Strigolactones released from plant roots induce hyphal branching of symbiotic arbuscular mycorrhizal (AM) fungi and germination of root parasitic weeds, Striga and Orobanche spp. We already demonstrated that, in red clover plants (Trifolium pratense L.), a host for both AM fungi and the root holoparasitic plant Orobanche minor Sm., reduced supply of phosphorus (P) but not of other elements examined (N, K, Ca, Mg) in the culture medium significantly promoted the secretion of a strigolactone, orobanchol, by the roots of this plant. Here we show that in the case of sorghum [Sorghum bicolor (L.) Moench], a host of both the root hemiparasitic plant Striga hermonthica and AM fungi, N deficiency as well as P deficiency markedly enhanced the secretion of a strigolactone, 5-deoxystrigol. The 5-deoxystrigol content in sorghum root tissues also increased under both N deficiency and P deficiency, comparable to the increase in the root exudates. These results suggest that strigolactones may be rapidly released after their production in the roots. Unlike the situation in the roots, neither N nor P deficiency affected the low content of 5-deoxystrigol in sorghum shoot tissues.

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Assembled metal complexes of platinum(ii) and gold(i) ions often exhibit a characteristic color and intense luminescence based on electronic metal±metal interactions. Such complexes have recently attracted particular interest as sensor materials. [1] Herein, we report a new dinuclear platinum(ii) complex, syn-[Pt 2 (bpy) 2 (pyt) 2 ][PF 6 ] 2 (bpy ¼ 2,2'-bipyridine, pyt ¼ pyridine-2-thiolate ion), which exhibits a remarkable change in its luminescence in the presence of organic vapors such as acetonitrile or ethanol.Dark-red polyhedral crystals of syn-[Pt 2 (bpy) 2 (pyt) 2 ]-[PF 6 ] 2 ¥CH 3 CN were isolated from an acetonitrile/ethanol solution as the minor component of two geometrical isomers containing the [Pt 2 (bpy) 2 (pyt) 2 ] 2þ ion. The syn isomer has a head-to-head configuration of two bridging pyridine-2-thiolate ions ( Figure 1), with the platinum ions adopting different coordination environments: Pt1 is bonded to four nitrogen atoms, whereas Pt2 has N 2 S 2 coordination. This arrangement is in contrast to the anti isomer, which has a head-to-tail configuration (Figure 2), and is obtained as orange needlelike crystals. The Pt¥¥¥Pt separations for the syn-and anti isomers are 2.923(1) and 2.997(1) ä, respectively. These separations are amongst the shortest Pt¥¥¥Pt interactions observed in divalent platinum complexes. [2] Similar dinuclear complexes have been reported by Che an co-workers. [3] Crystals of the anti isomer exhibit very intense orange luminescence (l max ¼ 603 nm), even at room temperature (Figure 3 a). On the basis of the spectral profile and the emission lifetime (t ¼ 240 ns), the luminescene can be assigned as emission from the triplet metal±metal-to-ligand charge-transfer state, which has been observed in diimineplatinum(ii) complexes with short Pt¥¥¥Pt separations. [4] The emission state is thought to also include a contribution from the sulfur orbitals of the pyridine-2-thiolate ligands. [3a, 5] Crystals of the syn isomer are initially dark-red in appearance, but appear to become lighter in color upon standing in air for several hours at room temperatue ( Figure 4). Luminescence spectroscopy of these materials shows the generation of a concomitant emission (Figure 3 b and c). It is noteworthy that the luminescent light-red form reverts back to the nonemissive darker immediately upon exposure to acetonitrile or ethanol vapor, formwhich could be monitored COMMUNICATIONS Figure 1. Single-crystal X-ray structure of the syn isomer. Figure 2. Single-crystal X-ray structure of the anti isomer.Figure 3. Luminescence spectra at room temperature for a) the anti isomer, b) the light-red (desolvated) form of the syn isomer, and c) the dark-red (solvated) form of the syn isomer.

