2019 - Issue 3

This InterView with Hailing Jin, Professor of Genetics at the University of California-Riverside, was conducted by Sowmya Ramachandran, a PhD candidate in the Department of Plant Pathology at Washington State University. If you are interested in completing your own InterView, please contact​ Interactions Editor-in-Chief Dennis Halterman.

Dr. Hailing Jin is the Cy Mouradick Endowed Chair and Professor of Genetics at The Institute of Integrative and Genome Biology, University of California (UC), Riverside. Her group works on plant–pathogen interactions with emphasis on cross-kingdom RNA interference and small RNA trafficking between plants and fungi.

Sowmya Ramachandran (SR): Thank you for giving me the opportunity to speak with you today. I would like to know what motivated you to enter plant science and eventually establish a program on host–pathogen interactions?

Hailing Jin (HJ): When I was young, my grandpa used to grow flowering plants at home. The beautiful and vivid flowers of jasmine and chrysanthemum attracted me toward plants from a very young age. More specifically, my interest in small RNAs started to develop when I was a post-doc at John Innes Center while studying transcription factors and gene regulation. Around this time, Andrew Fire and Craig Mello discovered RNA interference (RNAi) in Caenorhabditis elegans. A year later, David Balcombe, then a scientist at Sainsbury Laboratory, published a seminal study on posttranslational gene silencing, and together with Craig and Mello’s work, this opened up a new area of research. So, when I joined Barbara Baker’s lab at UC Berkeley Plant Gene Expression Center (PGEC), I utilized the RNA interference-based approach—specifically, virus-induced gene silencing to dissect the signaling transduction pathway of the N gene-mediated resistance to Tobacco mosaic virus in Nicotiana tabacum and N. benthamiana, which piqued my interest for small RNA-mediated gene regulation. By 2004, when I started my own lab at UC Riverside, studies had established the role of small RNAs in development, but very few had looked at their involvement in other processes, especially biotic stress responses. Combining my expertise in gene silencing and gene regulation in plants, I wanted to explore the role of small RNAs in plant–microbial interaction. I was particularly interested to understand plant endogenous small RNA silencing during bacterial and fungal infections. At the time, this was a unique niche and not many scientists were working in this area.

SR: Do you see small RNAs as effective management tools for plant diseases?

HJ: Yes, this is something our group is excited about. Now we can generate double-stranded RNAs or small RNAs that target fungal virulence genes in the plant. These small RNAs can be delivered into the plant and then enter the fungus to silence specific target genes. This strategy also allows us to custom design constructs for controlling diseases in different regions and against different pathogens at the same time. For example, if Botrytis and Sclerotinia are major pathogens in California, we can design constructs to target essential fungal genes, like Dicers, and control both diseases at the same time. The study was published in Nature Plants in 2016. Basically, now we can generate transgenic plants that can target multiple pathogens based on our needs in different regions and different seasons. But at the same time, transgenic plants and GMOs are still a technical challenge for many crops, such as tree crops, vegetables, and flowers, and some require a long time. It is also a concern for consumers in many regions of the world. So in this case, it would be ideal to develop an ecofriendly, easy-to-use, and non-GMO way to combat plant diseases. This led us to discover RNA uptake by fungal pathogens. 

Over the years, people have observed that Caenorhabditis elegans and other nematodes can take up RNAs from the environment. Since nobody had shown this for fungal cells, our group decided to give it a try. We put Botrytis spores on plates containing fluorescence-labeled RNA and saw RNA being efficiently taken up into fungal cells. This allowed us to use synthetic double-stranded RNAs or small RNA duplexes in the form of sprays on the plants or postharvest products, including vegetables, flowers, and fruits. This strategy offers an ecofriendly and natural alternative to fungicides. These small RNA fungicides will eventually degrade in the soil and leave no toxic residues, unlike chemical fungicides. They can also be designed in a way that they hardly have any off-target effect and at the same time be more durable. As most fungi have already developed complete or partial resistance to fungicides, there is an urgent need to develop a new generation of fungicides. So I think this discovery of RNA uptake will lead to development of a new class of RNA-based, ecofriendly fungicides.

SR: Through your research on Botrytis and cross-kingdom RNA, you show small RNAs can act as effectors that interfere with host processes, similar to protein effectors. Is it possible to develop resistance to small RNAs in the pathogen?

