Rice bundle sheath cell shape is regulated by the timing of light exposure during leaf development.

Plant leaves contain multiple cell types which achieve distinct characteristics whilst still coordinating development within the leaf. The bundle sheath possesses larger individual cells and lower chloroplast content than the adjacent mesophyll, but how this morphology is achieved remains unknown. To identify regulatory mechanisms determining bundle sheath cell morphology we tested the effects of perturbing environmental (light) and endogenous signals (hormones) during leaf development of Oryza sativa (rice). Total chloroplast area in bundle sheath cells was found to increase with cell size as in the mesophyll but did not maintain a 'set-point' relationship, with the longest bundle sheath cells demonstrating the lowest chloroplast content. Application of exogenous cytokinin and gibberellin significantly altered the relationship between cell size and chloroplast biosynthesis in the bundle sheath, increasing chloroplast content of the longest cells. Delayed exposure to light reduced the mean length of bundle sheath cells but increased corresponding leaf length, whereas premature light reduced final leaf length but did not affect bundle sheath cells. This suggests that the plant hormones cytokinin and gibberellin are regulators of the bundle sheath cell-chloroplast relationship and that final bundle sheath length may potentially be affected by light-mediated control of exit from the cell cycle.

APIP5 functions as a transcription factor and an RNA-binding protein to modulate cell death and immunity in rice.

Many transcription factors (TFs) in animals bind to both DNA and mRNA, regulating transcription and mRNA turnover. However, whether plant TFs function at both the transcriptional and post-transcriptional levels remains unknown. The rice (Oryza sativa) bZIP TF AVRPIZ-T-INTERACTING PROTEIN 5 (APIP5) negatively regulates programmed cell death and blast resistance and is targeted by the effector AvrPiz-t of the blast fungus Magnaporthe oryzae. We demonstrate that the nuclear localization signal of APIP5 is essential for APIP5-mediated suppression of cell death and blast resistance. APIP5 directly targets two genes that positively regulate blast resistance: the cell wall-associated kinase gene OsWAK5 and the cytochrome P450 gene CYP72A1. APIP5 inhibits OsWAK5 expression and thus limits lignin accumulation; moreover, APIP5 inhibits CYP72A1 expression and thus limits reactive oxygen species production and defense compounds accumulation. Remarkably, APIP5 acts as an RNA-binding protein to regulate mRNA turnover of the cell death- and defense-related genes OsLSD1 and OsRac1. Therefore, APIP5 plays dual roles, acting as TF to regulate gene expression in the nucleus and as an RNA-binding protein to regulate mRNA turnover in the cytoplasm, a previously unidentified regulatory mechanism of plant TFs at the transcriptional and post-transcriptional levels.

A New RING Finger Protein, PLANT ARCHITECTURE and GRAIN NUMBER 1, Affects Plant Architecture and Grain Yield in Rice.

Developing methods for increasing the biomass and improving the plant architecture is important for crop improvement. We herein describe a gene belonging to the RING_Ubox (RING (Really Interesting New Gene) finger domain and U-box domain) superfamily, PLANT ARCHITECTURE and GRAIN NUMBER 1 (PAGN1), which regulates the number of grains per panicle, the plant height, and the number of tillers. We used the CRISPR/Cas9 system to introduce loss-of-function mutations to OsPAGN1. Compared with the control plants, the resulting pagn1 mutant plants had a higher grain yield because of increases in the plant height and in the number of tillers and grains per panicle. Thus, OsPAGN1 may be useful for the genetic improvement of plant architecture and yield. An examination of evolutionary relationships revealed that OsPAGN1 is highly conserved in rice. We demonstrated that OsPAGN1 can interact directly with OsCNR10 (CELL NUMBER REGULATOR10), which negatively regulates the number of rice grains per panicle. A transcriptome analysis indicated that silencing OsPAGN1 affects the levels of active cytokinins in rice. Therefore, our findings have clarified the OsPAGN1 functions related to rice growth and grain development.

Identification and verification of grain shape QTLs by SNP array in rice.

