A new paper by researchers at the University of Pennsylvania published in the journal Science
has revealed that a plant hormone once believed to promote flower
formation in annual plants also plays a role in inhibiting flowers from
forming. The dual role of this hormone, gibberellin, could be exploited
to produce higher-yielding crop plants.
The study was led by
Nobutoshi Yamaguchi and Doris Wagner of the School of Arts and Sciences'
Department of Biology. Wagner is professor and graduate chair, and
Yamaguchi is a postdoctoral researcher. Department co-authors included
Cara M. Winter, Miin-Feng Wu and Ayako Yamaguchi. The Penn team
collaborated with Yuri Kanno and Mitsunori Seo of RIKEN Center for
Sustainable Resource Science in Japan.
Plant scientists used to
think that short-lived plants, annuals or bi-annuals, use a different
strategy from long-lived plants, perennials, to regulate flower
production.
"Anecdotal evidence was that the hormone gibberellin
promoted the switch to flower formation in short-lived plants, along
with other cues such as temperature, season and photoperiod," Wagner
said. "But in the long-lived plants, like in fruit trees, people have
known that if you sprayed them with the hormone it inhibited flower
production. So it was a big puzzle: why would the same hormone do one
thing in short-lived plants and another in long-lived plants?"
To
address this paradox, the Penn team began by looking for new genes
important to the flower-forming process. Specifically, they performed a
genome-wide search of the plant species Arabidopsis thaliana to find direct targets of the protein LEAFY, which is known to promote flower formation.
One
gene that turned up was called ELA1, which produces a cytochrome enzyme
and has been shown to play a role in breaking down gibberellin. Further
experiments showed that in plants that lost ELA1 function, flowers
formed much later than normal.
The researchers also found that
plants that lacked LEAFY had high levels of gibberellin, and plants
engineered to produce high levels of LEAFY had lower levels of the
hormone and were also shorter with greater levels of chlorophyll --
characteristics of gibberellin deficiency.
"At first we were
confused because gibberellin was supposed to promote all of this
activity that leads to flower formation," Wagner said. "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."
Plants that were genetically modified
to not produce gibberellin properly and plants that were treated with a
gibberellin inhibitor showed signs of a delayed first transition to
inflorescence but accelerated signs of flower formation. Spraying the
plants with gibberellin had the opposite effect.
The results
suggested that the two transition steps that lead plants to produce
flowers might be regulated distinctly, both involving gibberellin. While
gibberellin promotes the first transition, in which plants stop
producing stems and leaves and produce an inflorescence, it inhibited
the second stage, in which flowers were formed.
The mechanism,
the Penn team showed, involves rising and then falling levels of
gibberellin. High levels cause the plant to end the vegetative phase of
development. At that point, LEAFY and ELA1 activity cause gibberellin to
break down. Freed from the inhibitory effects of the hormone, a suite
of proteins are activated that trigger flower formation.
"When it
comes to determining the number of flowers formed and when they are
formed, we think this pathway is at the forefront," Wagner said.
Farmers
already use gibberellin-deficient breeds of rice to produce more
compact plants that don't topple over in wind and rain. The new
understanding of gibberellin's role gained from this study could help
create plant breeds that are even more productive
"We think it
can be used to enhance yield," Wagner said. "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."
The
Penn team plans to explore other plants to see if gibberellin operates
the same way across species and in perennials as well and to further
explore how different levels of the hormone trigger regulatory events
that either inhibit or promote flower production.
The National Science Foundation supported the research.
