In a groundbreaking study published in Scientific Reports, a team of researchers led by Parham Haddadi of Agriculture and Agri-Food Canada, Saskatoon Research and Development Centre, delves deep into the genetic interactions underlying the defense responses of Brassica napus (Bn), commonly known as canola, against the blackleg disease caused by the pathogen Leptosphaeria maculans (Lm). The study, titled “Dissecting R gene and host genetic background effect on the Brassica napus defense response to Leptosphaeria maculans,” sheds light on the intricate molecular dance between the plant’s resistance genes and its genetic background.
The Brassica-Leptosphaeria Pathosystem: A Decade of Advances
Over the past decade, substantial progress has been made in comprehending the genetics of the Brassica-Leptosphaeria pathosystem. This system is a model for studying the interactions between plants and their pathogens, which can lead to devastating diseases like blackleg in canola. Scientists have identified resistance (R) genes in Bn, which provide protection against specific Lm avirulence (Avr) genes. However, the impact of the host genetic background on these responses has remained a mystery—until now.
RNAseq Technology: The Key to Understanding Gene Expression
To understand how different R genes and host genetic backgrounds affect defense responses, the research team utilized high-throughput RNA sequencing (RNAseq) technology. By comparing the transcriptome profiles of Bn lines carrying one of four blackleg R genes (Rlm2, Rlm3, LepR1, and LepR2) in either the Topas or Westar genetic backgrounds, scientists could see the early stages of the plant’s response to infection by Lm.
Crucial Findings: Gene Expression and Hormone Signaling
The study’s findings revealed that all Bn lines with R genes showed an upsurge in genes associated with hormone signaling, cell wall reinforcement, chitin response, and glucosinolate production three days after inoculation (dai). However, it was noted that lines with LepR1 and Rlm2 exhibited a higher level of gene expression than those with Rlm3 and LepR2. In particular, Bn-SOBIR1, a receptor-like kinase involved in forming complex receptor-like proteins, showed high expression in LepR1 and Rlm2 lines at 3 dai.
The research team observed distinct patterns of salicylic acid (SA)- and jasmonic acid (JA)-related defense gene expression. Bn lines with LepR1 and Rlm2 showed enhanced expression of SA-related genes at 3 dai, while genes related to the JA pathway were more consistently induced across all Rlm and LepR lines at later stages of infection (6 and 9 dai).
The Topas Effect: A Stronger Defensive Stance
Crucially, the study also discovered the influence of the host genetic background on the induction of defense responses. When comparing LepR1 and LepR2 lines in the Topas and Westar backgrounds, a higher number of defense-related genes were induced in Topas at the earliest time point (3 dai).
Markers for both SA and JA, such as PR1 and PDF1.2, were more intensely induced in Topas than in the Westar lines. Furthermore, even without the presence of any R gene, the Topas genotype alone seemed to enhance defense, as indicated by the induction pattern of PDF1.2.
Conclusion: A Synergistic Effect of R Genes and Host Genetics
The data clearly suggest that the variation in the timing and intensity of defense-related gene expression depends on a combined influence of both the type of R gene present and the host genetic background. This level of genetic interaction complexity highlights the importance of considering both factors when breeding for disease resistance in crops.
Implications for Agricultural Practices and Future Research
This study is not simply an academic exercise; it has significant practical implications for the agricultural industry. By identifying the genetic interactions that lead to stronger defense responses in crops, plant breeders can develop more resilient varieties that may require fewer chemical interventions, resulting in more sustainable agricultural practices.
The elucidation of the relationship between R genes and host genetic backgrounds also opens the door for tailored disease management strategies that could enhance crop protection based on genetic profiles.
References
1. Haddadi, P., Larkan, N. J., & Borhan, M. H. (2019). Dissecting R gene and host genetic background effect on the Brassica napus defense response to Leptosphaeria maculans. Scientific Reports, 9, 6947. https://doi.org/10.1038/s41598-019-43419-9
2. Jones, J. D. G., & Dangl, J. L. (2006). The plant immune system. Nature, 444, 323–329. https://doi.org/10.1038/nature05286
3. Thomma, B. P. H. J., Nürnberger, T., & Joosten, M. H. A. J. (2011). Of PAMPs and Effectors: The Blurred PTI-ETI Dichotomy. The Plant Cell, 23, 4–15. https://doi.org/10.1105/tpc.110.082602
4. Becker, G. B., et al. (2017). Transcriptome analysis of the Brassica napus–Leptosphaeria maculans pathosystem identifies receptor, signalling and structural genes underlying plant resistance. The Plant Journal, 90, 573–586. https://doi.org/10.1111/tpj.13514
5. Larkan, N. J., et al. (2016). Single R Gene Introgression Lines for Accurate Dissection of the Brassica – Leptosphaeria Pathosystem. Frontiers in Plant Science, 7, 1771. https://doi.org/10.3389/fpls.2016.01771
Keywords
1. Brassica napus defense response
2. Leptosphaeria maculans resistance
3. R gene expression in canola
4. Host genetic background effect
5. Blackleg disease in crops