Plant pathogens, such as bacteria, fungi, oomycetes and nematodes, rely on wide range of virulent effectors delivered into host cells to suppress plant immunity. Although phytobacterial effectors have been intensively investigated, little is known about the function of effectors of plant-parasitic nematodes, such as Globodera pallida, a cyst nematode responsible for vast losses in the potato and tomato industries. Here, we demonstrate using in vivo and in vitro ubiquitination assays the potato cyst nematode (Globodera pallida) effector RHA1B is an E3 ubiquitin ligase that employs multiple host plant E2 ubiquitin conjugation enzymes to catalyze ubiquitination. RHA1B was able to suppress effector-triggered immunity (ETI), as manifested by suppression of hypersensitive response (HR) mediated by a broad range of nucleotide-binding leucine-rich repeat (NB-LRR) immune receptors, presumably via E3-dependent degradation of the NB-LRR receptors. RHA1B also blocked the flg22-triggered expression of Acre31 and WRKY22, marker genes of pathogen‐associated molecular pattern (PAMP)‐triggered immunity (PTI), but this did not require the E3 activity of RHA1B. Moreover, transgenic potato overexpressing the RHA1B transgene exhibited enhanced susceptibility to G. pallida. Thus, our data suggest RHA1B facilitates nematode parasitism not only by triggering degradation of NB-LRR immune receptors to block ETI signaling but also by suppressing PTI signaling via an as yet unknown E3-independent mechanism. Globodera pallida is a plant-parasitic cyst nematode that causes vast losses in economically important crops such as potato and tomato. To successfully parasitize host plants, G. pallida produces proteins called effectors to overcome plant defenses. Here, we report identification of a novel G. pallida effector RHA1B as an E3 ubiquitin ligase, which is responsible for ubiquitin-proteasome-mediated protein degradation in general. We found that RHA1B can suppress plant defense signaling via both E3-dependent and -independent manners. In particular, it promotes degradation of a broad range of NB-LRR immune receptors. In addition, expression of RHA1B in potato plants made the plants more susceptible to G. pallida infection, indicating that RHA1B acts as an effector that aids parasitism. Overall, we found RHA1B as the first effector with ubiquitin ligase activity identified from eukaryotic pathogen infecting plants or animals. Our data suggest nematode uses RHA1B as a powerful weapon to manipulate host cellular signaling pathways, thereby interfering with plant immunity for successful parasitism.

In the United States, potato cyst nematodes Globodera rostochiensis and G. pallida are quarantined pests. A new cyst nematode species, Globodera ellingtonae, discovered in Oregon and Idaho, reproduces well on potato but is not currently a quarantine pest. Identifying resistance to all three Globodera spp. would provide a valuable management tool. Thirteen breeding clones and nine cultivars were evaluated in Oregon, Idaho, and New York laboratories where the nematode populations are maintained. Minitubers or tissue culture plants were planted into pots and inoculated with eggs in replicated experiments. Results indicated that five entries were partially resistant or resistant to all three species, while another five were resistant or partially resistant to G. rostochiensis and G. ellingtonae. Resistance to G. rostochiensis pathotypes Ro1 and Ro4 is controlled by the H1 gene and this study suggests that H1 may confer resistance to G. ellingtonae as well. Observed resistance to G. pallida was lower relative to the levels of resistance observed for G. rostochiensis and G. ellingtonae. Germplasm with G. pallida or G. ellingtonae resistance will be used in hybridizations to develop russet-skinned cultivars with long tubers which represent the predominant market class in western U.S. production, and to further explore the basis of potato resistance to Globodera spp.

