PLANT REPRODUCTION GABA Guides the Pollen Tube

Plant fertilization involves the delivery of the sperm from the pollen, which lands at the top of the flower, to the ovule by the directed growth of the pollen tube. Palanivelu et al. determined that POP2, a gene implicated in pollination by genetic analysis, encodes a -aminobutyric acid (GABA) transaminase. In Arabidopsis, POP2 mutant plants have a 100-fold increase in the concentration of GABA in flowers compared with wild-type flowers. Analysis of POP2 mutants showed that the severity of the seed phenotype correlated with the GABA concentration. Analysis of wild-type flowers showed that GABA exists in a gradient--lowest at the peripheral tissues, the stigma, and highest in the cells surrounding the micropyle (integument cells). The micropyle is the place where the pollen tube enters the ovule. Fertilization of wild-type or POP2-mutant females by wild-type or mutant pollen showed that only one, either the male or female, needed to be wild type in order for proper pollen tube guidance and seed production. If mutant flowers were self-pollinated, the pollen tubes did not navigate properly, and seed generation was poor. In POP2-mutant females, a gradient of GABA still persists at the micropyle; however, GABA is more abundant in other parts of the flower. The authors suggest that, when pollinated by wild-type pollen, the POP2 GABA transaminase in the pollen tube can enhance the gradient and allow fertilization, whereas POP2-mutant pollen cannot create an adequate gradient. Surprisingly, in vitro or in vivo pollen tubes do not grow toward an exogenous GABA source. In vitro, pollen tubes do elongate in low concentrations of GABA, but do not elongate in high concentrations of GABA. The results suggest that, in plants, a combination of nonuniform GABA synthesis and controlled degradation serve to create a gradient that helps guide the pollen tube to the ovule.

R. Palanivelu, L. Brass, A. F. Edlund, D. Preuss, Pollen tube growth and guidance is regulated by POP2, an Arabidopsis gene that controls GABA levels. Cell 114, 47-59 (2003). [Online Journal]