Their very existence was once controversial but prions now move in the best scientific circles, having been documented in animals including human and in yeast as a non-genetic but heritable mechanism for cell regulation. But what about prions in plants? So far no one has identified a plant prion or shown that prions were even possible in the plant kingdom. Now Sohini Chakrabortee and colleagues in the Whitehead Institute/MIT lab of ASCB member Susan Lindquist report in PNAS the first example of a plant protein acting like a prion.
The demonstration, though, was done in a heterologous yeast model system where prion proteins have been well characterized and closely mapped to the prion domains (PrD) that give rise to them. First the researchers used a computational system to sift through nearly 500 Arabidopsis thaliana proteins, looking for candidate PrDs that resembled homologous PrDs in yeast. The three most likely PrD suspects from Arabidopsis were all involved in the plant’s flowering, a critically timed activity where environmental “memory” could be vital. The Arabidopsis proteins were plugged into the yeast model system to see if the Arabidopsis proteins would behave like a prion, in lieu of a known yeast prion. All three formed high-order oligomers but one Arabidopsis protein—Luminidependens (LD)—took on the full prion form and function of the yeast prion, Sup35.
In his fascinating commentary on Chakrabortee et al. in the same issue, Yory O. Chernoff of the Georgia Institute of Technology in Atlanta and the St. Petersburg State University in Russia, points out that the Arabidopsis LD protein is involved in “vernalization,” a process that triggers flowering by natural or artificial exposure of plants to cold. Vernalization has a sinister reputation in plant biology. It was used by Tofin Lysenko, the Ukranian agronomist, to improve Soviet grain production in the early 1930s and then to completely reject the chromosome theory of Mendelian inheritance. Vernalization is what we would call epigenetic memory since it passes along heritable traits outside of the DNA/RNA information stream. Since plants and potentially people could supposedly be improved by direct epigenetic influences like cold or politically correct thinking, it made Lysenko the scientific pet of Joseph Stalin. With Stalin’s backing, Lysenko ruthlessly hounded his opponents out of science (and to death in some cases). In the backlash that followed Lysenko’s fall in the 1960s, all his work on epigenetic vernalization went to the scrapheap. Chernoff suggests that intriguing work like Chakrabortee’s on the possibility of prions in plants will allow us to reconsider old ideas without fear or ideological preconceptions. “Novel ideas frequently recapitulate something that has been previously proposed and forgotten,” Chernoff notes.
