ultivated peanut or groundnut (A. hypogaea L.) is among the most important oil and food legumes, grown on 25 million ha between latitudes 40° N and 40° S with annual production of ~46 million tons (http://www.fao.org/faostat/en/#home). It presumably was domesticated in South America ~6,000 years ago and then was widely distributed in post-Columbian times 1. Combining richness in seed oil (~46-58%) and protein (~22-32%), peanut is important in fighting malnutrition and ensuring food security.

Xanthomonas oryzae pv. oryzicola, the causal agent of bacterial leaf streak in the model plant rice, possesses a hypersensitive response and pathogenicity (hrp), hrp-conserved (hrc), hrp-associated (hpa) cluster (hrp-hrc-hpa) that encodes a type III secretion system (T3SS) through which T3SS effectors are injected into host cells to cause disease or trigger plant defenses. Mutations in this cluster usually abolish the bacterial ability to cause hypersensitive response in nonhost tobacco and pathogenicity in host rice. In Xanthomonas spp., these genes are generally assumed to be regulated by the key master regulators HrpG and HrpX. However, we present evidence that, apart from HrpG and HrpX, HrpD6 is also involved in regulating the expression of hrp genes. Interestingly, the expression of hpa2, hpa1, hpaB, hrcC, and hrcT is positively controlled by HrpD6. Transcriptional expression assays demonstrated that the expression of the hrcC, hrpD5, hrpE, and hpa3 genes was not completely abolished by hrpG and hrpX mutations. As observed in analysis of their corresponding mutants, HrpG and HrpX exhibit contrasting gene regulation, particularly for hpa2 and hrcT. Other two-component system regulators (Zur, LrpX, ColR/S, and Trh) did not completely inhibit the expression of hrcC, hrpD5, hrpE, and hpa3. Immunoblotting assays showed that the secretion of HrpF, which is an HpaB-independent translocator, is not affected by the mutation in hrpD6. However, the mutation in hrpD6 affects the secretion of an HpaB-dependent TAL effector, AvrXa27. These novel findings suggest that, apart from HrpG and HrpX, HrpD6 plays important roles not only in the regulation of hrp genes but also in the secretion of TAL effectors.

Xanthomonas oryzae pv. oryzicola, the causative agent of bacterial leaf streak, injects a plethora of effectors through the type III secretion system (T3SS) into rice cells to cause disease. The T3SS, encoded by the hrp genes, is essential for the pathogen to elicit the hypersensitive response (HR) in nonhost tobacco and for pathogenicity in host rice. Whether or not a putative lytic transglycosylase, Hpa2, interacts with a translocon protein, HrpF, to facilitate bacterial pathogenicity remains unknown. Here we demonstrated that both the hpa2 and hrpF genes are required for the pathogenicity of X. oryzae pv. oryzicola strain RS105 in rice but not for HR induction in tobacco. The expression of hpa2 was positively regulated by HrpG and HrpD6 but not by HrpX. In vivo secretion and subcellular localization analyses confirmed that Hpa2 secretion is dependent on HpaB (a T3SS exit protein) and that Hpa2 binds to the host cell membrane. Protein-protein assays demonstrated that Hpa2 interacts with HrpF. In planta translocation of AvrXa10 indicated that the mutation in hpa2 and hrpF inhibits the injection of the HpaB-dependent transcriptional activator-like (TAL) effector into rice. These findings suggest that Hpa2 and HrpF form a complex to translocate T3S effectors into plant cells for pathogenesis in host rice.Xanthomonas oryzae pv. oryzicola, the causative agent of bacterial leaf streak disease in rice, is one of the model organisms for studying the molecular mechanisms of plant-pathogen pathosystems (41, 59). The bacterial ability to trigger the hypersensitive response (HR), a rapid and localized programmed cell death in nonhosts or in resistant hosts, and to be pathogenic in host plants depends on a type III secretion system (T3SS) encoded by a 27-kb hrp cluster containing 10 hrp, 9 hrc (hrp-conserved), and 8 hpa (hrp-associated) genes according to the homologous regions in other Xanthomonas species (9,41,59). Some of the hrp-hrc-hpa gene products comprise a pedestal-like T3SS structure that traverses the two bacterial membranes (21, 24), a pilus-like secretion channel (HrpE) outside HrcC (52), and a translocon protein (HrpF) in the eukaryotic host membrane (3, 4, 6, 21, 48). As a whole, the T3SS apparatus injects a number of effectors into the apoplast and cytosol of eukaryotic host cells, including harpins, which elicit HR induction in nonhost apoplasts (4, 45, 58), and transcriptional activator-like (TAL) effectors, which lead to disease susceptibility in hosts or trigger disease resistance in nonhosts upon interaction with a specific R gene product surveillance system (4,11,38,39,40,49,56). The virulence of Xanthomonas oryzae pv. oryzae is markedly reduced when hrpF is mutated (48). However, hrpF mutation has not been investigated in X. oryzae pv. oryzicola.The hpa genes contribute to virulence, but strains with mutations in hpa genes generally do not exhibit phenotypic changes in disease symptoms of the same severity as those with other hrp-hrc gene mutations (9,25,29). Some Hpa proteins, such as HpaB and HpaC ...

Plant pattern recognition receptors (PRRs) perceive pathogen‐associated molecular patterns (PAMPs) to activate immune responses. Medium‐chain 3‐hydroxy fatty acids (mc‐3‐OH‐FAs), which are widely present in Gram‐negative bacteria, were recently shown to be novel PAMPs in Arabidopsis thaliana. The Arabidopsis PRR LIPOOLIGOSACCHARIDE‐SPECIFIC REDUCED ELICITATION (LORE) is a G‐type lectin receptor‐like kinase that recognizes mc‐3‐OH‐FAs and subsequently mounts an immune response; however, the mechanisms underlying LORE activation and downstream signaling are unexplored. Here, we report that one of the mc‐3‐OH‐FAs, 3‐OH‐C10:0, induces phosphorylation of LORE at tyrosine residue 600 (Y600). Phosphorylated LORE subsequently trans‐phosphorylates the receptor‐like cytoplasmic kinase PBL34 and its close paralogs, PBL35 and PBL36, and therefore activates plant immunity. Phosphorylation of LORE Y600 is required for downstream phosphorylation of PBL34, PBL35, and PBL36. However, the Pseudomonas syringae effector HopAO1 targets LORE, dephosphorylating the tyrosine‐phosphorylated Y600 and therefore suppressing the immune response. These observations uncover the mechanism by which LORE mediates signaling in response to 3‐OH‐C10:0 in Arabidopsis.