Vienna (OTS) – Viruses are a threat to all organisms, including plants. However, a small group of plant stem cells successfully defends itself against infection. Marco Incarbone, now at MPIMP Golm, Gabriele Bradamante and their co-authors at the Gregor Mendel Institute for Molecular Plant Biology (GMI) now show that salicylic acid and RNA interference are responsible for this antiviral immunity of stem cells. The results were published on October 12 in the journal PNAS.
Plant viruses threaten the health of their hosts, can spread quickly and worldwide and endanger agricultural productivity. If the infection is successful, the virus often takes over the entire plant. Well, not quite. A small group of indomitable cells continues to resist the invader: the stem cells in the shoot tip. These cells give rise to the entire above-ground plant tissue and thus also the next generation of plants, and – for reasons that are not fully understood – viruses cannot multiply in these cells. Marco Incarbone, former postdoctoral researcher in Ortrun Mittelsten Scheid’s group at the Gregor Mendel Institute of the Austrian Academy of Sciences (GMI) and now group leader at the Max Planck Institute for Molecular Plant Physiology in Germany, clarified together with doctoral student Gabriele Bradamante and others GMI group members uncover the molecular basis of virus defense in these stem cells.
In order to understand the antiviral defenses of this cell group, Incarbone, Bradamante and colleagues first set up a screening platform. “We developed high-throughput microscopy techniques that allowed us to examine the shoot tips of many Arabidopsis plants at different times after virus infection. “This gives our study a temporal dimension,” explains Incarbone. Using this dynamic, semi-quantitative approach, the researchers observed that the Turnip Mosaic virus – their preferred model virus – advances to the stem cells in the shoot tip of the model plant Arabidopsis thaliana and even penetrates these cells , but is then quickly ruled out: “Surprisingly, these cells are really good at driving out the virus.”
Previous work on a close relative of tobacco provided evidence that RNA interference – a signaling pathway that inhibits virus replication in plants and many animals – and salicylic acid – an important plant defense hormone – also play a role in virus defense in plants. In their search for the molecular basis of defense in stem cells, the researchers examined mutant Arabidopsis plants that lacked certain components of the RNA interference signaling pathway, as well as plants that lacked salicylic acid.
Using targeted experiments, the researchers found that the production of salicylic acid is activated during a viral infection. “The plant recognizes the virus and uses salicylic acid as an alarm bell.” Salicylic acid, in turn, activates RDR1, a key factor for enhancing RNA interference. RDR1 increases the production of double-stranded RNA from viral RNA, giving plants more virus-specific sequences that direct the defense mechanism toward the invading virus.
In the fight against Turnip Mosaic Virus, both salicylic acid and RDR1 are necessary to expel the virus from the stem cells. However, RDR1 is not produced in the stem cells themselves, but in the tissue underneath and in the vessels, continues Incarbone. “There it generates the RNA-based and most likely mobile information that immunizes the stem cells against the invading virus. While we know that infection triggers a general increase in salicylic acid, we do not yet know where in the plant or at what point during the infection this occurs. We are currently trying to answer these questions.” But every virus is different, and when fighting other viruses, salicylic acid and RDR1 are activated but not necessarily needed. “However, from our experiments with other viruses we can conclude that RNA interference is always necessary to protect stem cells from infection.”
The stem cells still hold a mystery: plant viruses are known to be able to bypass or paralyze RNA interference. “Why can viruses suppress RNA interference in most of the plant, but not in these specific cells? That remains the big question.” In further studies, Incarbone will now investigate how viruses are prevented from penetrating the seeds and thus into the offspring of an infected plant – because these develop from the protected stem cells. “Our results provide important insights into how stem cell antiviral defense works and provide a molecular framework on which we can build.”
Original publication:
Incarbone, Bradamante et al. Salicylic acid and RNA interference mediate antiviral immunity of plant stem cells. PNAS. DOI: 10.1073/pnas.2302069120
The GMI:
The Gregor Mendel Institute for Molecular Plant Biology (GMI) of the Austrian Academy of Sciences (ÖAW) conducts cutting-edge research in molecular plant biology. Research topics include epigenetics, cell biology, plant-pathogen interactions, developmental biology and population genetics. The GMI is located at the Vienna BioCenter, one of the leading life science locations in Europe.
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