Garden enthusiasts know that the formation of broccoli heads can be likened to cauliflower when grown under high-temperature conditions. Groundbreaking research has recently shed light on the genetic reasons that cause broccoli heads to deform under heat. This discovery provides profound insights into the implications of climate change on crop yield, and paves the way for breeding heat-tolerant crop variants.
Broccoli thrives best when cultivated during the cool early spring or late summer transitioning into fall. The research showed that under average temperatures of 61 degrees Fahrenheit, broccoli grows normally. However, at 72 degrees F, its crowns start to deform, and at 82 degrees F, it forms dense, cauliflower-like heads, referred to as curds.
The scientists introduced 5-azacytidine, a chemical known to inhibit DNA methylation - a process in which a methyl group is appended to DNA, effectively switching genes on and off. This process inhibits a set of genes crucial for normal broccoli head formation. The application of 5-azacytidine led to the growth of regular broccoli heads even at 82 degrees F, indicating that methylation was the cause of abnormal growth in higher temperatures.
"Once we comprehend this mechanism more thoroughly, we can explore ways to create a new biotechnology that suppresses DNA methylation. This could help us breed crops that can withstand higher temperatures and grow in a wider range of regions," stated Susheng Gan, a professor at the School of Integrative Plant Science Plant Biology Section, in the College of Agriculture and Life Sciences. The study, co-authored by Gan and Liping Chen, a professor of vegetable science at Zhejiang University in Hangzhou, China, was published in the journal Molecular Horticulture on Dec. 22.
Rising temperatures influence all stages of plant growth and development. The process of flower development is particularly sensitive to temperature changes, which can affect the quality and yield of vegetables like broccoli where the entire flower head – stems, stalks, leaves, and flowers – is consumed. While further research is necessary, Gan suggests that the DNA methylation of genes involved in floral development could be common across all crops at higher temperatures.
In their study, the scientists analyzed broccoli plants (Brassica oleracea) using whole-genome sequencing technologies. They identified the plant’s methylome – the sites in the genome where methylation occurs – and its transcriptome – which identifies active genes. They discovered that abnormal flower development in broccoli was controlled by sets of floral development cessation-associated genes (FCGs). At 61 degrees F, broccoli heads developed normally. But at 82 degrees F, and to a lesser extent at 72 degrees F, the methylation of genetic elements that activated FCGs was suppressed.
When grown at warmer temperatures, floral development becomes restricted to earlier stages of development. So, at 72 degrees F, broccoli buds are restricted to a stage where they look like a cross between a broccoli head and cauliflower curd; and at 82 degrees, they are even further undeveloped, and resemble a cauliflower-like curd.
Future studies will examine the mechanism behind DNA methylation of FCGs at high temperatures.
Reference: Yao Z, Yuan L, Liu K, et al. Warming-induced changes of broccoli head to cauliflower-like curd in Brassica oleracea are regulated by DNA methylation as revealed by methylome and transcriptome co-profiling. Molec Hort. 2022;2(1):26. doi:10.1186/s43897-022-00047-8
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