Classical mutations involved in plant reproduction can theoretically be easily detected, because homozygous mutants obtained by self-fertilization show fertility abnormalities. Sterile mutants harbouring shorter siliques or a lower siliques number compared to wild-type are easy to screen. The study of some of these reproduction mutations has allowed important insights about pollen tube development. However, a more direct strategy to describe the gameto-phyte molecular machinery is to create and analyse true gametophytic mutants. Gametophytic mutants are affected in the developmental step of plant haploid structure, i.e., male or female gametophytes, and their screening implies specific strategies. When such mutation affects only one gametophyte, the other can ensure transmission to the progeny, but no individual ho-mozygous for the mutation can be obtained. Only heterozygous mutants are observed, producing 50% of wild-type gametophytes. However, in the case of male gametophytic mutant, half of wild-type pollen is sufficient to fertilize all the ovules, and heterozygous mutants therefore do not display a sterility phenotype. Moreover, phenotype analysis of affected tissue is complicated by the difficulties encountered in isolating gametophytes, and often needs extensive cell biology and cytology experiments. In terms of expected phenotypes on pollen tube development, a male gametophyte mutation can prevent or al ter pollen tube germination, or/and the pollen tube tip growth, and/or pollen tube guidance through the pistil, and/or gametes discharge. Finally, as pollen tubes are haploi'd cells, anomalies in their development might also correspond to mutations affecting genes that function is essential, but not specific to the progamic phase, and only the haploi'd status of the pollen makes the mutation lethal. This latter type of mutations is at least as much interesting as the others, as pollen tube offers the possibility to study specific processes in one living extending cell.
The first published pollen tube growth "mutants" were described in maize (Sprague 1933). The wx allele affected the gametophyte competitivity (estimated by measuring pollen tube length), that Sprague related to differential "establishment" performance (i.e., germination/attachment on silk). Twenty years later, the maize gametophytic factors Ga, whose ga recessive alleles lead to slower pollen tube growth, were reported by Schwartz (1950) and Nelson (1952). The level of cross-sterility was depending on the genotypes of the style (no fertilization on GaS/GaS, less fertilization on Ga/Ga). In these works, differences in pollen tube performance were deduced from the observation of distorted segregation of easy observable grains characters (e.g., colour).
Other reports of pollen tube growth mutations came from the analysis of embryo-lethal mutants. In Arabidopsis, following Ethane Methyl Sulfonate (EMS)-seed mutagenesis, Meinke (1982) observed higher rate of aborted vs wild-type seeds in the top half of the silique from two heterozygous embryo-lethal mutants. Those results were interpreted as possible evidence for slightly reduced pollen tube growth rate. Moreover, this work confirmed the suspected overlap of the sporophytic and gametophytic expression (Ottaviano et al. 1982) that has been largely corroborated in the following two decades.
In order to decipher the genetic and molecular control of pollen tube growth, as for other plant developmental processes, Arabidopsis is an attractive model: its genome size is small, the complete genomic sequence is available and numerous insertion mutant collections are accessible, permitting straightforward genetics. Moreover, cell biology of its reproductive process is very similar to that of many other plants, including crops, and is now very well described.
In this chapter, we will present the different strategies that have been developed to isolate so-called progamic mutations, affecting the pollen tube development, from germination to fertilization. We will not extend our report to self-incompatibility (SI), although works on SI brought much information on tip-growth key-signaling events (Barend et al., this volume). Similarly, the methods and tools used to analyze gametophytic mutants were recently reviewed (Johnson-Brousseau and McCormick 2004) and are discussed in Twell et al., this volume. Here we will focus on the different screening strategies that have been developed and the diversity of interesting mutant phenotypes and cloned genes that came out from these screens. We will include works that al though not aimed at studying pollen tubes, contributed also to increase the number of pollen-tube mutants.
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