Grain shape strongly influences the economic value and grain yield of rice. Thus, identifying quantitative trait loci (QTLs) for grain shape has been a longstanding goal in rice genetic research and breeding programs. Single nucleotide polymorphism (SNP) markers are ubiquitous in the rice genome and are more abundant and evenly distributed on the 12 rice chromosomes than traditional markers. An F2 population was genotyped using the RICE6K SNP array to elucidate the mechanisms governing grain shape. Thirty-five QTLs for grain shape were detected on 11 of 12 chromosomes over 2 years. The major QTL cluster qGS7 was detected in both years and displayed strong genetic effects on grain length and width, showing consistency with GL7/GW7. Some minor QTLs were also detected, and the effects of four QTLs on seed size were then validated using BC1F6 populations with residual heterozygous lines in each QTL region. Our findings provide insights into the molecular basis of grain shape as well as additional resources and approaches for producing hybrid high-yield rice varieties.

Evidence that synergism between potassium and nitrate enhances the alleviation of ammonium toxicity in rice seedling roots.

Ammonium toxicity in plants is considered a global phenomenon, but the primary mechanisms remain poorly characterized. Here, we show that although the addition of potassium or nitrate partially alleviated the inhibition of rice seedling root growth caused by ammonium toxicity, the combination of potassium and nitrate clearly improved the alleviation, probably via some synergistic mechanisms. The combined treatment with potassium and nitrate led to significantly improved alleviation effects on root biomass, root length, and embryonic crown root number. The aberrant cell morphology and the rhizosphere acidification level caused by ammonium toxicity, recovered only by the combined treatment. RNA sequencing analysis and weighted gene correlation network analysis (WGCNA) revealed that the transcriptional response generated from the combined treatment involved cellulose synthesis, auxin, and gibberellin metabolism. Our results point out that potassium and nitrate combined treatment effectively promotes cell wall formation in rice, and thus, effectively alleviates ammonium toxicity.

Reproductive cells and peripheral parietal cells collaboratively participate in meiotic fate acquisition in rice anthers.

In flowering plants, the transition from mitosis to meiosis is the precondition for gametogenesis, which is the most crucial event during sexual reproduction. Here, we report an intriguing mechanism whereby germ cells and surrounding somatic cells cooperatively involve in the meiotic switch during anther development in rice (Oryza sativa). In double mutants with loss function of both leptotene chromosome establishment- and somatic cell layer differentiation-associated genes, chromosome morphology in the reproductive cells remains the same as that in somatic cells, and sporogenous cells fail to differentiate into pollen mother cells. OsSPOROCYTELESS and MICROSPORELESS1, two pivotal genes involved in meiosis entry, are prominently downregulated in anthers of plants with mutations in both MULTIPLE SPOROCYTE1 and LEPTOTENE 1. In addition, the transcription of redox-related genes is also affected. Therefore, germ cells and the surrounding somatic cells collaboratively participate in meiosis initiation in rice.

OsMLH1 interacts with OsMLH3 to regulate synapsis and interference-sensitive crossover formation during meiosis in rice.

Meiotic recombination is essential for reciprocal exchange of genetic information between homologous chromosomes and their subsequent proper segregation in sexually reproducing organisms. MLH1 and MLH3 belong to meiosis-specific members of the MutL-homolog family, which are required for normal level of crossovers (COs) in some eukaryotes. However, their functions in plants need to be further elucidated. Here, we report the identification of OsMLH1 and reveal its functions during meiosis in rice. Using CRISPR-Cas9 approach, two independent mutants, Osmlh1-1 and Osmlh1-2, are generated and exhibited significantly reduced male fertility. In Osmlh1-1, the clearance of PAIR2 is delayed and partial ZEP1 proteins are not loaded into the chromosomes, which might be due to the deficient in resolution of interlocks at late zygotene. Thus, OsMLH1 is required for the assembly of synapsis complex. In Osmlh1-1, CO number is dropped by ~53% and the distribution of residual COs is consistent with predicted Poisson distribution, indicating that OsMLH1 is essential for the formation of interference-sensitive COs (class I COs). OsMLH1 interacts with OsMLH3 through their C-terminal domains. Mutation in OsMLH3 also affects the pollen fertility. Thus, our experiments reveal that the conserved heterodimer MutLγ (OsMLH1-OsMLH3) is essential for the formation of class I COs in rice.

A novel smartphone-based electrochemical cell sensor for evaluating the toxicity of heavy metal ions Cd2+, Hg2+, and Pb2+ in rice.

