THE SELF-INCOMPATIBILITY RESPONSE IN PAPAVER-RHOEAS IS MEDIATED BY CYTOSOLIC-FREE CALCIUM

The role of Ca2+ signalling during the self-incompatibility (SI) response in Papaver rhoeas L. has been investigated using Ca2+-sensitive dyes. Pollen tubes were micro-injected with Calcium Green-1 and cytosolic free calcium ([Ca2+]i) imaged using laser scanning confocal microscopy (LSCM). Addition of incompatible stigmatic S-glycoproteins induced a transient increase in the level of [Ca2+]i in pollen tubes. In contrast, no rise in [Ca2+]i was detectable after addition of either compatible or heat-denatured incompatible stigmatic S-glycoproteins. The elevation of [Ca2+]i was followed by the specific inhibition of pollen tube growth in incompatible reactions. It has been shown previously that gene expression in pollen tubes is switched on during an incompatible reaction. Since the [Ca2+]i transient appeared to originate from the region where the nuclei are located, Ca2+ may be involved in locally regulating the expression of these genes. The photoactivation of caged Ca2+ to artificially elevate [Ca2+]i resulted in the inhibition of pollen tube growth and thus mimicked the SI response. Taken together, the results provide an important link between a transient rise in [Ca2+]i and the biological phenomenon of inhibition of pollen tube growth and demonstrate, for the first time, direct evidence that the SI response in P. rhoeas is mediated by [Ca2+]i.