The great influence of this gaseous hormone on different plant organs made it an interesting target for pathogens. Indeed, the mold Penicillium digitatum has been known to produce ethylene since the mid-1950s (Wang et al., 1962) and its production was shown for prokaryotic plant pathogens in the early 1960s (Freebairn and Buddenhagen, 1964). Not surprisingly, the pathways found in these microorganisms are not analogous to the one in plants. So far, two distinct routes for ethylene production have been described in microbes: a 2-oxoglutarate-dependent pathway and the 2-keto-

4- methyl-thiobutyric acid (KMBA) pathway (Nagahama et al., 1991,1992). The latter is the most common among microorganisms, composed of a series of chemical and enzymatic reactions, by which only trace amounts of ethylene are usually produced (Ogawa et al., 1990). The former pathway has been found to be more efficient, with 2-oxoglutarate being used as substrate in a single­step reaction by the ethylene-forming enzyme (EFE). This pathway has been found in several different microor­ganisms, including P. digitatum, Chaetomium globosum, Phycomyces nitens, Fusarium oxysporum, and in different pathovars of Pseudomonas syringae, where a comparison study found the pv. phaseolicola to be the most efficient ethylene-producing strain (Weingart et al., 1999). This enzyme catalyzes simultaneously two reactions (Fukuda et al., 1992b):

2 — oxoglutarate 4 ethylene + 3CO2 + H2O (22.2)

2 — oxoglutarate + L — arginine + O2 4 succinate +CO2 + guanidine +(S) — 1 — pyrroline — 5 —carboxylate + H2O (22.3)

These reactions are rather interesting as they keep the tricarboxylic acid (TCA) cycle closed through a shortcut, converting 2-oxoglutarate directly into succinate with the formation of ethylene "as a by-product" (Figure 22.2), and therefore substituting for the steps catalyzed by 2-oxoglutarate dehydrogenase and succinyl-CoA synthe­tase (Figure 22.2). The original two-step reaction between 2-oxoglutarate and succinate generates one NADH and one guanosine triphosphate, which are not produced by EFE. Thus, competition of the two pathways for substrate 2-oxoglutarate would lower the formation of NADH. Since NADH also has a role as an inhibitor for four enzymes associated with the TCA cycle, pyruvate dehy­drogenase, isocitrate dehydrogenase, 2-oxoglutarate de­hydrogenase, and citrate synthase (Figure 22.2), this could potentially upregulate the reactions performed by these enzymes, from the decarboxylation of pyruvate to 2-oxoglutarate. The last is a direct and indirect substrate to the EFE, directly to generate ethylene (Eqn (22.2)) and indirectly, as it is also a substrate for the synthesis of argi­nine, required for the simultaneous reaction of this enzyme (Eqn (22.3) and Figure 22.2).