The improvements in techniques to collect pollen in adequate amounts and of proteome approaches facilitating polypeptide identification using a minimum of material have allowed researchers to engage in large-scale proteomic analyses of mature pollen. These analyses have been performed on the two reference plants Arabidopsis and O. sativa, since bioinformatic data from both of these species enable convenient protein identifications. So far, a total of 438 distinct polypeptides have been identified from mature pollen [3, 17, 18]. The majority (more than 60%) of these proteins are allocated to five main functional categories: metabolism, energy generation, protein processing, cytoskeleton dynamics and cell-wall remodeling, and cellular transport and signaling. These data confirm at the protein level that during maturation, pollen grains in flowering plants accumulate a large set of proteins dedicated to further biological processes. In complement to previous transcriptional analyses of Arabidopsis pollen [4, 5, 9, 23], these proteomic reports show that mature pollen stores, at least in part, the protein machinery that is needed for germination, as soon as the required environmental conditions are satisfied, and for further pollen tube elongation and guidance. Despite the presently incomplete description of the mature pollen proteome, a significant list of proteins is now available and can contribute to the understanding of the biochemistry of the pollen grain. A further consideration will be to shed light on the temporal and spatial coordination of this protein network in the pollen grain itself and then during its intimate interaction with the cells and tissues of the pistil. Furthermore, a relevant proportion of polypeptides identified in mature pollen is still of unclassified function. These proteins provide a basis for the use of reverse genetics to identify novel biological functions in plants by taking advantage of the reduced complexity of the pollen system.
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