Transcriptome Profiling of Barley Cultivar Hua 30 MDEC in Response to <i>Agrobacterium</i> Infection

Agrobacterium-mediated transformation has been widely used in plants. However, the mechanism in plant cells’ response to Agrobacterium infection was very complex. The mechanism of the determinants in host cell remains obscure, especially in barley, which is recalcitrant for Agrobacterium-mediated transformation. In the present study, microspore-derived embryogenic calli (MDEC) from barley elite cultivar were employed as unique subjects to characterize the mechanisms during the Agrobacterium infection process. Hua 30 MDEC can be successfully infected by Agrobacterium. RNA-sequencing at different infection points (0, 2, 6, 12, 24 hpi) was performed. The average expressional intensity of the whole genomics increased from 0 to 2 hpi, and then decreased subsequently. More upregulated than downregulated differentially expressed genes (DEGs) were counted at the same time. GO enrichment analysis showed that protein modification was significantly overrepresented in upregulated DEGs. Chromosome-related biological processes, gene expression and cellular metabolic processes were significantly overrepresented in downregulated DEGs. KEGG analysis showed that plant defense responses, phenylpropanoid biosynthesis and biosynthesis of amino acids were significantly enriched across the infection time course. Nine DEGs related to defense responses were identified. All DEGs were upregulated from 2 to 24 hpi. We speculate that these genes are possibly related to Agrobacterium infection. These findings will provide deep insights into the molecular events occurring during the process of Agrobacterium-mediated transformation.     

Tolerant and Susceptible Sesame Genotypes Reveal Waterlogging Stress Response Patterns

Waterlogging is a common adverse environmental condition that limits plant growth. Sesame (Sesamum indicum) is considered a drought-tolerant oil crop but is typically susceptible to harmful effects from waterlogging. The present study used comparative analysis to explore the waterlogging stress response associated with two sesame genotypes. The RNA-seq dataset generated during a time course of 0, 3, 9 and 15 h of waterlogging as well as 20 h post-drainage indicated that stress gradually suppressed the expression of sesame genes, with 9 h as the critical time point for the response of sesame to waterlogging stress. Of the 19,316 genes expressed during waterlogging, 72.1% were affected significantly. Sesame of both tolerant and susceptible genotypes showed decreased numbers of upregulated differentially expressed genes (DEGs) but increased numbers of downregulated DEGs at the onset of waterlogging. However, the tolerant-genotype sesame exhibited 25.5% more upregulated DEGs and 29.7% fewer downregulated DEGs than those of the susceptible-genotype strain between 3 and 15 h. The results indicated that the tolerant sesame displayed a more positive gene response to waterlogging. A total of 1,379 genes were significantly induced and commonly expressed in sesame under waterlogging conditions from 3 to 15 h regardless of tolerance level; of these genes, 98 are known homologous stress responsive genes, while the remaining 1,281 are newly reported here. This gene set may represent the core genes that function in response to waterlogging, including those related mainly to energy metabolism and phenylpropanoid biosynthesis. Furthermore, a set of 3,016 genes functioning in energy supply and cell repair or formation was activated in sesame recovery from waterlogging stress. A comparative analysis between sesame of the tolerant and susceptible genotypes revealed 66 genes that may be candidates for improving sesame tolerance to waterlogging. This study provided a comprehensive picture of the sesame gene expression pattern in response to waterlogging stress. These results will help dissect the mechanism of the sesame response to waterlogging and identify candidate genes to improve its tolerance.     

Transcriptome analysis provides insights into lignin synthesis and MAPK signaling pathway that strengthen the resistance of bitter gourd (Momordica charantia) to Fusarium wilt

Fusarium wilt is a typical soil-borne disease caused by Fusarium oxysporum f. sp. momordicae (FOM) in bitter gourd. In this study, by comparing sequencing data at multiple time points and considering the difference between resistant (R) and susceptible (S) varieties, differentially expressed genes were screened out. Short time-series expression miner analysis revealed the upregulated expression trend of genes, which were enriched in phenylpropanoid biosynthesis, plant–pathogen interaction, and mitogen-activated protein kinase signaling pathway. Further, observation of the microstructure revealed that the R variety may form tyloses earlier than the S variety to prevent mycelium diffusion from the xylem vessel. After Fusarium wilt infection, the enzymatic activities of superoxide dismutase, peroxidase, phenylalanine ammonia lyase, and catalaseas well as levels of superoxide anion and malondialdehyde were increased in the R variety higher than those in the S variety. This study provides a reference to elucidate the disease resistance mechanism of bitter gourd.     