HJ: There are several ways for pathogens to develop resistance to RNAi. One is to change the sequence of the RNA to escape RNAi. But we now know that small RNAs can tolerate many mismatches in their target region. We can also overcome this by targeting essential genes, which cannot mutate rapidly owing to the importance of protein functions. Another strategy that may be employed by the pathogens is to kick out their RNAi machinery. However, this will depend on the genome complexity of the pathogen, like the presence of transposons, and the importance of the RNAi machinery in the pathogen’s growth and defense. A third way of developing resistance is through eliminating the RNA uptake pathway. However, since RNA uptake is an important nutrient acquisition strategy, removing this pathway may not be feasible for the pathogen. Based on this, it seems it will be harder for pathogens to develop resistance to small RNA fungicides. These are some possibilities I can think of currently, based on which it will be harder for pathogens to develop resistance to small RNA fungicides. Even otherwise, we routinely use more than 100 bps dsRNA fragment for one gene, so even if there are a few mutations in this region, there is still enough homology to silence that gene. We also use a mix of small RNAs targeting multiple genes, which should make it harder for the fungus to develop resistance to the small RNAs.

SR: Since RNA is unstable in nature, how do you suppose small RNAs will remain stable when delivered in the field?

HJ: Our group recently published a paper in Science in which we reported that fungi can take up small RNA-containing extracellular vesicles from the plant hosts. This process is very efficient, as within 2 hours of delivery, all the vesicles are taken up by the fungus. Based on this finding, we are developing a way to package small RNAs into artificial vesicles to prolong its life in the environment and also increase the fungal uptake. Also, Neena Mitter’s lab at the University of Queensland, Australia, has developed nanoparticles that can increase the stability of these small RNAs and protect them. We are now collaborating with Neena’s lab to come up with formulations that will best protect these small RNA fungicides in the fields.

SR: RNAi machinery is under sophisticated regulation to ensure precise functions in growth and defense. Your lab recently found that Arabidopsis Argonaute 2 (AGO2) is regulated through arginine methylation upon bacterial infection. How does arginine methylation-mediated dual regulation modulate plant defense? 

HJ: This is an interesting question. In a recent paper published in Nature Communications, we show that the RNAi machinery is under a very sophisticated regulatory control. We have shown that Arabidopsis AGO2 protein is regulated by posttranslational modification. Quite a few years ago, our lab discovered that AGO2 protein is the only AGO in Arabidopsis that is highly induced by bacterial infection. In case of miR393, we know that it targets auxin receptors as well as functions in plant defense. We found that miR393 is loaded into AGO1, while the other strand of miR393 duplex, miR393*, is loaded into AGO2. This miR393* version can target a SNARE protein to promote secretion of pathogenesis-related protein PR1. Although we know AGO2 play an essential role in plant defense against bacterial pathogens, their regulation is poorly understood. To this end, our group recently found that the N-terminal of AGO2 has arginine-glycine (GR/RG) repeats. The arginine residues of these repeats are methylated by protein arginine methyltransferase 5 (PRMT5). This modification can lead to a dual regulation of AGO2: one leading to AGO2 degradation and another to recruit Tudor-domain proteins (TSNs), which degrade AGO2 bound to small RNAs. 

Under normal conditions, when plants don’t need immune responses to be activated, this mechanism can dampen AGO2-mediated plant immunity. However, upon bacterial infection, PRMT5 is down-regulated and the arginine methylation is reduced so AGO2 proteins can be accumulated to a high level, along with AGO2-associated small RNAs. Together, this dual regulation of AGO2 can precisely modulate RNAi and immune responses during infection. 

SR: Based on your experience as a professor and a biologist, can you identify some qualities that are needed to be successful in this field?

HJ: To be successful in science, one needs to have passion and dedication for their work. If you love what you are doing, you will invariably do a good job! Personally, I feel motivated when my work can benefit the environment and the society in some way. If I can help the world through my work and through my teaching, then my time is well spent and my life is meaningful! My work with plant protection and disease resistance, such as developing environmental-friendly fungicides, seems fulfilling, as it can directly help the planet. 

SR: Thank you very much for your time. It was fascinating to know more about your research. I am sure your inspiring words will be valuable to young scientists aspiring to careers in plant biology.

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