A novel smartphone-based electrochemical cell sensor was developed to evaluate the toxicity of heavy metal ions, such as cadmium (Cd2+), lead (Pb2+), and mercury (Hg2+) ions on Hep G2 cells. The cell sensor was fabricated with reduced graphene oxide (RGO)/molybdenum sulfide (MoS2) composites to greatly improve the biological adaptability and amplify the electrochemical signals. Differential pulse voltammetry (DPV) was employed to measure the electrical signals induced by the toxicity of heavy metal ions. The results showed that Cd2+, Hg2+, and Pb2+ significantly reduced the viability of Hep G2 cells in a dose-dependent manner. The IC50 values obtained by this method were 49.83, 36.94, and 733.90 µM, respectively. A synergistic effect was observed between Cd2+ and Pb2+ and between Hg2+ and Pb2+, and an antagonistic effect was observed between Cd2+ and Hg2+, and an antagonistic effect at low doses and an additive effect at high doses were found in the ternary mixtures of Cd2+, Hg2+, and Pb2+. These electrochemical results were confirmed via MTT assay, SEM and TEM observation, and flow cytometry. Therefore, this new electrochemical cell sensor provided a more convenient, sensitive, and flexible toxicity assessment strategy than traditional cytotoxicity assessment methods.

CBL-interacting protein kinase 31 regulates rice resistance to blast disease by modulating cellular potassium levels.

Rice blast disease caused by infection with Magnaporthe oryzae, a hemibiotrophic fungal pathogen, significantly reduces the yield production. However, the rice defense mechanism against blast disease remains elusive. To identify the genes involved in the regulation of rice defense to blast disease, dissociation (Ds) transposon tagging mutant lines were analyzed in terms of their response to M. oryzae isolate Guy11. Among them, CBL-interactingprotein kinase31 (CIPK31) mutants were more susceptible than wild-type plants to blast. The CIPK31 transcript was found to be insensitive to Guy11 infection, and the CIPK31-GFP was localized to the cytosol and nucleus. Overexpression of CIPK31 promoted rice defense to blast. Further analysis indicated that CIPK31 interacts with Calcineurin B-like 2 (CBL2) and CBL6 at the plasma membrane, and cbl2 mutants are more susceptible to blast compared with wild-type plants, suggesting that calcium signaling might partially through the CBL2-CIPK31 signaling regulate rice defense. Yeast two-hybrid results showed that AKT1-like (AKT1L), a potential potassium (K+) channel protein, interacted with CIPK31, and the K+ level was significantly lower in the cipk31 mutants than in the wild-type control. In addition, exogenous potassium application increased rice resistance to blast, suggesting that CIPK31 might interact with AKT1L to increase K+ uptake, thereby promoting resistance to blast. Taken together, the results presented here demonstrate that CBL2-CIPK31-AKT1L is a new signaling pathway that regulates rice defense to blast disease.

Leaf direction: Lamina joint development and environmental responses.

Plant architecture plays a major role in canopy photosynthesis and biomass production, and plants adjust their growth (and thus architecture) in response to changing environments. Leaf angle is one of the most important traits in rice (Oryza sativa L.) plant architecture, because leaf angle strongly affects leaf direction and rice production, with more-erect leaves being advantageous for high-density plantings. The degree of leaf bending depends on the morphology of the lamina joint, which connects the leaf and the sheath. In this review, we discuss cell morphology in different lamina joint tissues and describe the underlying genetic network that governs this morphology and thus regulates leaf direction. Furthermore, we focus on the mechanism by how environmental factors influence rice leaf angle. Our review provides a theoretical framework for the future genetic improvement of rice leaf orientation and plant architecture.

The URL1-ROC5-TPL2 transcriptional repressor complex represses the ACL1 gene to modulate leaf rolling in rice.

Moderate leaf rolling is beneficial for leaf erectness and compact plant architecture. However, our understanding regarding the molecular mechanisms of leaf rolling is still limited. Here, we characterized a semi-dominant rice (Oryza sativa L.) mutant upward rolled leaf 1 (Url1) showing adaxially rolled leaves due to a decrease in the number and size of bulliform cells. Map-based cloning revealed that URL1 encodes the homeodomain-leucine zipper (HD-Zip) IV family member RICE OUTERMOST CELL-SPECIFIC 8 (ROC8). A single-base substitution in one of the two conserved complementary motifs unique to the 3'-untranslated region of this family enhanced URL1 mRNA stability and abundance in the Url1 mutant. URL1 (UPWARD ROLLED LEAF1) contains an ethylene-responsive element binding factor-associated amphiphilic repression motif and functions as a transcriptional repressor via interaction with the TOPLESS co-repressor OsTPL2. Rather than homodimerizing, URL1 heterodimerizes with another HD-ZIP IV member ROC5. URL1 could bind directly to the promoter and suppress the expression of abaxially curled leaf 1 (ACL1), a positive regulator of bulliform cell development. Knockout of OsTPL2 or ROC5 or overexpression of ACL1 in the Url1 mutant partially suppressed the leaf-rolling phenotype. Our results reveal a regulatory network whereby a transcriptional repression complex composed of URL1, ROC5, and the transcriptional corepressor TPL2 suppresses the expression of the ACL1 gene, thus modulating bulliform cell development and leaf rolling in rice.