Expression of the Phytophthora infestans ipiB and ipi0 genes in planta and in vitro

The ipiB and ipiO genes of the potato late blight fungus Phytophthora infestans (Mont.) de Bary were isolated from a genomic library in a screen for genes induced in planta. Expression of these genes was studied during pathogenesis on various host tissues and different host plants, some of which show specific resistance against P. infestans infection. During pathogenesis on leaves and tubers of the fully susceptible potato cultivar (cv.) Ajax and on leaves of the fully susceptible tomato cv. Moneymaker, the P. infestans ipiB and ipiO genes show a transient expression pattern with highest mRNA levels in the early stages of infection. During the interaction with leaves of the partially resistant potato cv. Pimpernel, the expression is also transient but accumulation and disappearance of the mRNAs is delayed. Also in P. infestans inoculated onto a race-specific resistant potato cultivar and onto the nonhost Solanum nigrum, ipiB and ipiO mRNA is detectable during the initial stages of infection. Apparently, the expression of the ipiB and the ipiO genes is activated in compatible, incompatible and nonhost interactions. In encysted zoospores, ipiB and ipiO mRNA accumulation was not detectable, but during cyst germination and appressorium formation on an artificial surface the genes are highly expressed. Expression studies in mycelium grown in vitro revealed that during nutrient starvation the expression of the ipiB and ipiO genes is induced. For ipiO gene expression, carbon deprivation appeared to be sufficient. The ipiO gene promoters contain a sequence motif that functions as a glucose repression element in yeast and this motif might be involved in the regulation of ipiO gene expression.     

Plant integrity: An important factor in plant-pathogen interactions

The effect of plant integrity and of aboveground-belowground defense signaling on plant resistance against pathogens and herbivores is emerging as a subject of scientific research. There is increasing evidence that plant defense responses to pathogen infection differ between whole intact plants and detached leaves. Studies have revealed the importance of aboveground-belowground defense signaling for plant defenses against herbivores, while our studies have uncovered that the roots as well as the plant integrity are important for the resistance of the potato cultivar Sarpo Mira against the hemibiotrophic oomycete pathogen Phytophthora infestans. Furthermore, in the Sarpo Mira–P. infestans interactions, the plant’s meristems, the stalks or both, seem to be associated with the development of the hypersensitive response and both the plant’s roots and shoots contain antimicrobial compounds when the aerial parts of the plants are infected. Here, we present a short overview of the evidence indicating the importance of plant integrity on plant defense responses.     

The receptor-like kinase SERK3/BAK1 is required for basal resistance against the late blight pathogen phytophthora infestans in Nicotiana benthamiana

Background

The filamentous oomycete plant pathogen Phytophthora infestans causes late blight, an economically important disease, on members of the nightshade family (Solanaceae), such as the crop plants potato and tomato. The related plant Nicotiana benthamiana is a model system to study plant-pathogen interactions, and the susceptibility of N. benthamiana to Phytophthora species varies from susceptible to resistant. Little is known about the extent to which plant basal immunity, mediated by membrane receptors that recognise conserved pathogen-associated molecular patterns (PAMPs), contributes to P. infestans resistance.

Principal Findings

We found that different species of Phytophthora have varying degrees of virulence on N. benthamiana ranging from avirulence (incompatible interaction) to moderate virulence through to full aggressiveness. The leucine-rich repeat receptor-like kinase (LRR-RLK) BAK1/SERK3 is a major modulator of PAMP-triggered immunity (PTI) in Arabidopsis thaliana and N. benthamiana. We cloned two NbSerk3 homologs, NbSerk3A and NbSerk3B, from N. benthamiana based on sequence similarity to the A. thaliana gene. N. benthamiana plants silenced for NbSerk3 showed markedly enhanced susceptibility to P. infestans infection but were not altered in resistance to Phytophthora mirabilis, a sister species of P. infestans that specializes on a different host plant. Furthermore, silencing of NbSerk3 reduced the cell death response triggered by the INF1, a secreted P. infestans protein with features of PAMPs.

