Abstract
Arabinogalactan proteins (AGPs) have long been considered to be implicated in several steps of the reproductive process of flowering plants. Pollen tube growth along the pistil tissues requires a multiplicity of signaling pathways to be activated and turned off precisely, at crucial timepoints, to guarantee successful fertilization and seed production. In the recent years, an outstanding effort has been made by the plant reproduction scientific community in order to better understand this process. This resulted in the discovery of a fairly substantial number of new players essential for reproduction, as well as their modes of action and interactions. Besides all the indications of AGPs involvement in reproduction, there were no convincing evidences about it. Recently, several studies came out to prove what had long been suggested about this complex family of glycoproteins. AGPs consist of a large family of hydroxyproline-rich proteins, predicted to be anchored to the plasma membrane and extremely rich in sugars. These two last characteristics always made them perfect candidates to be involved in signaling mechanisms, in several plant developmental processes. New findings finally relate AGPs to concrete functions in plant reproduction. In this review, it is intended not only to describe how different molecules and signaling pathways are functioning to achieve fertilization, but also to integrate the recent discoveries about AGPs along this process.
Highlights
Arabinogalactan proteins (AGPs) have long been considered to be implicated in several steps of the reproductive process of flowering plants
AGPs are composed primarily of carbohydrates surrounding a small proteinaceous core (Seifert and Roberts, 2007). These glycoproteins are suggested to play important functions in plants as signaling molecules given their particular characteristics: (1) GPI-anchor- which is a possible mode of release of these molecules to the extracellular medium where they may exert their functions, similar to what happens with the glycoprotein SKU5 (Sedbrook et al, 2002); (2) the high diversity of glycans decorating AGPs are strong candidates to be released by proper enzymes and act as signaling molecules themselves
In these review we describe some of the already known molecules involved in plant reproduction, but we integrate AGPs findings along the different phases of this process
Summary
Arabinogalactan proteins (AGPs) have long been considered to be implicated in several steps of the reproductive process of flowering plants. Coimbra et al (2007) using several anti-AGP antibodies, identified AGPspecific sugar epitopes in the functional megaspore and in the female gametophyte: synergid cells and in the synergid filiform apparatus of Arabidopsis, confirming the previous immunolocalization studies and the concept of differential expression of AGPs in plant reproductive tissues.
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