Reactive carbonyls, especially ␣,␤-unsaturated carbonyls produced through lipid peroxidation, damage biomolecules such as proteins and nucleotides; elimination of these carbonyls is therefore essential for maintaining cellular homeostasis. In this study, we focused on an NADPH-dependent detoxification of reactive carbonyls in plants and explored the enzyme system involved in this detoxification process. Using acrolein (CH 2 ‫؍‬ CHCHO) as a model ␣,␤-unsaturated carbonyl, we purified a predominant NADPH-dependent acrolein-reducing enzyme from cucumber leaves, and we identified the enzyme as an alkenal/one oxidoreductase (AOR) catalyzing reduction of an ␣,␤-unsaturated bond. Cloning of cDNA encoding AORs revealed that cucumber contains two distinct AORs, chloroplastic AOR and cytosolic AOR. Homologs of cucumber AORs were found among various plant species, including Arabidopsis, and we confirmed that a homolog of Arabidopsis (At1g23740) also had AOR activity. Phylogenetic analysis showed that these AORs belong to a novel class of AORs. They preferentially reduced ␣,␤-unsaturated ketones rather than ␣,␤-unsaturated aldehydes. Furthermore, we selected candidates of other classes of enzymes involved in NADPH-dependent reduction of carbonyls based on the bioinformatic information, and we found that an aldo-keto reductase (At2g37770) and aldehyde reductases (At1g54870 and At3g04000) were implicated in the reduction of an aldehyde group of saturated aldehydes and methylglyoxal as well as ␣,␤-unsaturated aldehydes in chloroplasts. These results suggest that different classes of NADPH-dependent reductases cooperatively contribute to the detoxification of reactive carbonyls.When plants are subjected to abiotic and/or biotic stresses, oxidative stress often results; this leads to the production of reactive oxygen species, which damage biomolecules such as proteins and lipids. More than half of the fatty acids found in the membranes of chloroplasts and mitochondria, two of the most highly oxidative organelles, are linoleic and linolenic acid. Because linoleic and linolenic acid are sources of many short chain carbonyls through their peroxidation (1), biomolecules in both types of organelles are challenged by the toxicity of reactive compounds, including ␣,␤-unsaturated carbonyls, which are involved in the pathophysiological effects associated with oxidative stress in cells and tissues (2). In fact, ␣,␤-unsaturated carbonyls from peroxidized polyunsaturated fatty acids cause loss of functions in mitochondria (3); chloroplasts have recently been shown to be a major production center of reactive aldehydes (4), and photosynthetic functions are highly sensitive to ␣,␤-unsaturated carbonyls (5-7).The high reactivity of ␣,␤-unsaturated carbonyls is due to the ability of their ␣,␤-unsaturated bonds to form Michael adducts with thiol and amino groups in biomolecules; in the case of ␣,␤-unsaturated aldehydes, the aldehyde group also contributes to reactivity through the formation of Schiff base adducts with amino groups. Thus the targe...

Strigolactones (SLs) are carotenoid-derived phytohormones and rhizosphere signaling molecules for arbuscular mycorrhizal fungi and root parasitic weeds. Why and how plants produce diverse SLs are unknown. Here, cytochrome P450 CYP722C is identified as a key enzyme that catalyzes the reaction of BC-ring closure leading to orobanchol, the most prevalent canonical SL. The direct conversion of carlactonoic acid to orobanchol without passing through 4-deoxyorobanchol is catalyzed by the recombinant enzyme. By knocking out the gene in tomato plants, orobanchol was undetectable in the root exudates, whereas the architecture of the knockout and wild-type plants was comparable. These findings add to our understanding of the function of the diverse SLs in plants and suggest the potential of these compounds to generate crops with greater resistance to infection by noxious root parasitic weeds.

A simple and rapid analytical method for strigolactones, germination stimulants for the root parasitic weeds witchweed (Striga spp.) and broomrape (Orobanche spp.), has been developed using high-performance liquid chromatography connected to tandem mass spectrometry (LC/MS/MS). The natural strigolactones (strigol, sorgolactone, orobanchol, and alectrol) were clearly separated and identified by LC/MS/MS. As low as 0.1 pg/microL of strigol and 0.5 pg/microL of sorgolactone could be quantified, whereas 1 pg/microL was needed for the quantification of orobanchol (S/N > 10). Using this method, it was found that red clover produces orobanchol and alectrol but not strigol. The roots of red clover seedlings were found to produce 13, 70, 58, and 65 pg of orobanchol/plant 1, 2, 3, and 4 weeks after germination, respectively.

Coumarins are important compounds that contribute to the adaptation of plants to biotic or abiotic stresses. Among coumarins, umbelliferone occupies a pivotal position in the plant phenylpropanoid network. Previous studies indicated that umbelliferone is derived from the ortho-hydroxylation of p-coumaric acid by an unknown biochemical step to yield 2,4-dihydroxycinnamic acid, which then undergoes spontaneous lactonization. Based on a recent report of a gene encoding a 2-oxoglutarate-dependent dioxygenase from Arabidopsis thaliana that exhibited feruloyl CoA 6'-hydroxylase activity (Bourgaud et al., 2006), we combined a bioinformatic approach and a cDNA library screen to identify an orthologous ORF (Genbank accession number JF799117) from Ruta graveolens L. This ORF shares 59% amino acid identity with feruloyl CoA 6'-hydroxylase, was functionally expressed in Escherichia coli, and converted feruloyl CoA into scopoletin and p-coumaroyl CoA into umbelliferone with equal activity. Its bi-functionality was further confirmed in planta: transient expression of JF799117 in Nicotiana benthamiana yielded plants with leaves containing high levels of umbelliferone and scopoletin when compared to control plants, which contained barely detectable traces of these compounds. The expression of JF799117 was also tightly correlated to the amount of umbelliferone that was found in UV-elicited R. graveolens leaves. Therefore, JF799117 encodes a p-coumaroyl CoA 2'-hydroxylase in R. graveolens, which represents a previously uncharacterized step in the synthesis of umbelliferone in plants. Psoralen, which is an important furanocoumarin in R. graveolens, was found to be a competitive inhibitor of the enzyme, and it may exert this effect through negative feedback on the enzyme at an upstream position in the pathway.

Strigolactones, a novel class of phytohormones, regulate plant architecture and act as rhizosphere signals. Species-specific biosynthetic pathways convert a common precursor, carlactone, to strigolactones.