QNAS QNAS QnAs with Caroline Dean

Prashant Nair, Science Writer

As winter gives way to spring, many plants erupt in a understanding of vernalization. Dean spoke to PNAS riot of colors in a bid to attract pollinators and set about the work that led to the award. seed. As if by clockwork, these species time their bloom, delaying flowering until the onset of spring, PNAS: How did you become interested in vernalization? when conditions favor fertilization and seed develop- ment. Researchers have long known that plants delay Dean: I grew up in the north of England and enjoyed flowering until they have experienced a period of the distinct seasons, so when I went as a postdoc to prolonged cold, a process termed vernalization. For California, I missed the clear seasonal transitions. In decades, plant breeders have exploited this cold- California, I bought some tulip bulbs, and the person in dependence to develop varieties of cereals and veg- the shop said to me “Put them in the fridge for etables sown in the winter and spring, extending crop 6weeks.” I was so struck by the comment that I went range to ensure year-round supply. The molecular basis and read about why I would put them in the fridge, and of vernalization, however, is just beginning to come realized that [tulips] actually need the cold to bloom. into view. The question of how plants sense the passing That got me thinking generally about how plants use of winter and prepare to bloom preoccupies plant seasonal cues to time their development. Around this molecular biologist Caroline Dean at the John Innes time, the phenomenon was happening ’ Centre in Norwich, United Kingdom. Dean sworkand [plant geneticists had discovered the utility of focusing its implications for agriculture have gained increasing on one species as a model organism], and we could currency in the face of a changing climate. A foreign now clone genes important for complex traits. So when associate of the National Academy of Sciences, Dean I started my own laboratory, I chose to tackle the mo- received the 2018 L’Or ´eal-UNESCO For Women in Sci- lecular basis of vernalization in Arabidopsis. ence award for her contributions to the molecular PNAS: We are in the midst of cherry blossom season in Washington, DC, and the cherry trees burst into within days to weeks of each other seemingly on cue. Is that vernalization in action?

Dean: A related process; trees arrest their buds in autumn, with both temperature and photoperiod being important to break that arrest and promote the spring bloom that everyone enjoys. We know that many of the same molecular components are required to control flowering in trees and Arabidopsis, but still have to fully elucidate the regulatory network.

PNAS: What are some examples of commercially important plants that undergo vernalization?

Dean: Vernalization is important for many of our major crops. Winter and spring-sown varieties have been bred for many cereals, [for example], wheat and barley. On the other hand, for vegetable Brassica varieties, [such as] broccoli and cauliflower, breeding has focused Caroline Dean. Image courtesy of Thierry Bouët (photographer). on differing cold requirements that lead to flowering at

Published under the PNAS license. Published online May 29, 2018.

5824–5826 | PNAS | June 5, 2018 | vol. 115 | no. 23 www.pnas.org/cgi/doi/10.1073/pnas.1807547115 Downloaded by guest on October 3, 2021 different times throughout the winter, ensuring a con- andlookedatRNAexpression,aswehadbeendo- tinuous supply to the supermarkets. ing, it would look like a nice, slow decrease. This cold-induced epigenetic silencing is then main- PNAS: Before we delve into the molecular tained through many hundreds of cell divisions, even of vernalization, could you briefly explain how plants when plants return to warm conditions in the spring; adapt to different lengths of cold exposure before in other words, it is epigenetically silenced. The FLC flowering? gene does not switch back on again until seed development. Dean: We are still dissecting exactly how plants register winter exposure. Clearly, plants have to PNAS: But what is upstream of FLC? How does cold distinguish a short period of cold in autumn from trigger the silencing of FLC? full-blown winter. Different varieties of show different cold requirements, presum- Dean: We are finding that temperature influences a ably as an adaptation to the climate they are growing large number of steps in FLC silencing. The major in. Typically, varieties from southern Europe only need temperature steps characterized so far include early 4 weeks of cold to maximize acceleration of flowering, cold-induced up-regulation of antisense RNA tran- whereas those from northern Sweden need 10 to scripts at FLC, and up-regulation of the VIN3 protein 12 weeks to do the exact same thing. required for the epigenetic switch. In the cold, VIN3 associates with a protein complex at the FLC PNAS: And the variation is genetically encoded, correct? locus to promote the epigenetic switch.

