Elicitor-induced metabolic changes in cell cultures of chickpea (Cicer arietinum L.) cultivars resistant and susceptible to Ascochyta rabiei : II. Differential induction of chalcone-synthase-mRNA activity and analysis of in-vitro-translated protein patterns

Genetic Analysis of Partially Resistant and Susceptible Chickpea Cultivars in Response to Ascochyta rabiei Infection

The molecular mechanism involved in chickpea (Cicer arietinum L.) resistance to the necrotrophic fungal pathogen Ascochyta rabiei is not well documented. A. rabiei infection can cause severe damage in chickpea, resulting in significant economic losses. Understanding the resistance mechanism against ascochyta blight can help to define strategies to develop resistant cultivars. In this study, differentially expressed genes from two partially resistant cultivars (CDC Corinne and CDC Luna) and a susceptible cultivar (ICCV 96029) to ascochyta blight were identified in the early stages (24, 48 and 72 h) of A. rabiei infection using RNA-seq. Altogether, 3073 genes were differentially expressed in response to A. rabiei infection across different time points and cultivars. A larger number of differentially expressed genes (DEGs) were found in CDC Corinne and CDC Luna than in ICCV 96029. Various transcription factors including ERF, WRKY, bHLH and MYB were differentially expressed in response to A. rabiei infection. Genes involved in pathogen detection and immune signalings such as receptor-like kinases (RLKs), Leucine-Rich Repeat (LRR)-RLKs, and genes associated with the post-infection defence response were differentially expressed among the cultivars. GO functional enrichment and pathway analysis of the DEGs suggested that the biological processes such as metabolic process, response to stimulus and catalytic activity were overrepresented in both resistant and susceptible chickpea cultivars. The expression patterns of eight randomly selected genes revealed by RNA-seq were confirmed by quantitative PCR (qPCR) analysis. The results provide insights into the complex molecular mechanism of the chickpea defence in response to the A. rabiei infection.

Temperature and Soil Moisture Stress Modulate the Host Defense Response in Chickpea During Dry Root Rot Incidence

Dry root rot caused by the necrotrophic phytopathogenic fungus Rhizoctonia bataticola is an emerging threat to chickpea production in India. In the near future, the expected increase in average temperature and inconsistent rainfall patterns resultant of changing climatic scenarios are strongly believed to exacerbate the disease to epidemic proportions. The present study aims to quantify the collective role of temperature and soil moisture content (SMC) on disease progression in chickpea under controlled environmental conditions. In our study, we could find that both temperature and soil moisture played a decisive role in influencing the dry root rot disease scenario. As per the disease susceptibility index (DSI), a combination of high temperature (35°C) and low SMC (60%) was found to elicit the highest disease susceptibility in chickpea. High pathogen colonization was realized in chickpea root tissue at all time-points irrespective of genotype, temperature, and SMC. Interestingly, this was in contrast to the DSI where no visible symptoms were recorded in the roots or foliage during the initial time-points. For each time-point, the colonization was slightly higher at 35°C than 25°C, while the same did not vary significantly with respect to SMC. Furthermore, the differential expression study revealed the involvement of host defense-related genes like endochitinase and PR-3-type chitinase (CHI III) genes in delaying the dry root rot (DRR) disease progression in chickpea. Such genes were found to be highly active during the early stages of infection especially under low SMC.

