Arabinogalactan proteins: actors or spectators during abiotic and biotic stress in plants?

Arabinogalactan proteins (AGPs) are a family of hydroxyproline-rich glycoproteins (HRGP) ubiquitous in the plant kingdom. They are probably one of the most heterogeneous and complex families of macromolecules, making them able to perform different and multiple functions. Located at the plasma membrane-cell wall interface, AGPs are involved in several processes, from plant growth and development to reproduction. An additional function of AGPs in response to biotic and abiotic stress has been suggested by several studies. The purpose of this review is to summarize critically and analytically the available knowledge on the effects of abiotic stress (low and high temperatures, drought, flooding, anoxia and metal deficiency/toxicity) and biotic stress (bacteria, fungi, nematodes and viruses) on AGPs. A deeper understanding of the role of AGPs during these conditions can be an important tool for understanding AGP biology and for the possible development of efficient breeding strategies.