Introduction

The male gametophyte (or pollen) in higher plants is a key mediator of sexual reproduction. The last two decades have been marked by increasing efforts to decipher the genetic and molecular basis of pollen development and functions. Many of the biochemical events that occur in the vegetative cell of the pollen grain prior to anthesis (pollen maturation) are related to the preparedness of the gametophyte for the exigencies of germination and growth of the pollen tube in the pistil. In most plants, to achieve this considerable growth the pollen grain accumulates and stores large amounts of both mRNAs and proteins [1]. At present, many questions still remain. Which mRNAs are stored in mature pollen grains, and when are these mRNAs translated into proteins? Are there pre-synthesized proteins in mature pollen grains which are required for pollen germination and pollen tube elongation [1, 2]? If so, what are the proteins involved in signaling during pollen development or during fertilization [3]? Indeed, until recently, little biochemical and molecular genetic information was available about pollen functions. Recent studies have notably broken this bottleneck by performing transcriptomics analyses from pollen of the model plant A. thaliana

Plant Proteomics: Technologies, Strategies, and Applications. Edited by G. K. Agrawal and R. Rakwal Copyright © 2008 John Wiley & Sons, Inc.

[4, 5]. Global gene expression analysis is in fact useful for selecting candidates for functional studies. Nevertheless, a gene transcript in eukaryotic cells usually gives rise to multiple protein isoforms, possibly with different functions, after alternative splicing and/or essential PTMs [6]. Therefore proteomic studies have been acknowledged as essential in unraveling the biological complexity of the pollen grain and for obtaining a better understanding of pollen functions.

18.2 BIBLIOGRAPHIC REVIEW: POLLEN DEVELOPMENT AND FUNCTION

Pollen—The Male Gametophyte

In flowering plants (angiosperms), development of the male gametophyte is a well-programmed and elaborate process. Male sporogenesis begins with the division of a diploid sporophytic cell, giving rise to tapetal initials and sporogenic cells. Figure 18.1 illustrates the subsequent events leading to the formation of a pollen grain. These sporogenic cells then undergo several mitoses to differentiate into pollen mother cells. Subsequently, each diploid pollen mother cell forms a callose-encased tetrad of hap-loid microspores by two consecutive meiotic divisions. The haploid cells of the tetrad are released as free microspores by the action of an enzyme (callase) produced by the tapetum layer of the anther [7]. Each uninucleate microspore then undergoes an asymmetric mitotic division forming a large vegetative cell and a smaller generative cell entirely enclosed within the vegetative cell (i.e., bicellular pollen stage). In nearly 70% of flowering plants (e.g., Solanaceae, Liliaceae), the pollen grain is shed from the anther at the bicellular pollen stage; the second mitotic division of the generative cell, which gives rise to two sperm cells (i.e., tricellular pollen stage), occurs while the pollen tube grows through the female pistil. In the remaining plant families (e.g., Cruciferae, Graminae), this second division occurs before anther dehiscence proceeds [8].

When the pollen is released from the anther, it exists as a free organism until it is carried by wind, insects, or other agents to the stigma of the pistil, where it starts a new phase of its life cycle. At this stage of development, if the pistil is compatible, the vegetative cell controls the germination of the mature pollen grain and growth of the pollen tube into the style. During this phase of development, there is an intimate interaction between the pollen tube and the cells and tissues of the pistil. Finally, sperm cell nuclei are delivered to the embryo sac in the ovule, where they participate in double fertilization [1].

The Mature Pollen Grain: An Attractive Biological Model System

During the last few decades, the increased interest in understanding pollen biology has been primarily related to its obligatory function in plant reproduction. However, the extremely reduced haploid male plant (male gametophyte) of flowering plants also constitutes a very attractive biological model system. The pollen corresponds to

FIGURE 18.1. Schematic presentation of microsporogenesis typical of most angiosperms. (From McCormick [7], Copyright American Society of Plant Biologists.)

a homogeneous tissue type (only three cells) with a defined and rather small tran-scriptome and thus, very likely, proteome also. For instance, a total of 13,977 male gametophyte-expressed mRNAs have been identified from the different pollen stages of A. thaliana [9], which correspond to only around 40% of the full genome of the model plant. In addition, for a majority of monocots (e.g., maize, rice) and dicots (e.g., Solanaceous species), mature pollen is able to be easily collected at dehiscence in bulk, either by shaking tassels/panicles into paper bags or by agitating flowers with a mechanical buzzer [10]. In the case of the quite small flowers of the Arabidopsis model plant, Johnson-Brousseau and McCormick [10] developed a vacuum system allowing the harvest of milligram amounts of mature pollen. The isolation of earlier stages of pollen grains (i.e., microspore and bicellular pollen) requires heavier procedures, since grains are embedded in the anthers. Nevertheless, this is a perfectly feasible possibility [9]. Consequently, in addition to its role in plant reproduction, the pollen grain has progressively become a very useful model for studying fundamental aspects of plant biology such as cell fate determination [11], cell-cell interactions [12], or cell polarity and tip growth [13, 14]. Due to those attractive considerations, pollen constitutes an ideal experimental system for analyses of key biological processes in higher plants.

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