Plants respond to pathogen effectors in various ways depending on the genotypes of both the plant and the invading pathogen. In the first instance when the effector is recognized by pattern recognition receptors, plants can initiate processes associated with PAMP-triggered immunity, such as oxidative bursts, callose deposition and expression of pathogenesis-related genes. Effector proteins can, conversely, act to suppress these processes, promoting pathogen progression and manipulating plant physiological processes. In resistant plant genotypes, products of plant immune receptors, encoded by R genes, may recognize some of these effector proteins and mount ETI-associated processes to restrict pathogen colonization. A major challenge for the plant-pathogen research community is to link effector sequences to plant phenotypes and processes. Functional high-throughput screens of oomycete effector candidates can be performed by in planta transient expression [45, 60, 61] or by delivery of candidate effector proteins by the bacterial type III secretion system [62, 63]. These strategies can give valuable insights into the virulence activities of effector proteins, particularly regarding the suppression of host plant immunity [45, 60, 63]. However, some of these studies need to be analyzed with caution, as discussed by Bozkurt et al. . One example is the screening for suppression of immunity using the mammalian cell-death-inducing protein BAX with a heterologous expression system, which gave high frequencies of cell death suppression when testing P. sojae RXLR effectors . This is because the BAX-induced cell death can be readily suppressed following activation of the unfolded protein response . Another example is the screening of H. arabidopsidis RXLR effectors for the suppression of callose deposition, a component of PAMP-triggered immunity. Given that the frequency of effectors suppressing callose deposition found in this study was high (35 out of 62) , follow-up experiments are essential to determine whether this does truly represent identical and redundant defense suppression activities for most of the H. arabidopsidis effectors tested.
Heterologous transient expression of fluorescently tagged effectors in the model plant Nicotiana benthamiana is a useful tool for subcellular localization screens. Caillaud et al.  used this approach to test the localization of 49 RXLR effectors of H. arabidopsidis, showing that 33% of the tested proteins localize to the nucleus, another 33% accumulate in the nucleus and cytoplasm and the remaining effectors mainly target various plant cell membranes. In a similar way, Stam et al.  analyzed the subcellular localization of 11 diverse P. capsici CRN carboxy-terminal domains and found that all the tested domains target the host nucleus. These findings suggest that the plant cell nucleus has a crucial role in virulence and immunity, and highlights how effectors can help to elucidate the plant cell processes that take place during the interaction with oomycete pathogens.
High-throughput approaches can also be used to study the avirulence activity of effectors and can help to identify and assign functions to new immune receptors [60–62]. Vleeshouwers et al.  used a library of RXLR effector genes predicted computationally from the P. infestans genome to screen wild Solanum species (related to potato and tomato) for induction of hypersensitive responses indicating the presence of a plant R gene. A set of 54 effectors were expressed in planta using a Potato virus × (PVX) agroinfection assay optimized for Solanum. This led to the identification of AVRblb1, the effector protein recognized by the Solanum bulbocastanum (wild potato species) resistance protein Rpi-blb1. In a similar study, the S. bulbocastanum resistance gene Rpi-blb2 was co-expressed with a PVX-based library of 62 P. infestans RXLR effector clones in N. benthamiana. This approach allowed the identification of the corresponding effector Avrblb2 . Both Rpi-blb1 and Rpi-blb2 are considered broad-spectrum resistance genes, and the availability of the corresponding Avr genes could assist the use of these R genes in agriculture .
In addition, the use of effectors in large-scale screens of germplasm has facilitated the discovery of new resistance genes and their classification into discrete recognition specificities, accelerating the cloning of R genes while avoiding redundant cloning efforts [61, 62, 65]. Effectors can be used to identify R gene homologs in plant species that are more compatible for breeding. These strategies are nicely illustrated by the work of Vleeshouwers et al. , in which the screen of several wild Solanum species with a set of predicted P. infestans RXLR effectors led to the discovery and rapid cloning of Solanum stoloniferum
Rpi-sto1 and Solanum papita
Rpi-pta1, both functionally equivalent to S. bulbocastanum
Rpi-blb1, with the additional advantage that S. stoloniferum and S. papita are sexually more compatible with potato, which would facilitate the introgression of this resistance specificity into commercial potato cultivars.
Recently, high-throughput effector screens have also proved useful for dissecting the complex genetic basis of the late blight resistance in the potato cultivar Sarpo Mira, which has both qualitative and quantitative ('field' resistance) components . This analysis identified Rpi-Smira2, an R protein conferring partial resistance on recognition of the P. infestans RXLR effector AVRSmira2. AvrSmira2 is diagnostic of field resistance to late blight and can be used to accelerate the breeding and cloning of Rpi-Smira2, showing that effectors can be used to map linkage of quantitative traits, facilitating quantitative resistance breeding.
Another approach in which knowledge of effectors can assist the deployment of disease resistance against oomycetes is through the monitoring of effector allele diversity in pathogen populations [29, 67, 68]. This can provide valuable information to assess the potential of a given R gene regarding its spectrum and durability, and to design control strategies based on the dynamic distribution of virulence alleles in a given population, allowing the early detection of races that can overcome the deployed R genes. For example, the P. infestans avirulence gene Avrblb1 belongs to the highly ipiO RXLR effector family. The set of ipiO variants present in a given isolate determines the outcome of the interaction between this isolate and host plants carrying the Rpi-blb1 gene, with some combinations conferring avirulence and others overcoming this resistance gene [69, 70]. Therefore, monitoring ipiO diversity in P. infestans populations can tell us whether races are evolving in the field to overcome Rpi-blb1. In addition, genome and transcriptome analysis can be used to determine the set of effector genes present and expressed during infection by isolates of a given genotype, providing information on the R genes that can be deployed to control that particular genotype. This was illustrated by Cooke et al. , who showed that an isolate of the P. infestans 13_A2 genotype carries intact coding sequences of Avrblb1, Avrblb2 and Avrvnt1, and that these avirulence genes are induced during infection, suggesting that the cognate R genes (Rpi-blb1, Rpi-blb2 and Rpi-vnt1) could be used to control this aggressive genotype that is predominant in UK fields.
Finally, it is also possible to expand the effector recognition specificity of a given R gene to new virulent alleles by performing artificial evolution by random mutagenesis , an approach that has been previously successful when applied to the PVX resistance gene Rx .