Transcriptome analysis of root-knot nematode (Meloidogyne incognita)-resistant and susceptible sweetpotato cultivars.

MAIN CONCLUSION: Transcriptome analysis was performed on the roots of susceptible and resistant sweetpotato cultivars infected with the major root-knot nematode species Meloidogyne incognita. In addition, we identified a transcription factor-mediated defense signaling pathway that might function in sweetpotato-nematode interactions. Root-knot nematodes (RKNs, Meloidogyne spp.) are important sedentary endoparasites of many agricultural crop plants that significantly reduce production in field-grown sweetpotato. To date, no studies involving gene expression profiling in sweetpotato during RKN infection have been reported. Therefore, in the present study, transcriptome analysis was performed on the roots of susceptible (cv. Yulmi) and resistant (cv. Juhwangmi) sweetpotato cultivars infected with the widespread, major RKN species Meloidogyne incognita. Using the Illumina HiSeq 2000 platform, we generated 455,295,628 pair-end reads from the fibrous roots of both cultivars, which were assembled into 74,733 transcripts. A number of common and unique genes were differentially expressed in susceptible vs. resistant cultivars as a result of RKN infection. We assigned the differentially expressed genes into gene ontology categories and used MapMan annotation to predict their functional roles and associated biological processes. The candidate genes including hormonal signaling-related transcription factors and pathogenesis-related genes that could contribute to protection against RKN infection in sweetpotato roots were identified and sweetpotato-nematode interactions involved in resistance are discussed.

Transcriptomic changes in sweetpotato peroxidases in response to infection with the root-knot nematode Meloidogyne incognita.

A previous transcriptomic analysis of the roots of susceptible and resistant cultivars of sweetpotato (Ipomoea batatas) identified genes that were likely to contribute to protection against infection with the root-knot nematode Meloidogyne incognita. The current study examined the roles of peroxidase genes in sweetpotato defense responses during root-knot nematode infection, using the susceptible (cv. Yulmi) and resistant (cv. Juhwangmi) cultivars. Differentially expressed genes were assigned to gene ontology categories to predict their functional roles and associated biological processes. Comparison with Arabidopsis peroxidases identified a group of genes orthologous to Arabidopsis PEROXIDASE 52 (AtPrx52). An analysis of sweetpotato peroxidase genes determined their roles in protecting plants against root-knot nematode infection and enabled identification of important peroxidases. The interactions involved in sweetpotato resistance to nematode infection are discussed.

Differential Responses of Antioxidant Enzymes and Lignin Metabolism in Susceptible and Resistant Sweetpotato Cultivars during Root-Knot Nematode Infection.

Root-knot nematodes (RKN) cause significant damage to sweetpotato plants and cause significant losses in yield and quality. Reactive oxygen species (ROS) play an important role in plant defenses, with levels of ROS-detoxifying antioxidant enzymes tightly regulated during pathogen infection. In this study, ROS metabolism was examined in three RKN-resistant and three RKN-susceptible sweetpotato cultivars. The antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) were assessed, as was lignin-related metabolism. In RKN-infected roots, both resistant and susceptible cultivars increased SOD activity to produce higher levels of hydrogen peroxide (H2O2). However, H2O2 removal by CAT activity differed between cultivars, with susceptible cultivars having higher CAT activity and lower overall H2O2 levels. In addition, the expression of phenylpropanoid-related phenylalanine ammonia-lyase and cinnamyl alcohol dehydrogenase genes, which encode enzymes involved in lignin metabolism, were higher in resistant cultivars, as were total phenolic and lignin contents. Enzyme activities and H2O2 levels were examined during the early (7 days) and late (28 days) phases of infection in representative susceptible and resistant cultivars, revealing contrasting changes in ROS levels and antioxidant responses in the different stages of infection. This study suggests that differences in antioxidant enzyme activities and ROS regulation in resistant and susceptible cultivars might explain reduced RKN infection in resistant cultivars, resulting in smaller RKN populations and overall higher resistance to infection and infestation by RKNs.

Flooding Tolerance in Sweet Potato (Ipomoea batatas (L.) Lam) Is Mediated by Reactive Oxygen Species and Nitric Oxide.

Flooding is harmful to almost all higher plants, including crop species. Most cultivars of the root crop sweet potato are able to tolerate environmental stresses such as drought, high temperature, and high salinity. They are, however, relatively sensitive to flooding stress, which greatly reduces yield and commercial value. Previous transcriptomic analysis of flood-sensitive and flood-resistant sweet potato cultivars identified genes that were likely to contribute to protection against flooding stress, including genes related to ethylene (ET), reactive oxygen species (ROS), and nitric oxide (NO) metabolism. Although each sweet potato cultivar can be classified as either tolerant or sensitive to flooding stress, the molecular mechanisms of flooding resistance in ET, ROS, and NO regulation-mediated responses have not yet been reported. Therefore, this study characterized the regulation of ET, ROS, and NO metabolism in two sweet potato cultivars-one flood-tolerant cultivar and one flood-sensitive cultivar-under early flooding treatment conditions. The expression of ERFVII genes, which are involved in low oxygen signaling, was upregulated in leaves during flooding stress treatments. In addition, levels of respiratory burst oxidase homologs and metallothionein-mediated ROS scavenging were greatly increased in the early stage of flooding in the flood-tolerant sweet potato cultivar compared with the flood-sensitive cultivar. The expression of genes involved in NO biosynthesis and scavenging was also upregulated in the tolerant cultivar. Finally, NO scavenging-related MDHAR expressions and enzymatic activity were higher in the flood-tolerant cultivar than in the flood-sensitive cultivar. These results indicate that, in sweet potato, genes involved in ET, ROS, and NO regulation play an important part in response mechanisms against flooding stress.