Conclusions/Significance

We demonstrated that N. benthamiana NbSERK3 significantly contributes to resistance to P. infestans and regulates the immune responses triggered by the P. infestans PAMP protein INF1. In the future, the identification of novel surface receptors that associate with NbSERK3A and/or NbSERK3B should lead to the identification of new receptors that mediate recognition of oomycete PAMPs, such as INF1.     

Inoculation of susceptible and resistant potato plants with the late blight pathogen Phytophthora infestans: effects on an aphid and its parasitoid

Plants are exposed to microbial pathogens as well as herbivorous insects and their natural enemies. Here, we examined the effects of inoculation of potato plants, Solanum tuberosum L. (Solanaceae), with the late blight pathogen Phytophthora infestans (Mont.) de Bary (Peronosporales: Pythiaceae) on an aphid species commonly infesting potato crops and one of the aphid's major parasitoids. We observed the peach‐potato aphid, Myzus persicae Sulzer (Hemiptera: Aphididae), and its natural enemy, the biocontrol agent Aphidius colemani Viereck (Hymenoptera: Braconidae), on potato either inoculated with water or P. infestans. Population growth of the aphid, parasitism rate of its natural enemy, and other insect life‐history traits were compared on several potato genotypes, the susceptible cultivar Désirée and genetically modified (GM) isogenic lines carrying genes conferring resistance to P. infestans. Effects of P. infestans inoculation on the intrinsic rate of aphid population increase and the performance of the parasitoid were only found on the susceptible cultivar. Insect traits were similar when comparing inoculated with non‐inoculated resistant GM genotypes. We also tested how GM‐plant characteristics such as location of gene insertion and number of R genes could influence non‐target insects by comparing insect performance among GM events. Different transformation events leading to different positions of R‐gene insertion in the genome influenced aphids either with or without P. infestans infection, whereas effects of position of R‐gene insertion on the parasitoid A. colemani were evident only in the presence of inoculation with P. infestans. We conclude that it is important to study different transformation events before continuing with further stages of risk assessment of this GM crop. This provides important information on the effects of plant resistance to a phytopathogen on non‐target insects at various trophic levels.     

RNA sequencing analysis provides new insights into dynamic molecular responses to <Emphasis Type="Italic">Valsa mali</Emphasis> pathogenicity in apple ‘Changfu No. 2’

Valsa canker caused by the necrotrophic pathogen Valsa mali (Vm) severely affects apple production in Eastern Asia. The molecular basis underlying the apple response to Vm infection is poorly understood. Hence, we performed RNA sequencing (RNA-seq) to investigate the dynamic gene expression profiles of a major apple cultivar, ‘Changfu No.2’, during Vm infection. Compared with the control (C), 104, 313, and 1059 differentially expressed genes (DEGs) were detected from the phloem tissue within the range of 0.9–1.3 cm (T1), 0.5–0.9 cm (T2), and 0.1–0.5 cm (T3) beyond the lesion periphery, respectively. Gene ontology (GO) enrichment analysis revealed that the DEGs associated with plant growth and development were down-regulated, whereas those related to defense responses were up-regulated. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that hormonal and Ca²⁺ signaling and phenylpropanoid biosynthesis were involved in the defense responses. In conclusion, multiple defense responses associated with ABA, JA, ET, Ca²⁺, and cell wall signals contributed to the defense against Vm infection in ‘Changfu No.2’. In contrast, the DEGs with inhibited expression were involved in plant growth and development; auxin signaling and several resistance genes might weaken the resistance of ‘Changfu No.2’ to pathogens. Our results offer a new insight into plant responses against necrotrophs and could benefit programs aimed at breeding for Vm resistance.