According to researchers at the Salk Institute for Biological Studies and the Howard Hughes Medical Institute, primary metabolism likely arose from promiscuous primeval metabolic reactions and evolved toward greater catalytic precision and efficiency. This image illustrates the stepwise assembly of a specialized metabolic pathway using descendents from enzyme folds rooted in primary metabolism (indicated by circular phylogenetic trees and highlighted by Greek letters). Products of one reaction serve as substrates for another. Red arrowheads indicate the recruitment of single enzymes from protein families. The extensive radiation of the specialized metabolic system in plants have greatly contributed to plants’ dominance in the terrestrial environments. Credit: Jing-Ke Weng, Salk Institute

“Plants
produce a repository of structurally diverse chemicals …” That’s how a
new paper begins that proposes some provocative ideas about how plants
developed the wide assortment of chemicals they use to sustain life and
how they developed other chemicals that may or may not contribute to
their immediate survival, but instead often ensure reproductive success
in changing, earth environments.

In a June 29 journal Science
review paper, Joseph P. Noel, a lead investigator at the Salk Institute
for Biological Studies and the Howard Hughes Medical Institute, and
colleagues speculate that plant chemodiversity results from rapid and
sometimes unanticipated evolutionary steps.

Noel,
along with Jing-Ke Weng and Ryan Philippe also with the Salk Institute,
theorize that a very early form of chemical reactions that occurred in
the prebiotic soup paved the way for production of chemicals important
to the survival of the earliest cellular organisms—chemicals including
those essential for building nucleic acids—biological molecules such as
DNA, RNA and proteins necessary for encoding, transmitting and
expressing genetic information.

The
researchers speculate these chemical processes, now catalytically
robust, evolved into separate pathways both in early plants and in their
aquatic ancestors.

One pathway—having chemical processes termed primary metabolism—allowed the production of life-sustaining chemicals.

The
other pathway produced chemicals that no longer carry life sustaining
functions. Instead, these chemicals have more subtle effects on plants’
fitness, or reproductive success in their local environments.

In
the paper, the researchers hypothesize these secondary chemical
processes arose from the more conserved, life-sustaining processes and
term them “specialized metabolism.”

“Understanding
how plants evolved their ability to synthesize secondary
metabolites—such a vast and diverse array of chemicals—is a challenging
problem,” said Parag Chitnis, director of NSF’s Division of Molecular
& Cellular Biosciences, which funded Noel’s research. “In this
article, Dr. Noel and his colleagues present an attractive and plausible
explanation.”

Noel,
Weng and Philippe speculate that specialized metabolism is more
malleable than life-sustaining metabolic processes, and that specialized
metabolic systems can evolve rapidly to produce new “tailor-made
molecules” as means to adapt to ever-changing environments.

 

“Primary
metabolism likely arose from promiscuous primeval metabolic reactions
and evolved toward greater catalytic precision and efficiency,” the
researchers write in their article. “Specialized metabolism likely
emerged from primary metabolism.”

Salk Institute for Biological Studies researcher, Joseph Noel, and colleagues have been asking how has evolution shaped the complex natural chemistry of plants the world over and what role do these critical natural chemicals play in allowing plants to adapt to such a diverse set of complex and challenging ecosystems? This work funded for more than a decade by the National Science Foundation, is providing new discoveries in plant metabolism, their amazing chemistry and even the development of new technologies to benefit humankind in nutrition, disease prevention and the guilty pleasures of flavors and fragrances. More importantly though, this decade plus study, has provided unanticipated clues as to the rules governing the evolution of plant metabolism and their amazing repetoire of enzymes allowing humans to harness this information to engineer new kinds of metabolism for the burgeoning biorenewables and biofuels industries. Credit: Jing-Ke Weng, Salk Institute

According
to the researchers, specialized metabolism likely permitted more and
varied chemical reactions and natural products because the enzymes
responsible for their synthesis were more flexible in ways scientists
are only now beginning to understand at the molecular level.

The
upshot was the emergence of secondary chemical reactions that produce
color in flowers; rubber for vehicle tires; flavor, smells, nutrition
and browning in fruits and wine; natural plant antibiotics; fragrances
to attract pollinators and repel herbivores, and even the characteristic
aroma and flavor of the cabbage and tomato families.

“Plant
secondary metabolism generates a huge diversity of chemicals that are
not only very important to the plant, but also for humans,” said Greg
Warr, a program manager in NSF’s Division of Molecular & Cellular
Biosciences. “For example, we often depend on plant products for
nutrition, fuel, biorenewable chemicals, clothing, shelter and
pharmaceuticals.”

What’s
more, Noel and colleagues speculate the depth of specialized metabolism
likely mirrored the takeover of Earth by plants that form the essential
core of the global food network. As primary metabolism produced
life-sustaining chemical reactions, specialized metabolism gave rise to
secondary chemical reactions that allowed plants to adapt to
geographically dispersed environments, many of which are challenging to
other forms of life.

For
example, some metabolites, plant hormones regulate various aspects of
plant growth and development in response to environmental cues, while
others act as ultraviolet sunscreens and prevent dehydration.

“The
ability of these complex biological systems in plants to evolve quickly
to solve problems of plant survival and reproduction, will ultimately
teach us the lessons learned over a 500 million year old experiment
plants have been conducting since the dawn of terrestrial life,” said
Noel.

 

“Without this ongoing experiment, humankind and all the animal life we know of on the terrestrial earth would cease to exist.”

The researchers hope the Science
review paper will help provide a provocative and more informed set of
hypotheses regarding the amazing tapestry of plant chemistry while also
posing still unanswered but fundamental problems to life. “It will
certainly guide future research in this important area,” said Chitnis.

Source: National Science Foundation