The Defense Response Involved in Sweetpotato Resistance to Root-Knot Nematode Meloidogyne incognita: Comparison of Root Transcriptomes of Resistant and Susceptible Sweetpotato Cultivars With Respect to Induced and Constitutive Defense Responses.

Sweetpotato (Ipomoea batatas [L.] Lam) is an economically important, nutrient- and pigment-rich root vegetable used as both food and feed. Root-knot nematode (RKN), Meloidogyne incognita, causes major yield losses in sweetpotato and other crops worldwide. The identification of genes and mechanisms responsible for resistance to RKN will facilitate the development of RKN resistant cultivars not only in sweetpotato but also in other crops. In this study, we performed RNA-seq analysis of RKN resistant cultivars (RCs; Danjami, Pungwonmi and Juhwangmi) and susceptible cultivars (SCs; Dahomi, Shinhwangmi and Yulmi) of sweetpotato infected with M. incognita to examine the induced and constitutive defense response-related transcriptional changes. During induced defense, genes related to defense and secondary metabolites were induced in SCs, whereas those related to receptor protein kinase signaling and protein phosphorylation were induced in RCs. In the uninfected control, genes involved in proteolysis and biotic stimuli showed differential expression levels between RCs and SCs during constitutive defense. Additionally, genes related to redox regulation, lipid and cell wall metabolism, protease inhibitor and proteases were putatively identified as RKN defense-related genes. The root transcriptome of SCs was also analyzed under uninfected conditions, and several potential candidate genes were identified. Overall, our data provide key insights into the transcriptional changes in sweetpotato genes that occur during induced and constitutive defense responses against RKN infection.

Expression of both CuZnSOD and APX in chloroplasts enhances tolerance to sulfur dioxide in transgenic sweet potato plants.

We have previously reported that transgenic sweet potato (Ipomoea batatas) plants overexpressing both CuZn superoxide dismutase (CuZnSOD) and ascorbate peroxidase (APX) under the control of a stress-inducible SWPA2 promoter in chloroplasts (referred to as SSA plants) showed increased resistance to methyl viologen-mediated oxidative stress and chilling. To investigate whether SSA plants show enhanced tolerance to air pollutants, they were exposed to 500ppb of sulfur dioxide (SO2). SO2 caused visible damage to the leaves of sweet potato, but damage in the leaves of non-transgenic (NT) plants was more severe than in those of SSA plants. The photosynthetic activity (Fv/Fm) of the SSA plants decreased by only 7% on the 5th day after the treatment, whereas that of NT plants severely decreased by 63% after 5days of recovery. Moreover, the chlorophyll content in the oldest leaf of NT plants decreased by 69%, whereas that of SSA plants remained at a high level. APX activity in NT plants increased about three times under an SO2 stress, and in SSA plants about five times compared to the case with no stress conditions. These results suggest that the overexpression of both CuZnSOD and APX in chloroplasts reduces the oxidative stress derived from SO2.

Drought stress-induced compositional changes in tolerant transgenic rice and its wild type.

Comparing well-watered versus deficit conditions, we evaluated the chemical composition of grains harvested from wild-type (WT) and drought-tolerant, transgenic rice (Oryza sativa L.). The latter had been developed by inserting AtCYP78A7, which encodes a cytochrome P450 protein. Two transgenic Lines, '10B-5' and '18A-4', and the 'Hwayoung' WT were grown under a rainout shelter. After the harvested grains were polished, their levels of key components, including proximates, amino acids, fatty acids, minerals and vitamins were analysed to determine the effect of watering system and genotype. Drought treatment significantly influenced the levels of some nutritional components in both transgenic and WT grains. In particular, the amounts of lignoceric acid and copper in the WT decreased by 12.6% and 39.5%, respectively, by drought stress, whereas those of copper and potassium in the transgenics rose by 88.1-113.3% and 10.4-11.9%, respectively, under water-deficit conditions.

Host acceptance by aphids: probing and larviposition behaviour of the bird cherry-oat aphid, Rhopalosiphum padi on host and non-host plants.

The probing and larviposition behaviour of the bird cherry-oat aphid, Rhopalosiphum padi on summer and winter host plants were investigated using electrical penetration graph (EPG) coupled with simultaneous video recording. In this way the precise probing history prior to parturition can be monitored and the location of possible reproductive stimulants identified. On the host plant, all gynoparae (autumn winged females that give birth to sexual females on bird cherry, Prunus padus, the primary host) and 55% of winged virginoparae (summer females which produce further virginoparae on barley, Hordeum vulgare, a secondary host) initiated larviposition before phloem contact. However, 90% of wingless virginoparae (on barley) contacted the phloem before first larviposition whilst 10% did not. Thus, phloem contact does not appear to be a pre-requisite for these aphid forms to initiate reproduction.