Allene oxide synthase (AOS; hydroperoxide dehydratase; EC 4.2.1.92) catalyzes the first step in the biosynthesis of jasmonic acid from lipoxygenase-derived hydroperoxides of free fatty acids. Using theAOS cDNA from tomato (Lycopersicon esculentum), in which the role of jasmonic acid in wound-induced defense gene activation has been best described, we examined the kinetics of AOS induction in response to wounding and elicitors, in parallel with that of the wound-inducible PIN II (proteinase inhibitor II) gene. AOS was induced in leaves by wounding, systemin, 12-oxophytodienoic acid, and methyl jasmonate. The levels of AOS mRNA started declining by 4 h after induction, whereas the levels of PIN II mRNA continued to increase up to 20 h after induction. Salicylic acid inhibited AOS and PIN IIexpression, and the addition of 12-oxophytodienoic acid or methyl jasmonate did not prevent the inhibition of PIN IIexpression in the presence of salicylic acid. Ethylene induced the expression of AOS, but the presence of ethylene alone did not produce an optimal induction of PIN II. The addition of silver thiosulfate, an ethylene action inhibitor, prevented the wound-induced expression of both AOS and PIN II. Products of hydroperoxide lyase affected neitherAOS nor PIN II, but induced expression of prosystemin. Based on these results, we propose an updated model for defense gene activation in tomato.

Hydroperoxide lyase (HPL) cleaves lipid hydroperoxides to produce volatile flavor molecules and also potential signal molecules. We have characterized a gene from Arabidopsis that is homologous to a recently cloned HPL from green pepper (Capsicum annuum). The deduced protein sequence indicates that this gene encodes a cytochrome P-450 with a structure similar to that of allene oxide synthase. The gene was cloned into an expression vector and expressed in Escherichia coli to demonstrate HPL activity. Significant HPL activity was evident when 13S-hydroperoxy-9(Z),11(E),15(Z)-octadecatrienoic acid was used as the substrate, whereas activity with 13S-hydroperoxy-9(Z),11(E)-octadecadienoic acid was approximately 10-fold lower. Analysis of headspace volatiles by gas chromatography-mass spectrometry, after addition of the substrate to E. coli extracts expressing the protein, confirmed enzyme-activity data, since cis-3-hexenal was produced by the enzymatic activity of the encoded protein, whereas hexanal production was limited. Molecular characterization of this gene indicates that it is expressed at high levels in floral tissue and is wound inducible but, unlike allene oxide synthase, it is not induced by treatment with methyl jasmonate.In green plant tissue HPL cleaves C18-lipid hydroperoxides to form a C6-aldehyde and a 12-carbon oxoacid (Hatanaka, 1993). The C12 product of HPL leads to the formation of traumatin, which is implicated in wound signaling (Zimmerman and Coudron, 1979). The C6-aldehyde products of the HPL reaction depend on the substrate; cis-3-hexenal is formed from HPOT and hexanal is formed from HPOD. The lipid hydroperoxide substrates were generated from C18 free fatty acids by the enzymatic activity of lipoxygenase (see Fig. 1). HPL activity has been found in a variety of plants and is thought to be associated with the chloroplast envelope (Blée and Joyard, 1996), developmentally regulated (Vick and Zimmerman, 1976;Gardner et al., 1991;Riley et al., 1996), and discernible as two distinct activities in tea leaves (Matsui et al., 1991).Although the central lipoxygenase pathway is present in animal systems, and the formation of prostaglandins and leukotrienes in animals is analogous to the formation of jasmonates in plant tissue (Anderson, 1989), there is no pathway in animals analogous to the HPL branch pathway. The C6 compounds produced by HPL are released rapidly from disrupted plant tissue and form the basis for the "green note" flavor characteristic of plant tissue. The green note flavor is an important determinant of fresh fruit and vegetable quality, and C6 volatiles are widely used as a prepared food additive (Hatanaka, 1993). C6 volatiles produced from this pathway also have antimicrobial properties, suggesting that they may play a protective role in plant defense (Croft et al., 1993). They may also have an application as antimicrobial fumigants in postharvest storage (Archbold et al., 1997). In addition, C6 volatiles of the HPL pathway induce phytoalexin accumulation (Zeringue, 1992) and inhi...

Journal articleIFPRI3; ISIEPTDP

No abstract

Climate change during the last 40 years has had a serious impact on agriculture and threatens global food and nutritional security. From over half a million plant species, cereals and legumes are the most important for food and nutritional security. Although systematic plant breeding has a relatively short history, conventional breeding coupled with advances in technology and crop management strategies has increased crop yields by 56 % globally between 1965−85, referred to as the Green Revolution. Nevertheless, increased demand for food, feed, fiber, and fuel necessitates the need to break existing yield barriers in many crop plants. In the first decade of the 21st century we witnessed rapid discovery, transformative technological development and declining costs of genomics technologies. In the second decade, the field turned towards making sense of the vast amount of genomic information and subsequently moved towards accurately predicting gene-to-phenotype associations and tailoring plants for climate resilience and global food security. In this review we focus on genomic resources, genome and germplasm sequencing, sequencing-based trait mapping, and genomics-assisted breeding approaches aimed at developing biotic stress resistant, abiotic stress tolerant and high nutrition varieties in six major cereals (rice, maize, wheat, barley, sorghum and pearl millet), and six major legumes (soybean, groundnut, cowpea, common bean, chickpea and pigeonpea). We further provide a perspective and way forward to use genomic breeding approaches including marker-assisted selection, marker-assisted backcrossing, haplotype based breeding and genomic prediction approaches coupled with machine learning and artificial intelligence, to speed breeding approaches. The overall goal is to accelerate genetic gains and deliver climate resilient and high nutrition crop varieties for sustainable agriculture.

