Gibberellins (GAs) are crucial phytohormones involved in many aspects of plant growth and development, including plant-microbe interactions, which has led to GA production by plant-associated fungi and bacteria as well. While the GA biosynthetic pathways in plants and fungi have been elucidated and found to have independently arisen through convergent evolution, little has been uncovered about GA biosynthesis in bacteria. Some nitrogen-fixing/symbiotic, legume-associated rhizobia, including Bradyrhizobium japonicum, the symbiont of soybean, and Sinorhizobium fredii, a broad-host-nodulating species, contain a putative GA biosynthetic operon/gene cluster. Through functional characterization of five unknown genes, we demonstrate that this operon encodes the enzymes necessary to produce GA9, thereby elucidating bacterial GA biosynthesis. The distinct nature of these enzymes indicates that bacteria have independently evolved a third biosynthetic pathway for GA production. Furthermore, our results also reveal a central biochemical logic that is followed in all three convergently evolved GA biosynthetic pathways.
Sesquiterpenes play important roles in insect communication, for example as pheromones. However, no sesquiterpene synthases, the enzymes involved in construction of the basic carbon skeleton, have been identified in insects to date. We investigated the biosynthesis of the sesquiterpene (6R,7S)-himachala-9,11-diene in the crucifer flea beetle Phyllotreta striolata, a compound previously identified as a male-produced aggregation pheromone in several Phyllotreta species. A (6R,7S)-himachala-9,11-diene–producing sesquiterpene synthase activity was detected in crude beetle protein extracts, but only when (Z,E)-farnesyl diphosphate [(Z,E)-FPP] was offered as a substrate. No sequences resembling sesquiterpene synthases from plants, fungi, or bacteria were found in the P. striolata transcriptome, but we identified nine divergent putative trans-isoprenyl diphosphate synthase (trans-IDS) transcripts. Four of these putative trans-IDSs exhibited terpene synthase (TPS) activity when heterologously expressed. Recombinant PsTPS1 converted (Z,E)-FPP to (6R,7S)-himachala-9,11-diene and other sesquiterpenes observed in beetle extracts. RNAi-mediated knockdown of PsTPS1 mRNA in P. striolata males led to reduced emission of aggregation pheromone, confirming a significant role of PsTPS1 in pheromone biosynthesis. Two expressed enzymes showed genuine IDS activity, with PsIDS1 synthesizing (E,E)-FPP, whereas PsIDS3 produced neryl diphosphate, (Z,Z)-FPP, and (Z,E)-FPP. In a phylogenetic analysis, the PsTPS enzymes and PsIDS3 were clearly separated from a clade of known coleopteran trans-IDS enzymes including PsIDS1 and PsIDS2. However, the exon–intron structures of IDS and TPS genes in P. striolata are conserved, suggesting that this TPS gene family evolved from trans-IDS ancestors.
Isoprenyl diphosphate synthases (IDSs) produce the ubiquitous branched-chain diphosphates of different lengths that are precursors of all major classes of terpenes. Typically, individual shortchain IDSs (scIDSs) make the C 10 , C 15 , and C 20 isoprenyl diphosphates separately. Here, we report that the product length synthesized by a single scIDS shifts depending on the divalent metal cofactor present. This previously undescribed mechanism of carbon chainlength determination was discovered for a scIDS from juvenile horseradish leaf beetles, Phaedon cochleariae. The recombinant enzyme P. cochleariae isoprenyl diphosphate synthase 1 (PcIDS1) yields 96% C 10 -geranyl diphosphate (GDP) and only 4% C 15 -farnesyl diphosphate (FDP) in the presence of Co 2+ or Mn 2+ as a cofactor, whereas it yields only 18% C 10 GDP but 82% C 15 FDP in the presence of Mg 2+. In reaction with Co 2+ , PcIDS1 has a K m of 11.6 μM for dimethylallyl diphosphate as a cosubstrate and 24.3 μM for GDP. However, with Mg 2+ , PcIDS1 has a K m of 1.18 μM for GDP, suggesting that this substrate is favored by the enzyme under such conditions. RNAi targeting PcIDS1 revealed the participation of this enzyme in the de novo synthesis of defensive monoterpenoids in the beetle larvae. As an FDP synthase, PcIDS1 could be associated with the formation of sesquiterpenes, such as juvenile hormones. Detection of Co , or Mg 2+ in the beetle larvae suggests flux control into C 10 vs. C 15 isoprenoids could be accomplished by these ions in vivo. The dependence of product chain length of scIDSs on metal cofactor identity introduces an additional regulation for these branch point enzymes of terpene metabolism.cobalt | kinetic | prenyltransferase | secretions | silencing
Isoprenyl diphosphate synthases (IDSs) catalyze some of the most basic steps in terpene biosynthesis by producing the prenyl diphosphate precursors of each of the various terpenoid classes. Most plants investigated have distinct enzymes that produce the short-chain all-trans (E) prenyl diphosphates geranyl diphosphate (GDP, C10 ), farnesyl diphosphate (FDP, C15 ) or geranylgeranyl diphosphate (GGDP, C20 ). In the genome of Arabidopsis thaliana, 15 trans-product-forming IDSs are present. Ten of these have recently been shown to produce GGDP by genetic complementation of a carotenoid pathway engineered into Escherichia coli. When verifying the product pattern of IDSs producing GGDP by a new LC-MS/MS procedure, we found that five of these IDSs produce geranylfarnesyl diphosphate (GFDP, C25 ) instead of GGDP as their major product in enzyme assays performed in vitro. Over-expression of one of the GFDP synthases in A. thaliana confirmed the production of GFDP in vivo. Enzyme assays with A. thaliana protein extracts from roots but not other organs showed formation of GFDP. Furthermore, GFDP itself was detected in root extracts. Subcellular localization studies in leaves indicated that four of the GFDP synthases were targeted to the plastoglobules of the chloroplast and one was targeted to the mitochondria. Sequence comparison and mutational studies showed that the size of the R group of the 5th amino acid residue N-terminal to the first aspartate-rich motif is responsible for C25 versus C20 product formation, with smaller R groups (Ala and Ser) resulting in GGDP (C20 ) as a product and a larger R group (Met) resulting in GFDP (C25 ).
Summary Certain plant-associated microbes can produce gibberellin (GA) phytohormones, as first described for the rice fungal pathogen Gibberella fujikuroi and more recently for bacteria, including several rhizobia and the rice bacterial pathogen Xanthomonas oryzae pv. oryzicola. The relevant enzymes are encoded by a biosynthetic operon that exhibits both a greater phylogenetic range and scattered distribution among plant-associated bacteria. Here the phylogenetic distribution of this operon was investigated. To demonstrate conserved functionality, the enzymes encoded by the disparate operon from Xanthomonas translucens pv. translucens, along with those from the most divergent example, found in Erwinia tracheiphila, were biochemically characterized. In both of these phytopathogens the operon leads to production of the bioactive GA4. Based on these results, it seems that this operon is widely dedicated to GA biosynthesis. However, there is intriguing variation in exact product. In particular, although all plant pathogens seem to produce bioactive GA4, rhizobia generally only produce the penultimate hormonal precursor GA9. This is suggested to reflect their distinct interactions with plants, as production of GA4 counteracts the jasmonic acid-mediated defense response, reflecting the importance of wounds as the entry point for these phytopathogens, while such suppression presumably is detrimental in the rhizobial symbiotic relationship.
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