The discovery of a novel calcium sensor could present a potential strategy for breeding disease-resistant rice cultivars with high yield.
In our bodies, we have evolved specialised immune cells to ward off infections from pathogens. In the absence of such immune cells, how does one defend against pathogens? Plants are one such organism that does not possess specialised immune cells. Instead, they have evolved an elaborate immune machinery that can be triggered by microbe- or pathogen-associated molecular patterns, or effectors. However, such immune responses are costly and can impede growth. Therefore, plant immunity requires strict control to allow quick responses upon pathogen infection while minimising growth penalties under normal conditions. This would be especially important for staple food crops like rice, which feeds more than half the world’s population.
The quality and yield of rice are severely limited by the various pathogens it encounters. One major goal of rice breeding is broad-spectrum disease resistance without sacrificing its yield. But the molecular mechanisms coordinating rice disease resistance and growth fitness remains to be known. If we can understand how these plants adjust immune homeostasis in different environments, we may be able to secure crop production.
Here, a research team led by Dr. He Zuhua at the CAS Center for Excellence in Molecular Plant Sciences of the Chinese Academy of Sciences has revealed a novel Ca2+ sensor that balances rice immune homeostasis and inflorescence meristem growth. The discovery of this sensor presents a possible solution for breeding disease-resistant and high yield rice cultivars in the future.
The sheath blight fungus Rhizoctonia solani is one of the most destructive rice pathogens responsible for huge yield losses and yet, no high resistance germplasm has been discovered so far. In an effort to locate rice genetic resources with high sheath blight resistance, the researchers carried out large scale genetic screening of diverse rice germplasm and breeder’s collections and singled out a natural recessive variant line, resistance of rice to diseases 1 (rod1).
Upon further analyses, the team found that the rod1 variant displayed strong resistance to not only sheath blight but also two other major rice diseases, rice blast and bacterial blight. This finding suggests that ROD1 plays an important role in controlling rice immunity.
The team was also able to demonstrate that the ROD1 gene encodes a C2 domain that acts as a Ca2+ sensor that binds to lipids in a Ca2+ -dependent manner, enabling ROD1 localisation to the plasma membrane.
To identify ROD1 interacting proteins, Dr. He and colleagues conducted a yeast two-hybrid screen and found two E3 ligases, ROD1 interacting protein 1 (RIP1) and AvrPiz-t interacting protein 6 (APIP6). Both of these ligases are triggered by pathogen infection and fine-tune ROD1 protein stability. The yeast two-hybrid screen also revealed a catalase, CatB, as a ROD1 binding factor that promotes H2O2 degradation, thus inhibiting rice immunity. However, the team found that by generating a rice CR-catb mutant through CRISPR/Cas9, disease resistance was enhanced, suggesting a negative role for CatB in rice immune responses.
Given the important role of ROD1 in immune regulation, the team set out to study its variation in rice germplasm. A comparison of ROD1 coding sequences in different rice accessions revealed a resistant allele that is mainly found in low-latitude areas and prevalent in indica rice varieties, which correlates with the wide cultivation of indica rice in tropical and subtropical regions. This suggests a selection of immune alleles that are adapted to local climate and agroeconomical conditions.
The team’s work reveals a plant host-pathogen convergent immune suppression network that involves Ca2+ sensing, reactive oxygen species homeostasis, and immune homeostasis. The ROD1-mediated crosstalk between disease resistance and inflorescent meristem development suggests a new possible strategy for crop engineering and presents direct evidence supporting the theory of co-evolution between pathogen invading strategy and plant immune machinery. [APBN]
Source: Gao et al. (2021). Ca2+ sensor-mediated ROS scavenging suppresses rice immunity and is exploited by a fungal effector. Cell, 184(21), 5391-5404.