Pollen Tube Growth and Guidance Is Regulated by POP2, an Arabidopsis Gene that Controls GABA Levels

Abstract

During angiosperm reproduction, pollen grains form a tube that navigates through female tissues to the micropyle, delivering sperm to the egg; the signals that mediate this process are poorly understood. Here, we describe a role for gamma-amino butyric acid (GABA) in pollen tube growth and guidance. In vitro, GABA stimulates pollen tube growth, although vast excesses are inhibitory. The Arabidopsis POP2 gene encodes a transaminase that degrades GABA and contributes to the formation of a gradient leading up to the micropyle. pop2 flowers accumulate GABA, and the growth of many pop2 pollen tubes is arrested, consistent with their in vitro GABA hypersensitivity. Some pop2 tubes continue to grow toward ovules, yet they are misguided, presumably because they target ectopic GABA on the ovule surface. Interestingly, wild-type tubes exhibit normal growth and guidance in pop2 pistils, perhaps by degrading excess GABA and sharpening the gradient leading to the micropyle.

The COMATOSE ATP-Binding Cassette Transporter Is Required for Full Fertility in Arabidopsis

“…Succinate content was reduced in flowers of both cts-1 and kat2-1, but not in cts-2. Succinic semialdehyde, which is highly reactive, could not be detected, in agreement with a previous report (Palanivelu et al, 2003).…”

“…Because GABA is believed to be required for correct pollen tube growth and targeting (Palanivelu et al, 2003), we tested the hypothesis that CTS and, by extension, b-oxidation, might play a role in GABA catabolism, thereby assisting the generation of a GABA gradient in reproductive tissue, which permits optimal fertility. GABA and GABA shunt metabolites (Fig.…”

“…A defect in the first step of GABA catabolism in the pop2-1 mutant leads to GABA hypersensitivity of pollen tube growth (Palanivelu et al, 2003;Fig. 6A).…”

“…Pollen tube growth and guidance are separable genetically , but the identities of diffusible signals directing pollen tubes to the ovules and ultimately permitting entry to the micropyle have remained elusive until recently (Johnson and Preuss, 2002;Higashiyama et al, 2003;McCormick and Yang, 2005). However, recent studies have implied that g-aminobutyric acid (GABA) and NO are directional signals participating in pollen tube guidance in Arabidopsis (Palanivelu et al, 2003;Prado et al, 2004).…”

Deciphering the Possible Mechanism of GABA in Tobacco Pollen Tube Growth and Guidance

“…5 Palanivelu et al revealed that GABA also plays a critical role in pollen tube growth and orientation. 6 However, whether it acts via a pathway similar to that in animal cells by binding to specific receptors and thereby triggering downstream signaling, such as the Ca 2+ /CAM pathway, is still unknown. Corresponding genes of GABA receptors have not yet been cloned, and the Arabidopsis gene bank contains no sequences homologous to the animal counterparts.…”