Publikationen - Molekulare Signalverarbeitung

0

Lipoxygenases (LOXs) are members of a large enzyme family that catalyze oxygenation of free polyunsaturated fatty acids into diverse hydroperoxide compounds, collectively called oxylipins. Although LOXs have been well studied in dicot species, reports of the genes encoding these enzymes are scarce for monocots, especially maize. Herein, we reported the cloning, characterization and molecular functional analysis of a novel maize LOX gene, ZmLOX6. The ZmLOX6 nucleotide sequence encodes a deduced translation product of 892 amino acids. Phylogenetic analysis showed that ZmLOX6 is distantly related to previously reported 9- or 13-LOXs from maize and other plant species, including rice and Arabidopsis. Although sequence prediction suggested cytoplasmic localization of this protein, ZmLOX6 protein has been reportedly isolated from mesophyll cell chloroplasts, emphasizing the unique features of this protein. Plastidial localization was confirmed by chloroplast uptake experiments with the in vitro translated protein. Analysis of recombinant protein revealed that ZmLOX6 has lost fatty acid hydroperoxide forming activity but 13-LOX-derived fatty acid hydroperoxides were cleaved into odd-chain ω-oxo fatty acids and as yet not identified C5-compound. In line with its reported abundance in mesophyll cells, ZmLOX6 was predominantly expressed in leaf tissue. Northern blot analysis demonstrated that ZmLOX6 was induced by jasmonic acid, but repressed by abscisic acid, salicylic acid and ethylene and was not responsive to wounding or insects. Further, this gene was strongly induced by the fungal pathogen Cochliobolus carbonum during compatible interactions, suggesting that ZmLOX6 may contribute to susceptibility to this pathogen. The potential involvement of ZmLOX6 in maize interactions with pathogens is discussed.

0

Autoregulatory mechanisms have been reported in the rhizobial and the mycorrhizal symbiosis. Autoregulation means that already existing nodules or an existing root colonization by an arbuscular mycorrhizal fungus systemically suppress subsequent nodule formation/root colonization in other parts of the root system. Mutants of some legumes lost their ability to autoregulate the nodule number and thus display a supernodulating phenotype. On studying the effect of pre-inoculation of one side of a split-root system with an arbuscular mycorrhizal fungus on subsequent mycorrhization in the second side of the split-root system of a wild-type soybean (Glycine max L.) cv. Bragg and its supernodulating mutant nts1007, we observed a clear suppressional effect in the wild-type, whereas further root colonization in the split-root system of the mutant nts1007 was not suppressed. These data strongly indicate that the mechanisms involved in supernodulation also affect mycorrhization and support the hypothesis that the autoregulation in the rhizobial and the mycorrhizal symbiosis is controlled in a similar manner. The accumulation patterns of the plant hormones IAA, ABA and Jasmonic acid (JA) in non-inoculated control plants and split-root systems of inoculated plants with one mycorrhizal side of the split-root system and one non-mycorrhizal side, indicate an involvement of IAA in the autoregulation of mycorrhization. Mycorrhizal colonization of soybeans also resulted in a strong induction of ABA and JA levels, but on the basis of our data the role of these two phytohormones in mycorrhizal autoregulation is questionable.

Mycorrhizas are by far the most frequent occurring beneficial symbiotic interactions between plants and fungi. Species in >80% of extant plant families are capable of establishing an arbuscular mycorrhiza (AM). In relation to the development of the symbiosis the first molecular modifications are those associated with plant defense responses, which seem to be locally suppressed to levels compatible with symbiotic interaction (Gianinazzi-Pearson, 1996). AM symbiosis can, however, reduce root disease caused by several soil-borne pathogens. The mechanisms underlying this protective effect are still not well understood. In plants, products of the enzyme lipoxygenase (LOX) and the corresponding downstream enzymes, collectively named LOX pathway (Fig. 1B), are involved in wound healing, pest resistance, and signaling, or they have antimicrobial and antifungal activity (Feussner and Wasternack, 2002). The central reaction in this pathway is catalyzed by LOXs leading to formation of either 9- or 13-hydroperoxy octadeca(di/trien)oic acids (9/13-HPO(D/T); Brash, 1999). Thus LOXs may be divided into 9- and 13-LOXs (Fig. 1A). Seven different reaction branches within this pathway can use these hydroperoxy polyenoic fatty acids (PUFAs) leading to (i) keto PUFAs by a LOX; (ii) epoxy hydroxy-fatty acids by an epoxy alcohol synthase (EAS); (iii) octadecanoids and jasmonates via allene oxide synthase (AOS); (iv) leaf aldehydes and leaf alcohols via fatty acid hydroperoxide lyase (HPL); (v) hydroxy PUFAs (reductase); (vi) divinyl ether PUFAs via divinyl ether synthase (DES); and (vii) epoxy- or dihydrodiolPUFAs via peroxygenase (PDX; Feussner and Wasternack, 2002). AOS, HPL and DES belong to one subfamily of P450-containing enzymes, the CYP74 family (Feussner and Wasternack, 2002). Here, the involvement of this CYP74 enzyme family in mycorrhizal roots of M. truncatula during early stages of AM symbiosis formation was analyzed.

The jasmonic acid (JA)-dependent regulation of the Thi2.1 gene had previously been exploited for setting up a genetic screen for the isolation of signal transduction mutants of Arabidopsis thaliana (L.) Heynh. that constitutively express the thionin gene. Several cet mutants had been isolated which showed a constitutive expression of the thionin gene. These cet mutants, except for one, also showed spontaneous leaf cell necrosis and were up-regulated in the expression of the PR1 gene, reactions often associated with the systemic acquired resistance (SAR) pathway. Four of these cet mutants, cet1, cet2, cet3 and cet4.1 were crossed with the fad triple and coi1 mutants that are blocked at two steps within the JA-dependent signaling pathway, and with transgenic NahG plants that are deficient in salicylic acid (SA) and are unable to activate SAR. Analysis of the various double-mutant lines revealed that the four cet genes act within a signaling cascade at or prior to branch points from which not only JA-dependent signals but also SA-dependent signaling and cell death pathways diverge.

Jasmonic acid (JA) is an ubiquitously occurring plant growth regulator which functions as a signal of developmentally or environmentally regulated expression of various genes thereby contributing to the defense status of plants [1–5]. The formation of jasmonates in a lipid‐based signalling pathway via octadecanoids seems to be a common principle for many plant species to express wound‐ and stressinduced genes [4, 5].There are various octadecanoid‐derived signals [3]. Among them, jasmonic acid and its amino acid conjugates are most active in barley, supporting arguments that β‐oxidation is an essential step in lipid‐based JA mediated responses. Furthermore, among derivatives of 12‐oxophytodienoic acid (PDA) carrying varying length of the carboxylic acid side‐chain, only those with a straight number of carbon atoms are able to induce JA responsive genes in barley leaves after treatment with these compounds. Barley leaves stressed by treatment with sorbitol solutions exhibit mainly an endogenous rise of JA and JA amino acid conjugates suggesting that both of them are stress signals. Data on organ‐ and tissue‐specific JA‐responsive gene expression will be presented and discussed in terms of “JA as a master switch” among various lipid‐derived signals.

0

In plants, the oxylipin pathway gives rise to several oxygenated fatty acid derivatives such as hydroxy- and keto fatty acids as well as volatile aldehydes and cyclic compounds, which are, in part, physiologically active [1]. Among these, jasmonic acid is discussed as signalling molecule during several stress responses, wounding, senescense and plant pathogen interactions [2].