The eradication program for the potato cyst nematode (PCN), Globodera pallida, in the Northwest of the United States revolves around the use of soil fumigation. Alternative, integrated strategies are needed to continue to battle this invasive nematode. Laboratory, greenhouse, and field experiments were conducted with G. pallida and another cyst nematode found in the United States, Globodera ellingtonae, to evaluate the efficacy of a new formulated Brassica juncea seed meal extract, as well as a traditional B. juncea seed meal, as alternate eradication strategies. This is the first report on the efficacy of B. juncea seed meal extract against plant-parasitic nematodes. Rates of B. juncea seed meal greater than 2.2 t/ha and 4.5 t/ha for G. pallida and G. ellingtonae, respectively, were required for egg hatch suppression, as determined by a potato root diffusate (PRD) bioassay. Reproduction of G. pallida on potato after exposure to B. juncea seed meal at a rate of 2.2 t/ha was also significantly reduced. In the field, 8.9 t/ha B. juncea seed meal almost eliminated egg hatch of G. ellingtonae. Rates needed for Globodera spp. suppression were greatly reduced when using the B. juncea seed meal extract. When compared side-by-side, half as much B. juncea seed meal extract, 1.1 t/ha, was required to suppress G. ellingtonae egg hatch to the same extent as B. juncea seed meal. Exposure of G. pallida to B. juncea seed meal extract at 4.5 t/ha reduced egg hatch by 90% compared with a nonamended control. The ability to reduce the amount of material being applied to soil by using an extract has the potential for integration into a G. pallida eradication program.

The effect of the nematode trap crop Solanum sisymbriifolium was assessed against three Globodera spp., the potato cyst nematode Globodera pallida (in Idaho), the recently described Globodera ellingtonae (in Oregon), and the tobacco cyst nematode Globodera tabacum (in Connecticut) in field trials. At all locations the ability of S. sisymbriifolium to reduce Globodera encysted second-stage juveniles (J2) in egg densities compared to fallow was considered. For G. ellingtonae, the impact of planting and termination dates of S. sisymbriifolium on final egg densities was also evaluated; and for G. pallida, the ability of the nematode to reproduce on potato (Solanum tuberosum) after exposure to S. sisymbriifolium was determined. Encysted J2 in egg densities of all three Globodera spp. declined from 25 to 68% after trap cropping with S. sisymbriifolium. For G. pallida, S. sisymbriifolium reduced final encysted J2 in egg density by 23 to 50% compared to the fallow treatment, and significantly decreased G. pallida reproduction on potato after exposure to S. sisymbriifolium by 99 to 100% compared to the fallow treatment (P < 0.0001). For G. ellingtonae, the planting date of S. sisymbriifolium in May or June did not impact final egg densities (P = 0.32). Rather, percentage reduction in G. ellingtonae encysted J2 in egg density was most influenced by the length of time to which nematodes were exposed to S. sisymbriifolium, with 30 and 81% reduction after 6 vs 12 wk of exposure, respectively (P < 0.0001). Similar levels of nematode reduction after S. sisymbriifolium were observed for G. tabacum after 12 to 14 wk of exposure to the trap crop; G. tabacum density changes consisted of a 114% increase after susceptible tobacco, a 65% decrease after resistant tobacco, and an 88% decrease after S. sisymbriifolium compared to bare soil. In conclusion, this research demonstrates the widespread applicability of S. sisymbriifolium in reducing a diversity of Globodera spp. present in the USA.

Solanum glycoalkaloids are gaining increased scientific attention due to their bioactive potential in the defense of plants against pests and pathogens. The comprehensive glycoalkaloid profiling from the leaves, stems, and roots of seven underexploited Solanum species (S. caripense, S. melanocerasum, S. muricatum, S. nigrum, S. quitoense, S. retroflexum, and S. sisymbriifolium) was conducted using high-performance liquid chromatography–time-of-flight mass spectrometry. A total of 51 glycoalkaloids were shared among the studied Solanum species, with concentrations ranging from 7 to 5.63 × 10⁵ ng g⁻¹. Based on the glycoalkaloid composition, plants were separated into two clusters, Cluster 1 (S. melanocerasum, S. nigrum, and S. retroflexum) and Cluster 2 (S. caripense, S. muricatum, S. quitoense, and S. sisymbriifolium). The inhibition activity of glycoalkaloid extracts on acetylcholinesterase showed a half-maximal inhibitory concentration (IC₅₀), ranging from 0.4 (S. nigrum stems) to 344.9 µg mL⁻¹ (S. sisymbriifolium leaves), that was not directly correlated to the total glycoalkaloid contents. This suggests that the composition of glycoalkaloids in the plant extract, rather than the total concentration, is a driver of biological activity. The study provides a framework for the bioprospecting of underexploited Solanum species for exploring bioactive glycoalkaloids and other compounds with potential pesticidal activities for the development of green bioformulation. This is the first comprehensive report on the glycoalkaloid profiles of S. retroflexum.