Phytoalexins of the Pyrinae: Biphenyls and dibenzofurans

Biphenyls and dibenzofurans are the phytoalexins of the Pyrinae, a subtribe of the plant family Rosaceae. The Pyrinae correspond to the long-recognized Maloideae. Economically valuable species of the Pyrinae are apples and pears. Biphenyls and dibenzofurans are formed de novo in response to infection by bacterial and fungal pathogens. The inducible defense compounds were also produced in cell suspension cultures after treatment with biotic and abiotic elicitors. The antimicrobial activity of the phytoalexins was demonstrated. To date, 10 biphenyls and 17 dibenzofurans were isolated from 14 of the 30 Pyrinae genera. The most widely distributed compounds are the biphenyl aucuparin and the dibenzofuran γ-cotonefuran. The biosynthesis of the two classes of defense compounds is not well understood, despite the importance of the fruit crops. More recent studies have revealed simultaneous accumulation of biphenyls and dibenzofurans, suggesting sequential, rather than the previously proposed parallel, biosynthetic pathways. Elicitor-treated cell cultures of Sorbus aucuparia served as a model system for studying phytoalexin metabolism. The key enzyme that forms the carbon skeleton is biphenyl synthase. The starter substrate for this type-III polyketide synthase is benzoyl-CoA. In apples, biphenyl synthase is encoded by a gene family, members of which are differentially regulated. Metabolism of the phytoalexins may provide new tools for designing disease control strategies for fruit trees of the Pyrinae subtribe.

Enhanced plumbagin production in elicited Plumbago indica hairy root cultures

Elicitation of Plumbago indica hairy roots with yeast carbohydrate fraction, chitosan, manganese chloride, copper chloride and methyl jasmonate exhibited significant elevation (~1.2 to 2 fold) of plumbagin production in shake flask culture as compared with control. Chitosan and methyl jasmonate elicitation also caused simultaneous plumbagin leaching into culture media. Three days' exposure of chitosan (200 mg l(-1)) and methyl jasmonate (80 μM) together synergized total plumbagin yield to its maximum 11.96 ± 0.76 mg g(-l) DW in shake flask culture. In bioreactor cultivation, a significant raise in fresh root biomass was recorded on day 20 as compared with control shake flask culture. Three days' exposure of chitosan (200 mg l(-1)) and methyl jasmonate (80 μM) with 20 days old bioreactor-culture significantly improved total plumbagin production to 13.16 ± 1.72 mg g(-l) DW with simultaneous plumbagin leaching into bioreactor media.

Yeast extract and methyl jasmonate-induced silymarin production in cell cultures of Silybum marianum (L.) Gaertn

The biosynthesis of the flavonolignan silymarin, a constitutive compound of the fruits of Silybum marianum with strong antihepatotoxic and hepatoprotective activities, is severely reduced in cell cultures of this species. It is well known that elicitation is one of the strategies employed to increase accumulation of secondary metabolites. Our study here reports on the effect of several compounds on the production of silymarin in S. marianum cultures. Yeast extract (YE), chitin and chitosan were compared with respect to their effects on silymarin accumulation in S. marianum suspensions and only yeast extract stimulated production. Jasmonic acid (JA) potentiated the yeast extract effect. One of the jasmonic acid derivatives, methyl jasmonate (MeJA), strongly promoted the accumulation of silymarin. Methyl jasmonate acted in a number of steps of the metabolic pathway of flavonolignans and its stimulating effect was totally dependent of "de novo" protein synthesis. Chalcone synthase (CHS) activity was enhanced by methyl jasmonate; however there did not appear to be a temporal relationship between silymarin accumulation and increase in enzyme activity. Also, this increase was not blocked by the protein synthesis inhibitor cycloheximide (CH). This study indicates that elicitor treatment promotes secondary metabolite production in S. marianum cultures and that jasmonic acid and its functional analogue plays a critical role in elicitation.

Effects of isoflavonoids from Cicer on larvae of Heliocoverpa armigera

Four recently identified isoflavonoids, isolated from wild relatives of chickpea, Cicer arietinum, were shown to deter larval feeding by Heliocoverpa armigera at 100 ppm, judaicin and maackiain retained their antifeedant activity at 50 ppm and 10 ppm, respectively. The isoflavonoids were tested in combinations and with chlorogenic acid; the combinations containing judaicin and maackiain were most active, and chlorogenic acid enhanced the antifeedant activity of all four isoflavonoids. H. armigera was the only one of four noctuids to be deterred by all four isoflavonoids. Spodoptera littoralis was deterred by judaicin alone and S. frugiperda by maackiain alone. Heliothis virescens and S. exigua were not deterred from feeding by any of the isoflavonoids. When incorporated into a diet, isoflavonoids decreased the weight gain of early stadia larvae of H. armigera more than they did later stadia, and maackiain and judaicin were most potent. We conclude that the isoflavonoids. especially maackiain and judaicin, could play a role in decreasing the susceptibility of Cicer to attack by H. armigera.

