Flowers don't just catch our eyes, they catch those of pollinators like
bees as well. They have to, in order to reproduce. Because plants need
to maximize the opportunity for pollinators to gain access to their
seeds, variations in the timing of flowering can have profound effects
on flower, fruit, and seed production, and consequently agricultural
yields.
The major driving forces of flowering are external factors such as
light and temperature. However, new research from CSHL Assistant
Professor Zach Lippman, Ph.D. and his collaborators, published online
November 11 in Nature Genetics, shows there is a second, previously unknown mechanism controlling flowering.
Using the tomato plant as their model, Lippman and CSHL co-authors,
Cora MacAlister, Soon Ju Park and Ke Jiang, show that loss of control of
the timing of flowering, such that the flowering program turns on too
fast, results in production of only a single flower on each branch,
rather than the usual 7 to 10. Conversely, slowing down the flowering
program enables more flowering branches to grow, which means more fruit.
Such dissection of the timing mechanism of flowering in plants like
tomato is leading to new strategies for increasing agricultural yield in
important crops.
Timing of flowering is precise
During the flowering process, plants form reproductive shoot
structures called inflorescences. These structures derive from small
stem cell populations buried inside the tiny growing tips of plants
called meristems. As plants sense and respond to signals from light
and/or temperature, it is at the meristems where plant organs -- leaves
or flowers -- are formed.
Domesticated tomato plants, which we know and love for their shiny,
tasty red fruit, typically grow several multi-flowered inflorescences on
each shoot. Each inflorescence is arranged in a zigzag pattern of 7 to
10 flowers on a single branch. Curiously, many wild species of tomato
produce multiple branches on each inflorescence, with each branch having
many flowers, thereby increasing the reproductive potential of the
plant. In rare cases, genetic mutants of domesticated tomatoes form
broom-like inflorescences with dozens of branches like the wild species.
Interestingly, there is another class of mutants that produce just a
solitary, sometimes abnormal looking, flower.
In previous research Lippman and others reasoned that the timing of
flowering would be important in determining whether an inflorescence was
highly branched or not. By characterizing the activity of thousands of
genes involved in the flowering process of tomato, Lippman and members
of his laboratory revealed a "molecular clock" coordinating whether
meristems give rise to branched or unbranched inflorescences.
In their newly published research, they reveal that one of those
genes plays a critical role in keeping the clock from ticking too fast.
TMF controls synchronization of the flowering transition
"In order for a plant to determine when and where to switch from
making leaves to making flowers everything has to be timed perfectly,"
says Lippman. "We know that the flowering process is regulated by
temperature and day length; these control one aspect of the timing. But
now we've found a new timing mechanism."
The moment of insight for Lippman and his team, including colleagues
at the Unité de Recherche en Génomique Végétale in Evry, France and the
Weizmann Institute of Science in Rehovot, Israel, came when studying
mutant tomato plants. "We found a gene that when mutated converts the
typical tomato multi-flowered inflorescence into one with a single
flower," Lippman says. Interestingly, this caused the tomato plant to
mimic other single-flowered plants of the same family, called
Solanaceae, which includes the eggplant, tobacco, petunia, and pepper
plants.
The gene Lippman's team found, called TERMINATING FLOWER (TMF), had
not been previously known to have such a crucial role in plant growth.
This was despite the fact the flowering process and the genes that
control it have been studied in great depth over decades in many plant
systems, including the model plant Arabidopsis as well rice and corn
(maize).
"It seems TMF regulates a previously unknown pathway that is involved
in the timing of flowering. The reason that mutations in TMF cause
single-flower inflorescences is that the plant is tricked into thinking
it is time to make a flower when it is still in the vegetative state --
the phase of growth that precedes flowering when leaves are still being
made," explains Lippman.
Flowering is a tightly coordinated process, so when TMF function is
lost the process becomes de-synchronized and uncoordinated. The external
signals from light and temperature have not yet reached the critical
threshold to tell the plant it is ready to make flowers, yet the program
for making flowers starts anyway. Thus TMF acts as an internal check on
the flowering transition. "Its normal function is to delay flowering,
to gently slow it down, so that it doesn't happen too precociously,"
Lippman says.
If plants make flowers too quickly, there may not be enough energy
from leaves to support those flowers and fruits. But Lippman suggests
that some species of plants have taken advantage of this mechanism and
evolved to make more or less flowers per inflorescence. It may be that
in nature, some plants are more successful when making fewer flowers
over a longer period of time, for example.
The Solanaceae species to which tomato belongs contains examples of
all types of inflorescences, which is why Lippman finds the model is so
fascinating to study. By learning about the genetic switches controlling
flower production, the hope is that they can be manipulated in
agricultural crops like tomato to improve yield.
