EDS5 : A potential transporter of SA from the chloroplast to the cytosol
Author: Shailesh Kumar
Division of Plant Physiology, IARI, New Delhi
The word salicylic acid (SA) was derived from a latin word "salix" meaning willow tree and the name was given by Rafacle Piria, an Italian chemist in 1938. It is ubiquitously distributed in the whole plant kingdom and present in many plant species at various levels. In model plant Tobacco and Arabidopsis, which contain low basal levels of SA (less than 100 ng g-1 fresh weight), however, rice has basal SA levels (5000-30 000 ng g-1 fresh weight). Salicylic acid is considered to be a potent plant hormone because of its diverse regulatory roles in plant metabolism. Salicylic acid is an endogenous plant growth regulator of phenolic nature that possesses an aromatic ring with a hydroxyl group or its functional derivative. Salicylic acid has been found to play a key role in the regulation of plant growth, development, interaction with other organisms and in the responses to environmental stresses. SA, in particular, influences seed germination, seedling establishment, cell growth, respiration, stomatal closure, senescence-associated gene expression, responses to abiotic stresses, basal thermotolerance, nodulation in legumes, and fruit yield. Its effect on some of these processes may be indirect because SA alters the synthesis of and/or signaling by other plant hormones including jasmonic acid ( JA), ethylene (ET), and auxin. Salicylic acid is considered to be an important signalling molecule which is involved in local and endemic disease resistance in plants in response to various pathogenic attacks. Besides providing disease resistance to the plants, SA can modulate plant responses to a wide range of oxidative stresses.Genes regulate SA-mediated defense can be viewed in three types. Type I genes are responsible for SA biosynthesis, with SID2 contributing to the major SA production and Type II genes encode protein products that do not act directly as SA biosynthetic enzymes. Alternatively, they can affect SA stability, sequestration, transport, and/or conjugation. Among the known type II SA genes, EDS5 plays a major role in regulating SA accumulation. Other components also partially affect SA levels. Type III genes include NPR1 as the main SA signal transducer and other signal transducers independent of NPR1.
ENHANCED DISEASE SUSCEPTIBILITY5 (EDS5) a gene encodes a protein with a predicted series of nine to 11 membrane-spanning domains and a coil domain at the N terminus. EDS5 is homologous with members of the MATE (multidrug and toxin extrusion) transporter family. EDS5 expression is very low in unstressed plants and strongly induced by pathogens and UV-C light. The biological importance of SA was demonstrated using mutant or transgenic plants impaired in the accumulation of SA. For example, Arabidopsis (Arabidopsis thaliana) ENHANCED DISEASE SUSCEPTIBILITY5 (eds5)/SA INDUCTION-DEFICIENT1 (sid1) and ics1/sid2 mutants impaired in pathogen- and UV-induced SA accumulation are susceptible to Pseudomonas syringae; plants degrading SA upon expression of the bacterial enzyme naphthalene hydroxylase G (NahG) fail to accumulate SA and SA-dependent defense responses. A mutation in the ISOCHORISMATE SYNTHASE1 (ICS1) gene unveiled the essential role of isochorismate in the synthesis of SA
When plants are invaded by microorganisms (e.g. bacteria, fungi) or exposed to abiotic stress (UV light), an increase in the level of SA is observed. In plants, SA biosynthesis involves two steps: firstly, chorismate is converted to isochorismate by isochorismate synthase (ICS) and secondly, isochorismate is processed to SA by a yet unknown enzyme(s). Two isochorismate synthase genes, ICS1 and ICS2, are present in the genome of Arabidopsis. Majority of SA biosynthesis is driven by ICS1. Transient expression analysis of ICS1 and ICS2 fused to GFP showed that both enzymes are localized in the chloroplasts, suggesting that SA synthesized in chloroplasts. SA biosynthesis is negatively regulated by auto inhibitory feedback at ICS1. This raises the question of how SA is exported to the cytoplasm, from where it transits to its binding site in the nucleus. The eds5/sid1 mutation was mapped to EDS5, a gene encoding a member of the multidrug and toxin extrusion (MATE) transporter family. MATE transporters are found in prokaryotes and eukaryotes and represent conserved protein families. The kingdom of plants has the largest number of MATE genes; for example, there are 58 MATE genes in Arabidopsis. They are often H+/cation antiporters associated with the extrusion of secondary or toxic metabolites. In some cases, anions such as citrate can also be transported. Genetic studies indicated that the multidrug and toxin extrusion transporter ENHANCED DISEASE SUSCEPTIBILITY5 (EDS5) of Arabidopsis is necessary for SA accumulation after biotic and abiotic stress, recently it is shown that EDS5 colocalizes with a marker of the chloroplast envelope and that EDS5 functions as a multidrug and toxin extrusion-like transporter in the export of SA from the chloroplast to the cytoplasm in Arabidopsis, where it controls the innate immune response. In the eds5 mutant, stress-induced SA is trapped in the chloroplast and inhibits its own accumulation by auto inhibitory feedback. Now it is experimentally tested that EDS5 is localized at the chloroplast envelope membrane and catalyzes the export of SA from the chloroplast to the cytoplasm. Therefore EDS5 plays a major role in regulating SA accumulation.
- Yamasaki K, et al. Chloroplast envelope localization of EDS5, an essential factor for salicylic acid biosynthesis in Arabidopsis thaliana. Plant Signal Behav 2013;8
- Serrano M, et al. Export of salicylic acid from the chloroplast requires the multidrug and toxin extrusion-like transporter EDS5. Plant Physiol 2013; 162: 1815-182
- Hayat Q, Hayat S, Irfan M, Ahmad A. Effect of exogenous salicylic acid under changing environment. A review. Environ Exp Bot 2010;68:14-25
About Author / Additional Info:
An enthuiastic author from India