A review on the function of arabinogalactan-proteins during pollen grain development

Overview of the current understanding of PG development, PT growth and the role of AGPs in these processes. The pollen grain (PG) is a complex structure composed of three cells: the vegetative cell which develops into a pollen tube (PT) and two sperm cells that will fuse with the egg cell and central cell, giving rise to the embryo and endosperm, respectively. This resilient gametophyte is constantly subjected to selective pressures, leading to a diverse range of characteristics, with one of its defining features being the pollen cell wall. In this review, we closely examine the developmental stages of PG formation and PT growth, with a specific focus on the dynamic roles of arabinogalactan-proteins (AGPs) throughout these processes. AGPs are initially present in pollen mother cells and persist throughout PT growth. In the early stages, AGPs play a crucial role in primexine anchoring, followed by nexine and intine formation as well as cellulose deposition, thereby providing essential structural support to the PG. As PGs mature, AGPs continue to be essential, as their absence often leads to the collapse of PGs before they reach full maturity. Moreover, the absence of AGPs during PT growth leads to abnormal growth patterns, likely due to disruptions of cellulose, callose, and F-actin deposition, as well as perturbations in calcium ion (Ca²⁺) signalling. Understanding the intricate interplay between AGPs and PG development sheds light on the underlying mechanisms that drive reproductive success and highlights the indispensable role of AGPs in ensuring the integrity and functionality of PGs.