Homeostasis of cell wall damps and role in the growth-defense trade-off

Oligosaccharides derived from plant cell walls may activate plant immunity and behave as typical endogenous elicitors referred to as “damage-associated molecular patterns” (DAMPs). They may also behave as regulators of growth and development, for example by antagonizing the action of hormones such as auxin and leading to negative effects on growth and development. Due to the antithetic effect on immunity and growth, the concentrations, activity, time of formation and localization of DAMPs is critical for the so-called “growth-defense trade-off.” Moreover, like in animals, over accumulation of DAMPs in plants provokes deleterious physiological effects and may cause hyper-immunity if the cellular mechanisms controlling their homeostasis fail. Hyper-immunity is characterized by reduced growth and, sometimes, extensive cellular death that resembles that of the typical hypersensitive response. Recently, a mechanism has been discovered that controls the activity of two well-known cell wall DAMPs, i.e. the oligogalacturonides (OGs) released upon hydrolysis of homogalacturonan (HG), and the cellodextrins (CDs), released upon hydrolysis of cellulose. The potential homeostatic mechanism involves specific oxidases belonging to the family of berberine bridge enzyme-like (BBE-like) proteins. Oxidation of OGs and CDs not only inactivates their DAMP activity, but also makes them a significantly less desirable food source for some microbial pathogens. The oxidative inactivation of OGs and CDs may be part of a general strategy employed by plants to control the activity of DAMPs, and the possibility exists of discovering additional inactivating enzymes targeting other DAMP molecules. As a co-product of the enzymatic oxidation of OGs and CDs, H2O2 is formed. The accumulation of this reactive oxygen species is expected to occur locally. It can be argued that the enzymatic oxidation of the reducing ends of wall polysaccharides by the BBE-like oxidases is a mechanism for a strictly controlled and localized production of limited amounts of H2O2. This may occur where breaks are made by cell wall-degrading enzymes upon injuries or upon a localized loss of cell wall integrity during growth and development, and therefore often only in one or few cells, acting, for example, as a signal for a very short-range cell-cell communication. A balanced activation of transduction and metabolic pathways and a complex cross talk among the hormones and between hormones and other regulatory signals sustain the growth defense trade off. For example, auxin and salicylic acid may influence developmental processes and, at the same time, regulate immune defenses triggered by DAMPs. DAMP oxidases can be considered in the broadest sense as DAMP suppressors that maintain a balanced level of signals in plants. The identification and characterization of such suppressors may have significant biotechnological applications to overcome the limitations imposed by the growth/defense trade-off and by an excessive accumulation of DAMPs causing a deleterious hyper-immunity.