, Y. et al. Plasmodesmata mediate cell-to-cell transport of brassinosteroid hormones. Nat. Chem. Biol. 1–11 (2023). doi:10.1038/s41589-023-01346-x
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