Study reveals how fungus infects plants

Experts seek to prevent pathogens from suppressing plant’s immune system, making it vulnerable to infection

LINDELL BEACH, B.C. — Remember that dish of strawberries that got pushed to the back of the fridge and eventually grew a coat of grey, fuzzy mould? 


A research team at the University of California, Riverside, has discovered the mechanism by which a virulent fungus, botrytis cinerea, which causes grey mould disease, manages to infect fruit, vegetables and bulb crops.


Botrytis cinerea most notably affects grapes: its “botrytis” reference comes from ancient Greek meaning “diseased grapes.”


In viticulture, the destructive mould is known as botrytis bunch rot and can be triggered by tiny wounds to the fruit caused by insects, wind or accidental damage. In horticulture, the fungus is known as grey mould. Only occasionally, the fungus can cause “winegrower’s lung,” a rare allergic respiratory reaction in predisposed individuals.


Finding ways to prevent food spoilage is microbiology’s major contribution to the future of food security and distribution. The more researchers can understand the science behind the causes of food loss and waste, the greater the chance of reducing waste and helping feed a growing population. 


“Those very aggressive pathogens can really decimate plants,” said Hailing Jin, a professor in plant pathology and microbiology at UC Riverside. 


“They can affect more than 200 plant species and almost all the plants are for food. It is very common. If you leave your grapes on the table for several days, the pathogens will come. If you leave them at four degrees in the fridge for only a week, the pathogens come and you will see mould on the fruit.”


Jin’s research team discovered an aggressive “virulence mechanism” that can silence, or hamper, a host plant’s immunity genes.


According to the news release from UC Riverside, many fungal pathogens can deliver protein effectors that they secrete into the cells of host plants. These protein effectors suppress the host plant’s immune responses and promote infection. 


In the case of an aggressive fungal pathogen such as botrytis cinerea, small RNAs (technically known as Bc-sRNAs) are delivered into host cells to “hijack” the plant’s own RNA interference machinery to silence the plant’s immune responsive genes. The pathogen transfers aggressive small RNA effectors of its own into the plant’s cells to suppress its immunity so that the pathogen can establish itself and infect the plant. 


RNA is ribonucleic acid, a family of molecules that perform important roles in coding, regulating and expressing genes. Along with DNA and proteins, they are essential for all forms of life. 


This new study represents the first example of how a fungal pathogen uses small RNAs to suppress a host plant’s immunity and ultimately establish infection. Knocking out the host’s immune system is a battle tactic with enormously successful consequences for the virus. 


The goal of Jin’s research is to understand the molecular mechanism of a plant’s immune system in the fight against fungus pathogens. Her research seeks to understand how these pathogens manage to compromise a host plant’s immune system with a strategy of its own to gain control of the plant. 


As destructive as botrytis is, its infection has proven to have value in making dessert wines, notably a Hungarian sweet wine made from ripe grapes infected by the fungus. 


It is known as noble rot, and this fungal condition is harmless to humans. The grey fungus infects the grapes when they are still on the vine during wet weather. The grapes then become partially raisined during dry conditions following the high humidity. During this drying period, the fungus concentrates and intensifies the sugars and acids in the grapes, giving sweet wines their unique qualities. 


References to wines made from botrytised grapes appeared as long ago as 1576 and possibly earlier in Hungary. 


With better understanding, scientists will be able to develop more effective means of disease control. 


Jin’s lab is focused on the study of small RNA-mediated gene regulation in plants. The discovery of a bacteria-induced small RNA in her lab provided the first example of the regulatory role of small, interfering RNAs in plant immunity. 


“Our study discovered a novel new mechanism that the pathogen uses so we can explain why they are so successful,” said Jin. 


“What I mean is that the effector molecules produced by the pathogen are short RNA molecules, which don’t code any proteins but are regulating genes. They are used for silencing, or suppressing, (a host’s) genes. They can be delivered into the host cell.” 


Jin said the small RNAs interfered with the expression of genes not only in one species but across a broad spectrum of plant species. 


“Most of the research only studied what happened in one organism, but now several studies have shown that this can happen across different species,” she said. 


Jin said that more needs to be known about these small RNAs: how they work and how they can be blocked. 


The study, Fungal Small RNAs Suppress Plant Immunity by Hijacking Host RNA Interference Pathways, was published in the journal Science.