Dean: Yes, a small number of polymorphisms in the PNAS: Your recent article in Nature Communica- gene that underpins vernalization dictate the length of tions (1) explores how plants register fluctuating cold exposure a particular variety needs to flower. temperatures normally encountered in the field during vernalization. PNAS: You have uncovered the central role of the gene FLOWERING LOCUS C (FLC) in vernalization. Dean: What we showed in this paper is that plants are What does the gene do? monitoring distinct aspects of the fluctuating temper- ature to regulate different parts of the vernalization Dean: The FLC gene was cloned almost simulta- process. For example, FLC down-regulation occurs neously by two other groups. FLC represses flower- relatively quickly when temperatures drop below ing by blocking genes required to switch the plant’s 10 °C. In contrast, VIN3 accumulates gradually with apical meristem to a floral fate. We came to FLC time over a relatively wide temperature range (0– through our interest in the mechanism of vernaliza- 15 °C), but is degraded by transient high temperatures tion. Many natural variants of Arabidopsis have a win- (above 15 °C). So, it is the absence of warmth that is an ter annual habit and need prolonged cold exposure important trigger enabling accumulation of VIN3 and to flower. We mutagenized the winter annual variant epigenetic switching. This temperature registration to obtain vernalization mutants that could no longer strategy enables plants to “read and interpret” daily respond to cold. These mutants affected the silenc- temperature fluctuations, which often are as extreme ing of expression of FLC, which occurs gradually with as those over a whole season. cold exposure. We found that silencing of FLC expression hap- PNAS: Was that a surprising finding? pens through chromatin regulation: modifications of histone tails and chromatin remodeling [which are Dean: It wasn’t expected. Until recently, it was common routes of chemical modification that induce thought that plants slowly integrated cold tempera- conformational change in the genetic material to ture exposure to judge winter progression. In fact, switch off the expression of certain genes]. This par- horticulturalists have used day-degree calculations ticular type of gene-silencing mechanism is faith- to judge the progression of vernalization in the field. fully inherited through cell division (an epigenetic We now know these models will only work well when mechanism) and is conserved in plants, animals, the peak high and low temperatures are not too ex- and humans. treme. Instead of just averaging temperature over But if cold exposure silences FLC by essentially time, the plants are constantly monitoring maxi- switching off the gene’s expression, we wondered mum and minimum daily temperatures to judge why it was gradual with increasing cold exposure seasonal progression. [the length of cold exposure tracks time to flower- ing]. Through collaboration with computational biol- PNAS: Against the backdrop of climate change, does ogist Martin Howard, we showed that there is an that finding have any commercial implications for ON–OFFswitchateachindividualFLC gene. It takes food crops? all winter for all of the genes to be switched OFF, with the gradual silencing being a reflection of the Dean: The crop varieties that we grow are unlikely proportion of cells in which the FLC gene is switched to cope well with the predicted future extreme tem- OFF. So, naturally, if you ground up the whole plant perature fluctuations. Commercially, we have seen

Nair PNAS | June 5, 2018 | vol. 115 | no. 23 | 5825 Downloaded by guest on October 3, 2021 the consequences of such extremes. Last winter, for vernalize efficiently, it seems unlikely to play a major example, the unusually cold temperatures in the role in cold sensing. south of Spain triggered poor broccoli production. This led to a severe shortage in the supermarkets PNAS: Would you care to share your thoughts on winning around Europe. the L’Oreal-UNESCO For Women in Science award?

PNAS: The late molecular biologist Susan Lindquist Dean: I was thrilled. I just came back from the awards at the Whitehead Institute in Cambridge, Mas- events in Paris, and it was wonderful getting to know sachusetts, reported the role of a prion-like protein the other laureates and the rising talents: young, called LUMINIDEPENDENS (LD) in mediating plants’ talented, and enthusiastic women scientists from all memory of winter. Does the protein play a role in over the world, who [received fellowships and] were vernalization? also invited to the event. My hope is that this prize encourages other women to stay in research long- Dean: LD does affect FLC, but its role appears to be term; you can combine a fulfilling, high-level scientific chiefly in the warm. Since mutants defective in LD career with a relatively normal personal life.

1 Hepworth J, et al. (2018) Absence of warmth permits epigenetic memory of winter in Arabidopsis. Nat Commun 9:639.

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