Fungal and oomycete plant pathogens translocate effector proteins into host cells to establish infection. However, virulence targets and modes of action of their effectors are unknown. Effector AVR3a from potato blight pathogen Phytophthora infestans is translocated into host cells and occurs in two forms: AVR3a ^KI , which is detected by potato resistance protein R3a, strongly suppresses infestin 1 (INF1)-triggered cell death (ICD), whereas AVR3a ^EM , which evades recognition by R3a, weakly suppresses host ICD. Here we show that AVR3a interacts with and stabilizes host U-box E3 ligase CMPG1, which is required for ICD. In contrast, AVR3a ^KI/Y147del , a mutant with a deleted C-terminal tyrosine residue that fails to suppress ICD, cannot interact with or stabilize CMPG1. CMPG1 is stabilized by the inhibitors MG132 and epoxomicin, indicating that it is degraded by the 26S proteasome. CMPG1 is degraded during ICD. However, it is stabilized by mutations in the U-box that prevent its E3 ligase activity. In stabilizing CMPG1, AVR3a thus modifies its normal activity. Remarkably, given the potential for hundreds of effector genes in the P. infestans genome, silencing Avr3a compromises P. infestans pathogenicity, suggesting that AVR3a is essential for virulence. Interestingly, Avr3a silencing can be complemented by in planta expression of Avr3a ^KI or Avr3a ^EM but not the Avr3a ^KI/Y147del mutant. Our data provide genetic evidence that AVR3a is an essential virulence factor that targets and stabilizes the plant E3 ligase CMPG1, potentially to prevent host cell death during the biotrophic phase of infection.

Plant pathogens deliver effectors to alter host processes. Knowledge of how effectors target and manipulate host proteins is critical to understand crop disease. Here, we show that in planta expression of the RXLR effector Pi04314 enhances leaf colonization by Phytophthora infestans via activity in the host nucleus and attenuates induction of jasmonic and salicylic acid-responsive genes. Pi04314 interacts with three host protein phosphatase 1 catalytic (PP1c) isoforms, causing their re-localization from the nucleolus to the nucleoplasm. Re-localization of PP1c-1 also occurs during infection and is dependent on an R/KVxF motif in the effector. Silencing the PP1c isoforms or overexpression of a phosphatase-dead PP1c-1 mutant attenuates infection, demonstrating that host PP1c activity is required for disease. Moreover, expression of PP1c–1mut abolishes enhanced leaf colonization mediated by in planta Pi04314 expression. We argue that PP1c isoforms are susceptibility factors forming holoenzymes with Pi04314 to promote late blight disease.

Ethylene response factors (ERFs) are unique to the plant kingdom that plays crucial roles in plant response to various biotic and abiotic stresses. We show here that a potato StERF3, which contains ERF-associated amphiphilic repression (EAR) motif in C-terminal region, negatively regulates resistance to Phytophthora infestans and salt tolerance in potato. Promoter of the StERF3 responses to induction of salicylic acid (SA), abscisic acid (ABA), ethylene (ET) and NaCl, as well as P. infestans, the causal agent of potato late blight disease. StERF3 could bind to the GCC box element of the HIS3 promoter and activate transcription of HIS3 in yeast cells. Importantly, silencing StERF3 in potato showed an enhanced foliage resistance to P. infestans and elevated plant tolerance to NaCl stress accompanied by the activation of defense related genes (PR1, NPR1 and WRKY1), In contrast, StERF3-overexpressing plants showed reduced expression of these defense related genes and enhanced susceptibility to P. infestans, suggesting that StERF3 functions as a negative regulator of downstream defense- and/or stress-related genes in potato. StERF3 is localised to the nucleus. Interestingly, Yeast two-hybrid assay and bimolecular fluorescence complementation (BiFC) test clarified that the StERF3 could interact with other proteins in cytoplasm which may lead to its re-localization between the nucleus and cytoplasm, revealing a novel means of StERF3 regulation. Taken together, these data provide new insights into the mechanism underlying how StERF3 negatively regulates late blight resistance and abiotic tolerance in potato and may have a potential in engineering late blight resistance in potato.

Cold-induced sweetening (CIS) in potato is detrimental to the quality of processed products. Conversion of starch to reducing sugars (RS) by amylases is considered one of the main pathways in CIS but is not well studied. The amylase genes StAmy23, StBAM1, and StBAM9 were studied for their functions in potato CIS. StAmy23 is localized in the cytoplasm, whereas StBAM1 and StBAM9 are targeted to the plastid stroma and starch granules, respectively. Genetic transformation of these amylases in potatoes by RNA interference showed that β-amylase activity could be decreased in cold-stored tubers by silencing of StBAM1 and collective silencing of StBAM1 and StBAM9. However, StBAM9 silencing did not decrease β-amylase activity. Silencing StBAM1 and StBAM9 caused starch accumulation and lower RS, which was more evident in simultaneously silenced lines, suggesting functional redundancy. Soluble starch content increased in RNAi-StBAM1 lines but decreased in RNAi-StBAM9 lines, suggesting that StBAM1 may regulate CIS by hydrolysing soluble starch and StBAM9 by directly acting on starch granules. Moreover, StBAM9 interacted with StBAM1 on the starch granules. StAmy23 silencing resulted in higher phytoglycogen and lower RS accumulation in cold-stored tubers, implying that StAmy23 regulates CIS by degrading cytosolic phytoglycogen. Our findings suggest that StAmy23, StBAM1, and StBAM9 function in potato CIS with varying levels of impact.