Phytoalexin Production in an Apple Cultivar Resistant to Venturia inaequalis

ABSTRACT Cell suspension cultures of the scab-resistant apple (Malus x domestica) cultivar Liberty were challenged with yeast extract to mimic the effect of biological stress such as fungal invasion. The cells responded to the challenge by production of novel compounds. Suspension cultures of the scab-susceptible cultivar McIntosh, when similarly challenged, showed no detectable response. The major compound produced by scab-resistant cells in response to the challenge has been identified as the 2,4-methoxy-3-hydroxy-9-O-beta-D-glucosyloxydibenzofuran by UV, mass spectrometry, (1)H-nuclear magnetic resonance (NMR), and (13)C-NMR spectroscopy. We suggest the trivial name malusfuran for the compound. Malusfuran production was initiated approximately 24 h after being challenged. Malusfuran inhibited spore germination and growth of Venturia inaequalis at millimolar concentrations, indicating its role as a possible phytoalexin. The aglycone of malusfuran, 2,4-methoxy-3,9-hydroxy-dibenzofuran, showed higher toxicity to V. inaequalis than to the parent malusfuran. In vitro cultures of V. inaequalis produced a beta-glucosidase that hydrolyzed ortho- and para-substituted nitrophenyl-beta-glucosides, suggesting that the aglycone may act as the actual phytoalexin.

Tansley Review No. 86 Accumulation of phytoalexins: defence mechanism and stimulus response system

Phytoalexin synthesis is a defence-response- that is characterized by a requirement for a number of distinct elements, all of which must be present for the response to be expressed fully. These same elements: a signal, a cellular receptor, a signal transduction system and a responsive metabolic system, are also used to describe a stimulus-response system. A number of molecular species can function as signal molecules or elicitors of phytoalexin synthesis, including poly- and oligosaccharides, proteins and polypeptides, and fatty acids. Few receptors for elicitors have been identified but those that have been are proteins located on the plasma membrane of the plant. Induction of phytoalexin synthesis involves selective and co-ordinated activation of specific defence response genes, including those encoding the enzymes of phytoalexin synthesis, and these genes constitute the responsive metabolic system. The separate, and distant, locations of the receptor and the responsive genes means that the event in which the signal is perceived by the receptor must be relayed to the genes by means of a second messenger system. Several second messengers are candidates for such a coupling- or signal transduction-system, including udenosine-3',5'-cyclic monophosphate, Ca²⁺ , diacylglycerol and inositol 1,4,5-trisphosphate, active oxygen species and jasmonic acid. Each has been examined as a possible component of the signal transduction system mediating between the elicitor receptor interaction and the phytoalexin synthesis it induces. Analysis of the signalling events is made complex by the simultaneous solicitation by the invading micro-organism of several defence responses, each of which might involve elements of a different signal system. The same complexity is evident which the role of phytoalexin accumulation in resistance is analysed. Evaluation of the contribution made by phytoalexin accumulation towards resistance has been attempted by the use of various inhibitors and enhancers of the process. Transgenic and mutant plants with specific alterations in one or more ot those elements necessary for the plant to respond to the signals for phytoalexin synthesis and other defence responses, are beginning to aid resolution of the complex pattern ot signalling events and the respective roles of the inducible defence mechanisms in resistance. CONTENTS Summary 1 I. Introduction 2 II. Chemistry of phytoalexins 3 III. Phytoalexin accumulation as a determinant of resistance 6 IV. Elicitation of phytoalexin accumulation 11 References 34.

