Jasmonates: An Update on Biosynthesis, Signal Transduction and Action in Plant Stress Response, Growth and Development

Despite its importance in a variety of plant defense responses, our understanding of how jasmonic acid (JA) functions at the biochemical level is limited. Several amino acid conjugates of JA were tested for their ability to complement the JA-insensitive Arabidopsis thaliana mutant jar1-1. Unlike free JA, JA-Ile inhibited root growth in jar1-1 to the same extent as in the wild type, whereas JA-Val, JA-Leu, and JA-Phe were ineffective inhibitors in both genotypes. Thin-layer chromatography and gas chromatography–mass spectrometry (GC-MS) analysis of products produced in vitro by recombinant JAR1 demonstrated that this enzyme forms JA-amido conjugates with several amino acids, including JA-Ile. JA-Val, -Leu, -Ile, and -Phe were each quantified in Arabidopsis seedlings by GC-MS. JA-Ile was found at 29.6 pmole g−1 fresh weight (FW) in the wild type but was more than sevenfold lower in two jar1 alleles. JA-Leu, -Val, and -Phe were present at only low levels in both genotypes. Expression of wild-type JAR1 in transgenic jar1-1 plants restored sensitivity to JA and elevated JA-Ile to the same level as in the wild type. The ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) conjugated to JA was also found in plant tissue at 18.4 pmole g−1 FW. JA-ACC was determined not be an effective jasmonate root inhibitor, and surprisingly, was twofold higher in the mutants than in the wild type. This suggests that another JA-conjugating enzyme(s) is present in Arabidopsis. Synthesis of JA-ACC might provide a mechanism to coregulate the availability of JA and ACC for conversion to the active hormones JA-Ile and ethylene, respectively. We conclude that JAR1 is a JA-amino synthetase that is required to activate JA for optimal signaling in Arabidopsis. Plant hormone activation by conjugation to amino acids and the enzymes involved in their formation were previously unknown.

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The oxylipin signal jasmonic acid is activated by an enzyme that conjugates it to isoleucine in Arabidopsis.

Roots of grasses in response to iron deficiency markedly increase the release of chelating substances (;phytosiderophores') which are highly effective in solubilization of sparingly soluble inorganic Fe(III) compounds by formation of Fe(III)phytosiderophores. In barley (Hordeum vulgare L.), the rate of iron uptake from Fe(III)phytosiderophores is 100 to 1000 times faster than the rate from synthetic Fe chelates (e.g. Fe ethylenediaminetetraacetate) or microbial Fe siderophores (e.g. ferrichrome). Reduction of Fe(III) is not involved in the preferential iron uptake from Fe(III)phytosiderophores by barley. This is indicated by experiments with varied pH, addition of bicarbonate or of a strong chelator for Fe(II) (e.g. batho-phenanthrolinedisulfonate). The results indicate the existence of a specific uptake system for Fe(III)phytosiderophores in roots of barley and all other graminaceous species. In contrast to grasses, cucumber plants (Cucumis sativus L.) take up iron from Fe(III)phytosiderophores at rates similar to those from synthetic Fe chelates. Furthermore, under Fe deficiency in cucumber, increased rates of uptake of Fe(III)phytosiderophores are based on the same mechanism as for synthetic Fe chelates, namely enhanced Fe(III) reduction and chelate splitting. Two strategies are evident from the experiments for the acquisition of iron by plants under iron deficiency. Strategy I (in most nongraminaceous species) is characterized by an inducible plasma membrane-bound reductase and enhancement of H(+) release. Strategy II (in grasses) is characterized by enhanced release of phytosiderophores and by a highly specific uptake system for Fe(III)phytosiderophores. Strategy II seems to have several ecological advantages over Strategy I such as solubilization of sparingly soluble inorganic Fe(III) compounds in the rhizosphere, and less inhibition by high pH. The principal differences in the two strategies have to be taken into account in screening methods for resistance to ;lime chlorosis'.

Evidence for a specific uptake system for iron phytosiderophores in roots of grasses.

Hormone-triggered activation of the jasmonate signaling pathway in Arabidopsis thaliana requires SCFCOI1-mediated proteasome degradation of JAZ repressors. (−)-JA-L-Ile is the proposed bioactive hormone, and SCFCOI1 is its likely receptor. We found that the biological activity of (−)-JA-L-Ile is unexpectedly low compared to coronatine and the synthetic isomer (+)-JA-L-Ile, which suggests that the stereochemical orientation of the cyclopentanone-ring side chains greatly affects receptor binding. Detailed GC-MS and HPLC analyses showed that the (−)-JA-L-Ile preparations currently used in ligand binding studies contain small amounts of the C7 epimer (+)-7-iso-JA-L-Ile. Purification of each of these molecules demonstrated that pure (−)-JA-L-Ile is inactive and that the active hormone is (+)-7-iso-JA-L-Ile, which is also structurally more similar to coronatine. In addition, we show that pH changes promote conversion of (+)-7-iso-JA-L-Ile to the inactive (−)-JA-L-Ile form, thus providing a simple mechanism that can regulate hormone activity through epimerization.

(+)-7- iso -Jasmonoyl- L -isoleucine is the endogenous bioactive jasmonate

Higher plants differ considerably in their capability for mobilization of iron in the rhizospere of soils with low iron availability. In the plant kingdom at least two different Strategies exist in the Fe deficiency‐induced root responses which lead to enhancement of both iron mobilization in the rhizosphere and uptake rate of iron. Strategy I is found in all dicots and in monocots, with the exception of the grasses (graminaceous species, e.g. barley, corn, sorghum). Strategy I is characterized in all instances, by an increase in the activity of a plasma membrane‐bound reductase leading to enhanced rates of Fe‐III reduction and corresponding splitting of Fe‐III‐chelates at the plasma membrane. Often, the net rate of H+ extrusion, i.e. acidification of the rhizosphere, is also increased. This acidification facilitates iron uptake by both enhancement of the reductase activity and solubilization of iron in the rhizosphere. An additional mobilization of sparingly soluble iron in the rhizosphere may o...

Klaus Schreiber

Papers

Synthesis of n-(jasmonoyl)amino acid conjugates

(+)-7-Iso-jasmonic acid and related compounds from botryodiplodia theobromae

Nicotianamine, a possible phytosiderophore of general occurrence

Nicotianamine and Analogous Amino Acids, Endogenous Iron Carriers in Higher Plants

Occurrence of jasmonic acid analogues in Vicia faba