Root Secretion of Defense-related Proteins Is Development-dependent and Correlated with Flowering Time

Abstract

Proteins found in the root exudates are thought to play a role in the interactions between plants and soil organisms. To gain a better understanding of protein secretion by roots, we conducted a systematic proteomic analysis of the root exudates of Arabidopsis thaliana at different plant developmental stages. In total, we identified 111 proteins secreted by roots, the majority of which were exuded constitutively during all stages of development. However, defense-related proteins such as chitinases, glucanases, myrosinases, and others showed enhanced secretion during flowering. Defense-impaired mutants npr1-1 and NahG showed lower levels of secretion of defense proteins at flowering compared with the wild type. The flowering-defective mutants fca-1, stm-4, and co-1 showed almost undetectable levels of defense proteins in their root exudates at similar time points. In contrast, root secretions of defense-enhanced cpr5-2 mutants showed higher levels of defense proteins. The proteomics data were positively correlated with enzymatic activity assays for defense proteins and with in silico gene expression analysis of genes specifically expressed in roots of Arabidopsis. In conclusion, our results show a clear correlation between defense-related proteins secreted by roots and flowering time.The plant root system serves many roles, including anchorage and uptake of nutrients and water. The ability of roots to communicate with roots of neighboring plants and other organisms in the rhizosphere has been the focus of increasing attention (1, 2). Root secretions of secondary metabolites and volatile organic compounds have been shown to play offensive, defensive, and symbiotic roles (3-7). Likewise, the symbioses between Rhizobium species and various members of the legume family have been extensively studied and involve the initial secretion of specific flavonoids by plant roots (8 -10). Similarly, a multitrophic interaction has been uncovered, in which volatile organic compounds released by Medicago truncatula were shown to attract soil nematodes that could in turn bring Rhizobia, used as food by the nematodes, to the roots and thus facilitate the symbiotic process (11). All these interactions involve carbon-containing compounds secreted in the root exudates.Although numerous reports show changes in the protein profile of leaves and root tissues in response to wounding (12, 13), insect attack (14), fungal or oomycete infection (15-20), or the defense-related hormone jasmonic acid (21, 22), fewer reports have discussed the biological roles of proteins secreted by the roots (23-25). One recent study has shown that the composition of proteins secreted in the root exudates changes with the presence of a given microbial neighbor and that the exudation of proteins by a given bacterium is altered by the presence of a specific plant neighbor (26). These studies examined root secretion of proteins under inducible conditions, but there is no information available about the protein profiles in the root exudates under normal, nonstressed...

Environmental Metabolomics of the Tomato Plant Surface Provides Insights on Salmonella enterica Colonization

“…The release of plant secondary metabolites increases at later plant developmental stages (38). Chaparro et al (39) reported a decrease in the quantities of sugars and sugar alcohols in Arabidopsis root exudates with a concomitant increase in the quantities of phenolics and amino acids.…”

Rhizosphere microbiome assemblage is affected by plant development

“…However, there are some indirect correlations; for example, cotton (Zaki et al, 1998) or bean (Nicoli et al, 2012) plants are more susceptible to root disease (Rizoctonia solani or Fusarium root rot, respectively) at the seedling stage. Recent studies have shown that root secretion of defense-related proteins is enhanced during flowering time (De-la-Pena et al, 2010), suggesting an involvement of plant roots in ARR. Similarly, Chaparro et al (2013) have shown that Arabidopsis roots release more phenolic-related compounds at later stages of life which might be correlated to defense strategies against pathogens as secondary metabolites are involved in plant immunity against bacterial and fungal pathogens (Rogers et al, 1996;Clay et al, 2009;Millet et al, 2010;An and Mou, 2011;Bednarek, 2012).…”

Root Exudation: The Ecological Driver of Hydrocarbon Rhizoremediation

“…For example, lectins function as defense factors and recognition in symbiotic interactions [223]. During the flowering period of Arabidopsis thaliana, roots excrete more protein involved in defense, such as chitinases, glucanases and myrosinases [224]. Furthermore, protein patterns released as root exudates depend on the identity of microorganisms exposed to the roots of A. thaliana [222].…”