Broccoli Looks more like Cauliflower in Warmer World

By Krishna Ramanujan

As seasoned gardeners know, broccoli heads don't develop properly and can resemble cauliflower when grown in higher temperatures.

A new study identifies the genetic underpinnings for why broccoli heads become abnormal when it's hot, providing insight into effects of climate-induced warming for all crops and pointing the way for breeding heat-resistant new varieties.

Broccoli grows best when planted in cool early spring or late summer, into fall. The researchers found that while broccoli grows normally at average temperatures of 61 degrees Fahrenheit, its crowns begin to deform at 72 degrees F and it forms dense cauliflower-like heads (called curds) at 82 degrees F.

The researchers then applied 5-azacytidine, a chemical known to inhibit a process called DNA methylation, where a methyl group—a small molecule—gets added to DNA. Methylation is one mechanism for turning genes on and off; in this case it suppresses a cluster of genes required for normal broccoli head production. When 5-azacytidine was administered, normal broccoli heads grew even at 82 degrees F, suggesting that methylation was behind the abnormal growth in the presence of heat.

"Once we understand the mechanism better, we should be able to devise ways to develop a new biotechnology, a molecular genetics approach to suppress DNA methylation, in order to breed crops to grow in much warmer temperatures and in wider regions," said Susheng Gan, professor in the School of Integrative Plant Science Plant Biology Section, in the College of Agriculture and Life Sciences, and a co-corresponding author of the paper published Dec. 22 in the journal Molecular Horticulture.

Liping Chen, a professor of vegetable science at Zhejiang University in Hangzhou, China, is the other corresponding author.

Warming temperatures impact all aspects of growth and development in plants. Flower development is especially complex and sensitive to temperatures, with heat reducing the quality and yield of vegetables such as broccoli where the complete flower head—including stems, stalks, leaves and flowers—is eaten. Though more study is needed, Gan believes that at higher temperatures, DNA methylation of genes involved in floral development may be conserved across all crops.