Rice and Arabidopsis homologs of yeast CHROMOSOME TRANSMISSION FIDELITY PROTEIN 4 commonly interact with Polycomb complexes but exert divergent regulatory functions.

Both genetic and epigenetic information must be transferred from mother to daughter cells during cell division. The mechanisms through which information about chromatin states and epigenetic marks like histone 3 lysine 27 trimethylation (H3K27me3) are transferred have been characterized in animals; these processes are less well understood in plants. Here, based on characterization of a dwarf rice (Oryza sativa) mutant (dwarf-related wd40 protein 1, drw1) deficient for yeast CTF4 (CHROMOSOME TRANSMISSION FIDELITY PROTEIN 4), we discovered that CTF4 orthologs in plants use common cellular machinery yet accomplish divergent functional outcomes. Specifically, drw1 exhibited no flowering-related phenotypes (as in the putatively orthologous Arabidopsis thaliana eol1 mutant), but displayed cell cycle arrest and DNA damage responses. Mechanistically, we demonstrate that DRW1 sustains normal cell cycle progression by modulating the expression of cell cycle inhibitors KIP-RELATED PROTEIN 1 (KRP1) and KRP5, and show that these effects are mediated by DRW1 binding their promoters and increasing H3K27me3 levels. Thus, although CTF4 orthologs ENHANCER OF LHP1 1 (EOL1) in Arabidopsis and DRW1 in rice are both expressed uniquely in dividing cells, commonly interact with several Polycomb complex subunits, and promote H3K27me3 deposition, we now know that their regulatory functions diverged substantially during plant evolution. Moreover, our work experimentally illustrates specific targets of CTF4/EOL1/DRW1, their protein-proteininteraction partners, and their chromatin/epigenetic effects in plants.

Structural Alteration of Rice Pectin Affects Cell Wall Mechanical Strength and Pathogenicity of the Rice Blast Fungus Under Weak Light Conditions.

Pectin, a component of the plant cell wall, is involved in cell adhesion and environmental adaptations. We generated OsPG-FOX rice lines with little pectin due to overexpression of the gene encoding a pectin-degrading enzyme [polygalacturonase (PG)]. Overexpression of OsPG2 in rice under weak light conditions increased the activity of PG, which increased the degradation of pectin in the cell wall, thereby reducing adhesion. Under weak light conditions, the overexpression of OsPG decreased the pectin content and cell adhesion, resulting in abnormally large intercellular gaps and facilitating invasion by the rice blast fungus. OsPG2-FOX plants had weaker mechanical properties and greater sensitivity to biotic stresses than wild-type (WT) plants. However, the expression levels of disease resistance genes in non-infected leaves of OsPG2-FOX were more than twice as high as those of the WT and the intensity of disease symptoms was reduced, compared with the WT. Under normal light conditions, overexpression of OsPG2 decreased the pectin content, but did not affect cell adhesion and sensitivity to biotic stresses. Therefore, PG plays a role in regulating intercellular adhesion and the response to biotic stresses in rice.

Interaction between endogenous H2O2 and OsVPE3 in the GA-induced PCD of rice aleurone layers.