In Trifolium subterraneum, chalcone synthase is encoded by a multigene family

Chalcone synthase (CHS) catalyzes the first and key regulatory step in flavonoid biosynthesis. We report the existence and characterization of a CHS multigene family present in Trifolium subterraneum L. cultivar Karridale. The CHS family consists of at least four members, which are tightly clustered in a 15-kb region. The complete sequences of two of these genes (CHS1 and CHS2) are presented. The putative promoters of these genes have sequences which are homologous to those known, or implicated, in regulation of the expression of phenylpropanoid-encoding genes.

Stress responses in alfalfa (Medicago sativa L.). 15. Characterization and expression patterns of members of a subset of the chalcone synthase multigene family

We have identified five different full length chalcone synthase (CHS) cDNA clones from a cDNA library produced from transcripts isolated from an elicitor-treated alfalfa cell suspension culture. Nucleotide sequence similarity between the clones varied from 88-93%. Oligonucleotides based on divergent sequences in the 5'-untranslated regions of the clones could distinguish individual genes, or groups of genes, and their corresponding transcripts. Developmentally regulated expression of the CHS transcripts was predominantly in roots and root nodules; other unidentified members of the CHS gene family are expressed in stems, leaves and nodules. One of the CHS transcripts was strongly expressed in floral tissue. All the CHS transcripts studied were induced in elicitor-treated cell suspension cultures. Transcripts were also induced in roots in response to wounding or spraying with various elicitors, and in leaves infected with Phoma medicaginis (but not in wounded leaves). The induction kinetics of CHS2 transcripts were more rapid and/or transient than those of other members of the CHS family in CuCl2-treated roots and Phoma-infected leaves. The results are discussed in terms of the evolution and functions of the CHS gene family in legumes.

Pterocarpan phytoalexin biosynthesis in elicitor-challenged chickpea (Cicer arietinum L.) cell cultures. Purification, characterization and cDNA cloning of NADPH:isoflavone oxidoreductase

NADPH:isoflavone oxidoreductase (IFR) is the first soluble enzyme of the pterocarpan-specific part of phytoalexin biosynthesis in chickpea (Cicer arietinum L.). The enzyme was purified to apparent homogeneity by a five-step procedure from chickpea cell cultures treated with yeast extract as elicitor. Analysis by gel filtration and SDS/PAGE showed that the enzyme consists of a single polypeptide with a molecular mass of 36 kDa. Km values for the substrates 2'-hydroxyformononetin, 2'-hydroxypseudobaptigenin and NADPH were 6, 6 and 20 microM, respectively. The IFR showed pronounced specificity for the substitution pattern of isoflavones. We found a 2'-hydroxy group and a 4',5'-methylenedioxy or 4'-methoxy function to be essential for acceptance as substrate. The isoelectric point of the protein was determined as 6.3 by IEF and there is no evidence for the existence of isoenzymes. Partial amino acid sequences of IFR were determined from internal peptides obtained by tryptic digestion of the protein and corresponding oligonucleotides were synthesized. A lambda gt10 cDNA library was constructed using poly(A)-rich RNA isolated from chickpea cell cultures treated with Ascochyta rabiei elicitor. 150 positive clones were obtained by screening 2 x 10(5) clones with an IFR-specific oligonucleotide. The identity of sequenced clones was confirmed by comparison of the deduced amino acid sequence with the internal peptide sequences of purified IFR. The sequence of a 1183-bp clone contained a continuous open reading frame of 954 bases encoding a polypeptide of 318 amino acids with a calculated molecular mass of 35.4 kDa, indicating that a full-length cDNA coding for IFR was isolated.

