Growth promotion induced by the endosymbiont Piriformospora indica has been observed in various plants; however, except growth phytohormones, specific functional metabolites involved in P. indica-mediated growth promotion are unknown. Here, we used a gas chromatography-mass spectrometry-based untargeted metabolite analysis to identify tomato (Solanum lycopersicum) metabolites whose levels were altered during P. indica-mediated growth promotion. Metabolomic multivariate analysis revealed several primary metabolites with altered levels, with putrescine (Put) induced most significantly in roots during the interaction. Further, our results indicated that P. indica modulates the arginine decarboxylase (ADC)-mediated Put biosynthesis pathway via induction of SlADC1 in tomato. Piriformospora indica did not promote growth in Sladc1-(virus-induced gene silencing of SlADC1) lines of tomato and showed less colonization. Furthermore, using LC–MS/MS we showed that Put promoted growth by elevation of auxin (indole-3-acetic acid) and gibberellin (GA4 and GA7) levels in tomato. In Arabidopsis (Arabidopsis thaliana) adc knockout mutants, P. indica colonization also decreased and showed no plant growth promotion, and this response was rescued upon exogenous application of Put. Put is also important for hyphal growth of P. indica, indicating that it is co-adapted by both host and microbe. Taken together, we conclude that Put is an essential metabolite and its biosynthesis in plants is crucial for P. indica-mediated plant growth promotion and fungal growth.

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Background Bipolaris sorokiniana is a filamentous fungus that causes spot blotch disease in cereals like wheat and has severe economic consequences. However, information on the identities and role of the cell wall-degrading enzymes (CWDE) in B. sorokiniana is very limited. Several fungi produce CWDE like glycosyl hydrolases (GHs) that help in host cell invasion. To understand the role of these CWDE in B. sorokiniana, the first step is to identify and annotate all possible genes of the GH families like GH3, GH6, GH7, GH45 and AA9 and then characterize them biochemically.ResultsWe confirmed and annotated the homologs of GH3, GH6, GH7, GH45 and AA9 enzymes in the B. sorokiniana genome using the sequence and domain features of these families. Quantitative real-time PCR analyses of these homologs revealed that the transcripts of the BsGH7-3 (3rd homolog of the GH 7 family in B. sorokiniana) were most abundant. BsGH7-3, the gene of BsGH7-3, was thus cloned into pPICZαC Pichia pastoris vector and expressed in X33 P. pastoris host to be characterized. BsGH7-3 enzyme showed a temperature optimum of 60 °C and a pHopt of 8.1. BsGH7-3 was identified to be an endoglucanase based on its broad substrate specificity and structural comparisons with other such endoglucanases. BsGH7-3 has a very long half-life and retains 100% activity even in the presence of 4 M NaCl, 4 M KCl and 20% (v/v) ionic liquids. The enzyme activity is stimulated up to fivefold in the presence of Mn+2 and Fe+2 without any deleterious effects on enzyme thermostability.ConclusionsHere we reanalysed the B. sorokiniana genome and selected one GH7 enzyme for further characterization. The present work demonstrates that BsGH7-3 is an endoglucanase with a long half-life and no loss in activity in the presence of denaturants like salt and ionic liquids, and lays the foundation towards exploring the Bipolaris genome for other cell wall-degrading enzymes.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0822-0) contains supplementary material, which is available to authorized users.

Stem rust caused by Puccinia graminis f. sp. tritici is one of the most devastating diseases of wheat. Breakdown of host resistance under field conditions triggered by the evolution of new pathogenic races and pathotypes is a perennial threat for wheat cultivation. Rice, often grown in a rice-wheat cropping system, is immune to rust infection. Our microscopic studies revealed that P. graminis f. sp. tritici, although displaying nearly identical uredospore germination, stomatal entry, and epi-and endophytic mycelial growth in rice and wheat, failed to sporulate to cause rust disease in rice.We identified 18 key defence signalling genes in rice and unravelled their elicitation dynamics in time-course studies during infection. ICS1, NPR1-3, PRs, EDS1, PAD4, FMO1 (salicylic acid [SA] signalling), and ethylene-related genes (ACO4 and ACS6) were strongly elicited in rice. However, genes from the jasmonic acid (JA) signalling pathway (LOX2, AOS2, MYC2, PDF2.2, JAZ8, JAZ10) showed a delayed response during colonization in rice compared to an early or no induction in wheat. However, the JA/ ethylene marker gene PDF2.2 was strongly induced in wheat as early as 12 hr postinoculation. Furthermore, rice and wheat displayed specific profiles of accumulation of various phenolic acids during P. graminis f. sp. tritici 40A infection. We propose a model where a differential modulation of the SA/JA-dependent defence network may modulate nonhost resistance. A deeper understanding of the molecular mechanism governing differential elicitation of defence signalling may provide a novel resistance mechanism for the sustainable management of rust diseases.

Plants, in nature are challenged by various environmental stresses (both biotic and abiotic), which distort the plant growth, development and crop yield. A wide range of insect herbivores attack plants across the globe, throughout the year, causing huge loss to the productivity of crop species. Being sessile, the plants respond to the crisis using a wide range of defense responsive elements which at the transcriptional level, includes the transcription factors, like WRKY, NAC, AP2/ERF, MYB, MYC, bZIP, bHLH and others. Increasing evidences suggest a pivotal role of WRKY TFs in the regulation of plant defense response against herbivory in Arabidopsis and other important crop plants. Unlike pathogenic attack, very limited information is available on the role of different WRKYs in herbivore-plant interaction. In this review, we will thus majorly focus on the role of WRKY transcription factors and its regulation in response to herbivore attack while summarizing the recent developments from the field of transcriptional, post-transcriptional and epigenetic regulation of WRKYs in response to herbivory attack with the prospects for future implementation in research.

Growth promotion by endosymbiont Piriformospora indica has been observed in various plants; however, specific functional metabolites involved in P. indica mediated growth promotion are unknown. A GC-MS based untargeted metabolite analysis was used to identify Solanum lycopersicum metabolites altered during P. indica mediated growth promotion. Metabolomic analysis showed primary metabolites altered and specifically putrescine to be maximally induced in roots during the interaction. P. indica induced putrescine biosynthetic gene SlADC1 in S. lycopersicum and acts via arginine decarboxylase (ADC) mediated pathway. P. indica did not promote growth in Sladc-VIGS (virus induced gene silencing of SlADC gene) lines of S. lycopersicum and when the ADC enzyme was inhibited with an inhibitor, DL-α-(Difluoromethyl) arginine. In Arabidopsis adc knock-out mutants, P. indica do not promote growth and this response was rescued upon exogenous application of putrescine. Putrescine promoted growth by elevation of auxin (indole-3-acetic acid) and gibberellin (GA4, GA7) levels in S. lycopersicum. Putrescine is also important for P. indica hyphal growth indicating that it is co-adapted by both host and microbe. Hence, we conclude that putrescine is an essential metabolite and its biosynthesis in plants is crucial for P. indica mediated growth promotion and fungal growth.

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