Functional in planta analysis of a plasma membrane-associated brassinosteroid response pathway

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Brassinosteroids (BRs) are essential hormones for plant growth and development. The generally accepted “standard” model of the BR response pathway in Arabidopsis includes the perception of BR by the plasma membrane receptor BRI1, its oligomerisation with the coreceptor BAK1 followed by the activation of a signalling cascade finally resulting in the expression of BR-responsive genes. We recently identified a fast, gene expressionindependent BR response pathway that modifies the plasma membrane potential (Em) and the expansion of the cell wall – both crucial processes preceding cell elongation growth. This signal-response pathway comprises at least BRI1 and the plasma membrane P-type ATPase. However, the molecular mechanisms underlying the pathway are not yet fully understood. Using the Em of the plasma membrane and the expansion of the cell wall as quantitative cell physiological readouts, we therefore aim to address the following questions: (i) what are the molecular determinants which enable BRI1 to activate the P-ATPases in the plasma membrane; (ii) does phosphorylation of tyrosines or serine/threonine play a role; (iii) are other elements of the “standard” BR signalling pathway and (iv) further elongation growth-promoting substances required? The majority of the proposed experiments will be carried in living plant cells at quantitative level applying novel spectro-microscopic technologies.