Controlled free radical attack in the apoplast: A hypothesis for roles of O, N and S species in regulatory and polysaccharide cleavage events during rapid abscission by Azolla

Abstract

Shedding of organs by abscission is a key terminal step in plant development and stress responses. Cell wall (CW) loosening at the abscission zone can occur through a combination chain breakage of apoplastic polysaccharides and tension release of cellulose microfibrils. Two distinctly regulated abscission cleavage events are amenable to study in small water ferns of the genus Azolla; one is a rapid abscission induced by environmental stimuli such as heat or chemicals, and the other is an ethylene-induced process occurring more slowly through the action of hydrolytic enzymes. Although free radicals are suggested to be involved in the induction of rapid root abscission, its mechanism is not fully understood. The apoplast contains peroxidases, metal-binding proteins and phenolic compounds that potentially generate free radicals from H2O2 to cleave polysaccharides in the CW and middle lamella. Effects of various thiol-reactive agents implicate the action of apoplastic peroxidases having accessible cysteine thiols in rapid abscission. The Ca2+ dependency of rapid abscission may reflect the stabilization Ca2+ confers to peroxidase structure and binding to pectin. To spur further investigation, we present a hypothetical model for small signaling molecules H2O2 and NO and their derivatives in regulating, via modification of putative protein thiols, free radical attack of apoplastic polysaccharides.

Breakdown of middle lamella pectin by ●OH during rapid abscission in Azolla

“…This is currently being studied. Recently, Cohen et al (2014) hypothesized in a review about rapid abscission that • OH generated by peroxidase and catalase is involved in this phenomenon.…”

“…; Cohen et al . ). Like many other plants, Azolla was shown to abscise its roots and branches by treatment with ethylene.…”

“…Azolla, a small water fern with a global distribution, exhibits two types of abscission, (1) ethylene-induced protracted abscission; and (2) stress-induced rapid abscission Uheda et al , 1995Uheda & Nakamura 2000;Fukuda et al 2013;Cohen et al 2014). Like many other plants, Azolla was shown to abscise its roots and branches by treatment with ethylene.…”

Oxidative Metabolism in ‘Valencia’ Sweet Orange (Citrus sinensis Osbeck) Abscission Zone Tissue Treated with the Abscission Agent 5-Chloro-3-Methyl-4-Nitro-1H-Pyrazole

“…Free radical-mediated cell wall loosening has been observed during abscission, seed germination, cell elongation, and fruit ripening (Cohen et al, 2015;Fry et al, 2001). In addition, there is a possibility of generation of highly reactive OH · in AZ through the action of PODs (Cohen et al, 2014). PODs are heme-containing enzymes and can generate OH · (Cohen et al, 2014).…”

Role of peroxidase activity and Ca2+ in axis growth during seed germination

“…POX is one of the key enzymes towards metabolism of ROS whose functional diversity let us presume and study for its role in the process of axis growth during germination. Among the various functions, Prx actively produces OH Á (in presence of H 2 O 2 and electron donors such as O 2 Á-through the hydroxylic cycle) that participates in cleavage of cell wall polymers in the apoplast required for cell extension growth (Liszkay et al 2003;Kukavica et al 2009;Cohen et al 2014). Initial axis growth i.e.…”