Ca 2+ ‐independent phosphorylation of a 68 kDa pollen protein is stimulated by the self‐incompatibility response in Papaver rhoeas

Summary The self‐incompatibility (SI) response in Papaver rhoeas involves a Ca 2+ ‐based signalling pathway, which mediates the SI‐specific inhibition of incompatible pollen. We have previously reported the identification of p26.1, a pollen protein whose phosphorylation was increased specifically as a consequence of the SI response. We have investigated whether further specific protein phosphorylation events are induced in P. rhoeas pollen. Here we report the identification of an additional pollen protein, p68, which also responds to S proteins by an increase in its phosphorylation state. This phosphorylation event occurs in living pollen tubes grown in vitro , and can be observed specifically when pollen is challenged with biologically active S proteins that are incompatible with the S alleles carried by the pollen and not when pollen was challenged with compatible S proteins. The timing of the increase in phosphorylation of p68 is temporally later than that of p26.1, occurring between 240 sec and 400 sec after challenge. This suggests that its phosphorylation is downstream of p26.1 in the SI signalling pathway(s). Surprisingly, the kinases responsible for the phosphorylation of p68 are not Ca 2+ ‐dependent. This, and the later timing of the p68 response, suggests that a ‘second wave’ of Ca 2+ ‐independent signalling may follow the initial Ca 2+ ‐dependent SI signalling. This indicates that the SI signalling pathway(s) in pollen may be quite complex.