KEY MESSAGE: Endogenous hydrogen peroxide (H2O2) is involved in regulating the gibberellic acid-induced programmed cell death (PCD) of the aleurone layers by cooperating with OsVPE3 during rice seed germination. Preliminary experiments revealed that H2O2 produced by the NOX pathway is the key factor affecting rice germination. Histochemical analysis indicated that H2O2 is located in the aleurone layer. Both the H2O2 scavenger DMTU and the NOX inhibitor DPI decreased H2O2 content and significantly slowed down vacuolation in a dose-dependent manner. Interestingly, DMTU down-regulated the OsNOX8 transcript or DMTU and DPI decreased the intracellular H2O2 level, resulting in a delay of PCD. In contrast, GA and H2O2 up-regulated the OsNOX8 transcript and intracellular H2O2 level, leading to premature PCD, and the effects of GA and H2O2 were reversed by DMTU and DPI, respectively. These results showed that the imbalance of intracellular H2O2 levels leads to the delayed or premature PCD. Further experiments indicated that GA up-regulated the OsVPE3 transcript and VPE activity, and the effect was reversed by DPI. Furthermore, Ac-YVAD-CMK significantly blocked H2O2 accumulation, and DPI + Ac-YVAD-CMK had a more significant inhibitory effect compared with DPI alone, resulting in the delayed PCD, suggesting that OsVPE3 regulates PCD by promoting H2O2 generation. Meanwhile, DPI significantly inhibited the OsVPE3 transcript and VPE activity, and in turn delayed PCD occurrence, suggesting that the H2O2 produced by the NOX pathway may regulate PCD by up-regulating the OsVPE3 transcript. Thus, the endogenous H2O2 produced by the NOX pathway mediates the GA-induced PCD of rice aleurone layers by interacting with OsVPE3.

Expression of Recombinant Human Octamer-Binding Transcription Factor 4 in Rice Suspension Cells.

The rice cell suspension culture system is a good way to produce recombinant human proteins, owing to its high biosafety and low production cost. Human Octamer-binding Transcription Factor 4 (Oct4) is a fundamental transcription factor responsible for maintaining human pluripotent embryonic stem cells. Recombinant Oct4 protein has been used to induce pluripotent stem cells. In this study, recombinant Oct4 proteins are produced via a sugar starvation-inducible αAmy3/RAmy3D promoter-signal peptide-based rice recombinant protein expression system. Oct4 mRNAs accumulate in the transgenic rice suspension cells under sugar starvation. The Oct4 recombinant protein is detected in the transgenic rice suspension cells, and its highest yield is approximately 0.41% of total cellular soluble proteins after one day of sugar starvation. The rice cell-synthesized recombinant human Oct4 protein show DNA-binding activity in vitro, which implies that the protein structure is correct for enabling specific binding to the target DNA motif.

Dynamics of cell wall structure and related genomic resources for drought tolerance in rice.

KEY MESSAGE: Cell wall plasticity plays a very crucial role in vegetative and reproductive development of rice under drought and is a highly potential trait for improving rice yield under drought. Drought is a major constraint in rice (Oryza sativa L.) cultivation severely affecting all developmental stages, with the reproductive stage being the most sensitive. Rice plants employ multiple strategies to cope with drought, in which modification in cell wall dynamics plays a crucial role. Over the years, significant progress has been made in discovering the cell wall-specific genomic resources related to drought tolerance at vegetative and reproductive stages of rice. However, questions remain about how the drought-induced changes in cell wall made by these genomic resources potentially influence the vegetative and reproductive development of rice. The possibly major candidate genes underlying the function of quantitative trait loci directly or indirectly associated with the cell wall plasticization-mediated drought tolerance of rice might have a huge promise in dissecting the putative genomic regions associated with cell wall plasticity under drought. Furthermore, engineering the drought tolerance of rice using cell wall-related genes from resurrection plants may have huge prospects for rice yield improvement. Here, we review the comprehensive multidisciplinary analyses to unravel different components and mechanisms involved in drought-induced cell wall plasticity at vegetative and reproductive stages that could be targeted for improving rice yield under drought.

Reproductive phasiRNAs regulate reprogramming of gene expression and meiotic progression in rice.

Plant spermatogenesis is a complex process that directly affects crop breeding. A rapid change in gene abundance occurs at early meiosis prophase, when gene regulation is selective. However, how these genes are regulated remains unknown. Here, we show that rice reproductive phasiRNAs are essential for the elimination of a specific set of RNAs during meiotic prophase I. These phasiRNAs cleave target mRNAs in a regulatory manner such that one phasiRNA can target more than one gene, and/or a single gene can be targeted by more than one phasiRNA to efficiently silence target genes. Our investigation of phasiRNA-knockdown and PHAS-edited transgenic plants demonstrates that phasiRNAs and their nucleotide variations are required for meiosis progression and fertility. This study highlights the importance of reproductive phasiRNAs for the reprogramming of gene expression during meiotic progression and establishes a basis for future studies on the roles of phasiRNAs with a goal of crop improvement.

Rice glutaredoxin GRXS15 confers broad-spectrum resistance to Xanthomonas oryzae pv. oryzae and Fusarium fujikuroi.