lhe accumulation and reduction of nitrate in the presence of the nitrogen metabolites asparagine (Asn) and glutamine (Cln) and the carbon metabolite sucrose (Suc) were examined i n maize (Zea mays L.) seedlings in an attempt to separate their effects on the nitrate uptake system and the nitrate reduction system. After 8 h of exposure to nitrate in the presence of 1 mM Asn, tissue nitrate accumulation was reduced at 250 ~L M external nitrate, but not at 5 mM Asn.The induction of nitrate reductase (NR) activity was reduced at both external nitrate concentrations. In the presence of 1 mM Cln or 1 '/ O SUC, tissue nitrate concentration was not significantly altered, but the induction of root NR activity was reduced or enhanced, respectively. l h e induction of root nitrite reductase (NiR) activity was also reduced in the presence of Asn or Cln and enhanced i n the presence of SUC. Transcript levels of NR and NiR in roots were reduced in the presence of the amides and enhanced i n the presence of SUC. When Suc was present i n combination with either amide, there was complete relief from the inhibition of NiR transcription observed in the presence of amide alone. In the case of NR, however, this relief from inhibition was negligible. l h e inhibition of the induction of NR and NiR activities in the presence of Cln and Asn is a direct effect and is not the result of altered nitrate uptake in the presence of these metabolites.Nitrate, the primary nitrogen source for most land plants, is absorbed by a specific nitrate uptake permease(s) in plant roots, reduced to nitrite by NR in the cytoplasm, and then reduced to ammonium by NiR in the plastid. Therefore, nitrate uptake permease(s), NR, and NiR constitute the first three enzymes of the nitrate assimilatory pathway and are subject to regulation by severa1 endogenous and environmental stimuli, including nitrate, Gln, light, and SUC (for review, see Crawford and Arst, 1993; Oaks, 1994; Sivasankar and Oaks, 1996). Nitrate induces the expression of both the uptake and reduction systems, whereas SUC and light enhance this induction. Gln represses the induction of the nitrate assimilatory system. The regulation of NR by nitrate, SUC, and Gln occurs at the transcriptional level, whereas the regulation by light occurs at both the transcriptional and posttranslational levels (Melzer et al., 1989; Cheng et al., 1992; Vincentz et al., 1993; Li et al., 1995; MacKintosh et al., 1995). Because the effec- tors Gln and SUC influence both the uptake and reduction systems, and the presence of nitrate is required for the induction of NR and NiR, it is possible that any effect of Gln and SUC on NR and NiR is an indirect effect arising out of a direct effect on nitrate uptake. Separation of the effects of these metabolites on the uptake and reduction systems has not yet been attempted. Nitrate uptake is mediated by two or more classes of nitrate transporters, referred to as the low-and highaffinity classes, which are present at the plasmalemma of root cells. In barley at least three transp...

Expression of the wheat dehydrin gene Cor410b is induced several fold above its non-stressed levels upon exposure to stresses such as cold, drought and wounding. Deletion analysis of the TdCor410b promoter revealed a single functional C-repeat (CRT) element. Seven transcription factors (TFs) were shown to bind to this CRT element using yeast one-hybrid screens of wheat and barley cDNA libraries, of which only one belonged to the DREB class of TFs. The remaining six encoded ethylene response factors (ERFs) belong to three separate subfamilies. Analysis of binding selectivity of these TFs indicated that all seven could bind to the CRT element (GCCGAC), and that three of the six ERFs could bind both to the CRT element and the ethylene-responsive GCC-box (GCCGCC). The TaERF4 subfamily members specifically bound the CRT element, and did not bind either the GCC-box or DRE element (ACCGAC). Molecular modeling and site-directed mutagenesis identified a single residue Pro42 in the Apetala2 (AP2) domain of TaERF4-like proteins that is conserved in monocotyledonous plants and is responsible for the recognition selectivity of this subfamily. We suggest that both DREB and ERF proteins regulate expression of the Cor410b gene through a single, critical CRT element. Members of the TaERF4 subfamily are specific, positive regulators of Cor410b gene expression.

Crowth systems that either permit (wet system) or prevent (dry system) the hydrolysis of endosperm reserves in maize (Zea mays) seedlings were developed to study the effect of endosperm reserves on the acquisition of externa1 nitrogen. Three-day-old seedlings treated with 5 mM KNO, for 24 h had higher levels of nitrate reductase (NR) activity and protein in shoot and root tissues in the dry relative to the wet system. This suggests that the induction of NR is sensitive to products of hydrolysis of endosperm reserves. Asparagine (1 mM) or glutamine (1 mM), potential products of that hydrolysis, inhibited the induction of NADH-dependent root NR in the dry system by about i'O0/0. The inhibition of the induction of NR activity in the wet system was only about 35%, suggesting that the enzyme in the wet system was already partially repressed at 3 d. At 5 d, when asparagine and glutamine levels in the plant tissue had decreased, the induction of root NR activity was inhibited to a similar extent in the two growth systems by amide additions. l h e shoot enzyme was less sensitive to amide additions, and 10 mM concentrations of either amide was required for a 65% inhibition.