Elicitor-induced metabolic changes in cell cultures of chickpea (Cicer arietinum L.) cultivars resistant and susceptible to Ascochyta rabiei : II. Differential induction of chalcone-synthase-mRNA activity and analysis of in-vitro-translated protein patterns

Cell-suspension cultures of two chickpea (Cicer arietinum L.) cultivars, resistant (ILC 3279) and susceptible (ILC 1929) to the fungus Ascochyta rabiei (Pass.) Lab., showed differential accumulation of the phytoalexins medicarpin and maackiain, and transient induction of related enzyme activities after application of an A. rabiei-derived elicitor. The chalcone-synthase (CHS) activity (EC 2.3.1.74) which is involved in the first part of phytoalexin biosynthesis exhibited a maximum 8-12 h after elicitation in the cells of both cultivars. Concomitant with the fivefold-higher phytoalexin accumulation, CHS activity increased twofold in the cells of the resistant cultivar. The maximum of the elicitor-induced CHS-mRNA activity was determined 4 h after onset of induction in the cultures of both cultivars, although in cells of cultivar ILC 3279 this mRNA activity was induced at a level twofold higher than that in cells of the susceptible race ILC 1929. Investigations of CHS isoenzymes by two-dimensional gel electrophoresis of immunoprecipitated in-vitro-translated protein indicated the presence of five proteins. In the cells of both cultivars only two of the isoenzymes were induced after elicitor treatment. Analysis of the total in-vitro-translated proteins by two-dimensional gel electrophoresis showed that the constitutively expressed patterns of mRNA activities in the cell cultures of the two cultivars were identical. After elicitation, considerably more translatable mRNAs were induced in the cells of cultivar ILC 3279. The few induced proteins, and their respective mRNA activities, which could be detected in the cells of the susceptible cultivar, all existed in the cells of the resistant cultivar, too. One highly induced protein (Mr 18 kDa) found in the cells of cultivar ILC 3279 reached its maximum mRNA activity 6 h after elicitor application. The amount of this protein was hardly increased in the cells of the susceptible cultivar. This protein appears to be excreted from the cells into the growth medium.

Elicitor-induced metabolic changes in cell cultures of chickpea (Cicer arietinum L.) cultivars resistant and susceptible to Ascochyta rabiei : I. Investigations of enzyme activities involved in isoflavone and pterocarpan phytoalexin biosynthesis

Cell-suspension cultures of Ascochyta rabiei-resistant (ILC 3279) and -susceptible (ILC 1929) chickpea (Cicer arietinum L.) cultivars were compared with regard to their elicitor-induced accumulation of pterocarpan phytoalexins and increases in the activities of biosynthetic enzymes. The growth performances and protein patterns of the two cell-culture lines were essentially identical. Treatment of cell cultures with a polysaccharide elicitor from A. rabiei induced fivefold-higher amounts of the phytoalexins medicarpin and maackiain in the cells of the resistant than in the susceptible cultivar. Glucose 6-phosphate dehydrogenase and eight enzymes representing the general phenylpropanoid pathway, the flavonoid-forming steps and the pterocarpanspecific branch of phytoalexin biosynthesis were found to be elicitor-induced. Phenylalanine ammonia-lyase and chalcone synthase reached sharp, transient optima some 8 h after elicitor application in the cells of both cultivars. The activities of isoflavone 2'- and 3'-hydroxylases were only induced in cells of the resistant cultivar with a maximum after 8 h. Cinnamic acid 4-hydroxylase, chalcone isomerase, 2'-hydroxyisoflavone reductase and pterocarpan synthase showed a later or no sharp optimum. The isoflavone-specific 7-O-glucosyltransferase was not induced in either cell-culture line. Cells of the susceptible cultivar failed to induce significant activities of isoflavone 2'-hydroxylase and these cells produced only very low amounts of phytoalexins. Isoflavone 2'-hydroxylase is postulated to be the main limiting enzyme for pterocarpan biosynthesis in cells of the susceptible cultivar. The pterocarpan biosynthetic pathway in chickpea cells represents a suitable model for investigations of differential gene activation in connection with the expression of antimicrobial defence reactions.