Of the three classes of enzymes involved in ubiquitination, ubiquitin-conjugating enzymes (E2) have been often incorrectly considered to play merely an auxiliary role in the process, and few E2 enzymes have been investigated in plants. To reveal the role of E2 in plant innate immunity, we identified and cloned 40 tomato genes encoding ubiquitin E2 proteins. Thioester assays indicated that the majority of the genes encode enzymatically active E2. Phylogenetic analysis classified the 40 tomato E2 enzymes into 13 groups, of which members of group III were found to interact and act specifically with AvrPtoB, a Pseudomonas syringae pv tomato effector that uses its ubiquitin ligase (E3) activity to suppress host immunity. Knocking down the expression of group III E2 genes in Nicotiana benthamiana diminished the AvrPtoB-promoted degradation of the Fen kinase and the AvrPtoB suppression of host immunity-associated programmed cell death. Importantly, silencing group III E2 genes also resulted in reduced pattern-triggered immunity (PTI). By contrast, programmed cell death induced by several effector-triggered immunity elicitors was not affected on group III-silenced plants. Functional characterization suggested redundancy among group III members for their role in the suppression of plant immunity by AvrPtoB and in PTI and identified UBIQUITIN-CONJUGATING11 (UBC11), UBC28, UBC29, UBC39, and UBC40 as playing a more significant role in PTI than other group III members. Our work builds a foundation for the further characterization of E2s in plant immunity and reveals that AvrPtoB has evolved a strategy for suppressing host immunity that is difficult for the plant to thwart.Ubiquitination as a major posttranslational modification of proteins in eukaryotes has emerged in recent years as an important regulatory mechanism underlying plant innate immunity Fu et al., 2012;Marino et al., 2012;Li et al., 2014). The ubiquitination process involves a consecutive, threestep enzymatic cascade that is catalyzed by three different classes of enzymes: ubiquitin-activating (E1), ubiquitin-conjugating (E2), and ubiquitin ligase (E3) enzymes. The first step of the process activates ubiquitin, a highly conserved 76-amino acid protein, in an ATP-dependent manner by attaching ubiquitin to an E1 enzyme. The activated ubiquitin is then transferred from the E1 to the Cys residue at the active site of an E2 conjugating enzyme. An E3 ligase then recruits the substrate protein to the E2 to transfer the ubiquitin molecule from E2 to the substrate. Through the action of E1, E2, and E3, ubiquitin is covalently attached usually to the Lys residue of a substrate through an isopeptide bond (Hershko and Ciechanover, 1998). Of the three enzymes involved in ubiquitination, E3 ubiquitin ligases have been the focus of many studies due to their key role in determining substrate specificity for the ubiquitination

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SUMMARYSporamin is a tuberous storage protein with trypsin inhibitory activity in sweet potato (Ipomoea batatas Lam.), which accounts for 85% of the soluble protein in tubers. It is constitutively expressed in tuberous roots but is expressed in leaves only after wounding. Thus far, its wound-inducible signal transduction mechanisms remain unclear. In the present work, a 53-bp DNA region, sporamin wound-response ciselement (SWRE), was identified in the sporamin promoter and was determined to be responsible for the wounding response. Using yeast one-hybrid screening, a NAC domain protein, IbNAC1, that specifically bound to the 5 0 -TACAATATC-3 0 sequence in SWRE was isolated from a cDNA library from wounded leaves.IbNAC1 was constitutively expressed in root tissues and was induced earlier than sporamin following the wounding of leaves. Transgenic sweet potato plants overexpressing IbNAC1 had greatly increased sporamin expression, increased trypsin inhibitory activity, and elevated resistance against Spodoptera litura. We further demonstrated that IbNAC1 has multiple biological functions in the jasmonic acid (JA) response, including the inhibition of root formation, accumulation of anthocyanin, regulation of aging processes, reduction of abiotic tolerance, and overproduction of reactive oxygen species (ROS). Thus, IbNAC1 is a core transcription factor that reprograms the transcriptional response to wounding via the JA-mediated pathway in sweet potato.

Sweet potato (Ipomoea batatas [L.] Lam) is an important food crop, an excellent fodder crop, and a new type of industrial raw material crop. The lack of genomic resources could affect the process of industrialization of sweet potato. Few detailed reports have been completed on the mitochondrial genome of sweet potato. In this research, we sequenced and assembled the mitochondrial genome of sweet potato and investigated its substructure. The mitochondrial genome of sweet potato is 270,304 bp with 23 unique core genes and 12 variable genes. We detected 279 pairs of repeat sequences and found that three pairs of direct repeats could mediate the homologous recombination into four independent circular molecules. We identified 70 SSRs in the whole mitochondrial genome of sweet potato. The longest dispersed repeat in mitochondrial genome was a palindromic repeat with a length of 915 bp. The homologous fragments between the chloroplast and mitochondrial genome account for 7.35% of the mitochondrial genome. We also predicted 597 RNA editing sites and found that the rps3 gene was edited 54 times, which occurred most frequently. This study further demonstrates the existence of multiple conformations in sweet potato mitochondrial genomes and provides a theoretical basis for the evolution of higher plants and cytoplasmic male sterility breeding.