IDENTIFICATION OF GENES REQUIRED FOR POLLEN-STIGMA RECOGNITION IN ARABIDOPSIS-THALIANA

In higher plants, cell-cell recognition reactions taking place followi ng pollination allow the selective restriction of self-pollination and /or interspecific pollination. Many of these systems function by regul ating the process of water transfer from the cells found at the stigma tic surface to the individual pollen grain. Interspecific pollination studies on the cruciferous weed Arabidopsis thaliana revealed only a b road specificity of pollen recognition such that pollen from all teste d members of the crucifer family were recognized, whereas pollen from almost all other species failed to hydrate. Genetic analysis of A. tha liana has identified three genes that are essential for this recogniti on process. Recessive mutations in any of these genes result in male s terility due to the production of pollen grains that fail to hydrate w hen placed on the stigma, but that are capable of hydrating and growin g a pollen tube in vitro. Results from mixed pollination experiments s uggest that the mutant pollen grains specifically lack a functional po llen-stigma recognition system. All three mutations described also res ult in a defect in the wax layer normally found on stems and leaves, s imilar to previously described eceriferum (cer) mutations. Genetic com plementation and mapping experiments demonstrated that the newly ident ified mutants are allelic to the previously identified genes cer1, cer 3 and cer6. TEM analysis of the ultrastructure of the pollen coating r evealed that all of the mutant pollen grains bear coatings of normal t hickness and that tryphine lipid droplets are missing in cer1-147, are reduced in size in cer6-2654 and appear normal in cer3-2186. Temperat ure shift experiments revealed that the block in the recognition step of the mutant pollen grains can be suppressed by pollination at lower temperatures but not by reduced temperatures during pollen development . These results suggest that the lipids which are altered in the cer m utations may be important in regulating some biophysical property of t he pollen coating.