Rice is a particularly widely consumed food crop globally, but its yield is seriously damaged by bacterial blight due to Xanthomonas oryzae pv. oryzae (Xoo) and bakanae disease due to Fusarium fujikuroi (F. fujikuroi). However, broad-spectrum resistance (BSR) to both Xoo and F. fujikuroi remains largely elusive. In this study, we showed that rice monothiol glutaredoxin GRXS15 localizes in mitochondria and the nucleus, and its transcription is induced by Xoo. Transgenic rice lines constitutively expressing OsGRXS15 showed enhanced disease resistance to Xoo and F. fujikuroi, while CRISPR/Cas9-based knockout mutants showed reduced resistance compared with the wild-type plants. The transcription of pathogenesis-related (PR) genes was significantly induced in OsGRXS15-expressing plants. The rice transcription factor OsWRKY65 was identified as a binding partner, and it directly interacted with OsGRXS15 in the nucleus. Moreover, we revealed that the interaction of OsGRXS15 and OsWRKY65 results in the upregulation of OsPR1. These results suggested that OsGRXS15 interacts with transcription factors, and it confers BSR through regulating the expression of genes related to pathogen response. This is the first report on the nuclear function associated with the monothiol glutaredoxin GRXS15.

MERISTEM ACTIVITYLESS (MAL) is involved in root development through maintenance of meristem size in rice.

KEY MESSAGE: Rice MERISTEM ACTIVITYLESS (MAL), a RING-H2 finger domain (RFD)-containing gene, regulates meristem cell viability after the initiation of root primordia mediated by cytokinin signaling. Genes in the RING-H2 finger domain (RFD) family play various roles during plant development and in biotic/abiotic stress responses. Rice gene MERISTEM ACTIVITYLESS (MAL), being contained in the RING-H2 finger domain (RFD), is characterized by a transmembrane domain at the N-terminal and a C3H2C3 zinc finger domain at the C-terminal. To elucidate the physiological and molecular functions of MAL, we generated MAL knockdown transgenic plants by RNA interference. MAL RNA-interfered (MRi) transgenic plants exhibited a phenotype with shorter crown root length and lower crown root number, accompanied by a lower cell division rate. The low division rate was observed in the root meristem exactly where MAL was expressed. Furthermore, transcriptome data revealed that cell wall macromolecule metabolism-related genes and redox-related genes were enriched in MAL RNAi lines. Most of these differentially expressed genes (DEGs) were induced by exogenous cytokinin. Hence, we conclude that MAL, as a novel regulatory factor, plays a major role in maintaining cell viability in the meristem after the initiation of root primordial formation, mediated by cytokinin signaling and reactive oxygen species (ROS).

The rice NLR pair Pikp-1/Pikp-2 initiates cell death through receptor cooperation rather than negative regulation.

Plant NLR immune receptors are multidomain proteins that can function as specialized sensor/helper pairs. Paired NLR immune receptors are generally thought to function via negative regulation, where one NLR represses the activity of the second and detection of pathogen effectors relieves this repression to initiate immunity. However, whether this mechanism is common to all NLR pairs is not known. Here, we show that the rice NLR pair Pikp-1/Pikp-2, which confers resistance to strains of the blast pathogen Magnaporthe oryzae (syn. Pyricularia oryzae) expressing the AVR-PikD effector, functions via receptor cooperation, with effector-triggered activation requiring both NLRs to trigger the immune response. To investigate the mechanism of Pikp-1/Pikp-2 activation, we expressed truncated variants of these proteins, and made mutations in previously identified NLR sequence motifs. We found that any domain truncation, in either Pikp-1 or Pikp-2, prevented cell death in the presence of AVR-PikD, revealing that all domains are required for activity. Further, expression of individual Pikp-1 or Pikp-2 domains did not result in cell death. Mutations in the conserved P-loop and MHD sequence motifs in both Pikp-1 and Pikp-2 prevented cell death activation, demonstrating that these motifs are required for the function of the two partner NLRs. Finally, we showed that Pikp-1 and Pikp-2 associate to form homo- and hetero-complexes in planta in the absence of AVR-PikD; on co-expression the effector binds to Pikp-1 generating a tri-partite complex. Taken together, we provide evidence that Pikp-1 and Pikp-2 form a fine-tuned system that is activated by AVR-PikD via receptor cooperation rather than negative regulation.