Yariv binding of plant arabinogalactan proteins induces wound-like transcript accumulation and cell wall deposition

Plant arabinogalactan proteins (AGPs) are proteoglycans in the plant cell surface. AGPs are expressed in all plant species examined to date. In Arabidopsis, at least 47 genes encoding AGPs have been identified. The large AGP gene family and the ubiquitous distribution of AGPs among plant species suggest that AGPs have important roles in plant physiology. AGPs have been suggested to be involved in many plant cell activities. A chemical, Yariv phenylglycoside, can specifically bind to AGPs and perturb AGP functions. We used this synthetic chemical to treat Arabidopsis cell cultures. Aggregation of AGPs by Yariv phenylglycoside induces plant cell death in some cases, while in other cases the effects of the chemical are reversible. Ultrastructural studies showed that AGP aggregation induced intracellular vesiculation and cell wall deposition. DNA microarray analysis of transcript accumulation suggested that AGP aggregation induces a wound-like response. The appearance of cell wall deposits and the wound-like transcript accumulation suggested that the composition of the cell wall deposits may resemble the composition of the wound plugs. Histochemical studies showed that callose and AGPs are abundant in the cell wall deposits in the Yariv-treated Arabidopsis and rose cells. Further chemical purification of the cell wall deposits and chemical analysis of carbohydrates showed that callose, AGPs, arabinan, and other polysaccharides are accumulated in the cell wall deposits. The large amount of callose production prompted us to study which callose synthase is specifically activated by AGP aggregation. One callose synthase mutant, pmr4, showed significantly reduced callose production in response to Yariv treatment. Taken together, these studies show that plant cell surface AGP aggregation triggers wound-like responses, including cell wall thickening, callose synthesis, and accumulation of certain transcripts.