Penn biologists find on-off switch for flower formation

April showers bring May flowers, or so the saying goes, but flower formation actually relies on a number of other cues, including day length, temperature, and season.

Penn researchers recently clarified yet another one of the signals that tells plants when to form flowers and when to grow vegetation. Their findings do more than help people understand the process that gives rise to pretty spring blooms; the study’s results also suggest ways that farmers could coax plants to produce more flowers, and thus more seeds and fruits, improving their bottom lines.

The study, published earlier this month in the journal Science, was led by Nobutoshi Yamaguchi, a postdoctoral researcher, and Doris Wagner, professor and graduate chair of the Department of Biology in Penn Arts & Sciences.

The researchers started with the seeming paradox that long-lived plants, or perennials, appeared to have a different mechanism for regulating flower formation than short-lived plants, or annuals. Whereas the hormone gibberellin was believed to inhibit flowers from forming in perennials, most scientists believed the hormone promoted flower formation in annuals.

“It was a big puzzle,” Wagner says. “Why would the same hormone do one thing in short-lived plants and another in long-lived plants?”

Hoping to find an answer, the team screened the genome of the plant Arabidopsis thaliana for genes that interact with the LEAFY gene, which is known to promote flower formation.

One of the genes they found produces an enzyme called cytochrome 450 that breaks down gibberellin. Plants that lacked a working version of this protein had flowers that formed much later than normal.

“At first we were confused because gibberellin was supposed to promote all of this activity that leads to flower formation,” Wagner says. “Then when we found a direct target of LEAFY that is linked to gibberellin catabolism, that gave us the clue that gibberellin must have a role in inhibiting flower formation, as well.”

When the researchers sprayed plants with gibberellin, the plants showed signs that they were getting ready to make flowers—they stopped making stems and leaves—but never actually formed the blooms.

The results indicated that gibberellin levels rise and then fall to promote flower formation. High levels cause the plant to cease making stems and leaves. Then cytochrome 450 breaks down the hormone, which frees up another set of proteins to tell the plant to make flowers.

This new understanding of gibberellin’s role could help create plant breeds that are even more productive.

“We think it can be used to enhance yield,” Wagner says. “Seeds are the product of a flower so if you want more seed, you want more flowers. Being able to modulate the accumulation or degradation of gibberellin could allow one to optimize or enhance the seed set and yield in crop plants.”