Plant hormones

Abscisic acid and auxin signal transduction pathways

We were the first to show that protons function as second messengers in abscisic acid (ABA), jasmonic acid (JA) and auxin signal transduction. For example, ABA and JA raise cytosolic pHc and due to the sensitivity of K+ channels to pHc, stomata will close. Conversely, auxins lower pHc, K⁺ channels are differently affected, and stomata open.

We are studying the mechanisms by which the hormones regulate pHc and the role of pHc in hormone-induced gene regulation.

The auxin binding protein (ABP) is thought to bind auxin in the cell wall, causing ABP to activate a putative transmembrane protein (“docking protein”) which induces the appropriate signal transduction pathway. Using ABP-specific antibodies, we have shown extracellular ABP is involved in auxin-induced stomatal opening.

Most recently we have been down-regulating ABP expression at various stages of development in Arabidopsis thaliana and examining the effects on phenotype.

We are using protoplasts to examine the ABA, JA and auxin signal transduction pathways, in particular the mechanisms underlying pHc regulation.

Gene regulation by ABA and cold

The dehydrin XERO2 is thought to protect certain plant tissue against abiotic stresses, in particular cold. The gene is induced approximately 10-fold by ABA and 70-fold by cold. Separate signalling pathways appear to be involved.

Using an insertion mutant we are studying XERO2 function.

We have ascertained which DNA sequences in the XERO2 gene promoter are involved in ABA and cold activation. Regulation is complex, involving both activators and repressors and we have proposed an enhancesome model. We are presently identifying the proteins that bind to the regulatory elements in the promoter.