Annexin 1 regulates the H 2 O 2 ‐induced calcium signature in A rabidopsis thaliana roots

Hydrogen peroxide is the most stable of the reactive oxygen species ( ROS ) and is a regulator of development, immunity and adaptation to stress. It frequently acts by elevating cytosolic free C a 2+ ([ C a 2+ ] cyt ) as a second messenger, with activation of plasma membrane C a 2+ ‐permeable influx channels as a fundamental part of this process. At the genetic level, to date only the C a 2 + ‐permeable S telar K + Outward Rectifier ( SKOR ) channel has been identified as being responsive to hydrogen peroxide. We show here that the ROS ‐regulated C a 2+ transport protein Annexin 1 in A rabidopsis thaliana ( A t ANN 1) is involved in regulating the root epidermal [ C a 2+ ] cyt response to stress levels of extracellular hydrogen peroxide. Peroxide‐stimulated [ C a 2+ ] cyt elevation (determined using aequorin luminometry) was aberrant in roots and root epidermal protoplasts of the A tann1 knockout mutant. Similarly, peroxide‐stimulated net C a 2+ influx and K + efflux were aberrant in A tann1 root mature epidermis, determined using extracellular vibrating ion‐selective microelectrodes. Peroxide induction of GSTU 1 ( G lutathione‐ S ‐ T ransferase1 T au 1 ), which is known to be [ C a 2+ ] cyt ‐dependent was impaired in mutant roots, consistent with a lesion in signalling. Expression of A t ANN 1 in roots was suppressed by peroxide, consistent with the need to restrict further C a 2+ influx. Differential regulation of annexin expression was evident, with At ANN 2 down‐regulation but up‐regulation of A t ANN 3 and A t ANN 4 . Overall the results point to involvement of A t ANN 1 in shaping the root peroxide‐induced [ C a 2+ ] cyt signature and downstream signalling.