Symposium III: Advances in plant reproductive biology
Abs #
30003: GABA and Arabidopsis reproduction: Novel roles in pollen tube growth and guidance
The Arabidopsis POP2 gene encodes a transaminase that degrades g-amino butyric acid (GABA), a small molecule signal. The mutant flowers accumulate GABA, but not other GABA pathway compounds suggesting that the pop2 phenotype is a direct consequence of GABA accumulation rather than downstream deficiencies. pop2 mutants are self-sterile due to (i) reduced pollen tube elongation in the septum and (ii) misguided pollen tube growth during final stages of pollen tube growth. To understand how increased GABA levels results in these pollen tube behavior abnormalities, we performed in vitro pollen tube growth and in vivo GABA localization experiments. High GABA impairs tube growth in vitro, but lower levels stimulate migration. pop2 tubes are hypersensitive to GABA-mediated inhibition, presumably because they cannot metabolize GABA. Consequently, although pop2 tubes can successfully grow in wild type septum, elevated GABA in pop2 pistils causes reduced elongation of mutant tubes.Wild-type pistils have a steep GABA gradient between the septum and the micropyle, the pollen tube target. In mutant, despite elevated GABA levels, GABA gradient between septum and micropyle still exists. Wild-type tubes equipped with a functional POP2 enzyme continually senses the gradient by degrading exogenous GABA and thus distinguish the micropyle from rest of ovule cells, in both wild type and pop2 pistils. However, the mutant with a defective POP2 enzyme cannot sense the gradient and consequently is not properly guided to the micropyle. The cumulative effect of these growth and guidance defects in pop2 results in sterility. These results establish a novel role for GABA in pollen tube growth and guidance.
