1. Characterizing Auxin Biosynthetic Mutants in Arabidopsis thaliana
Nicole Colón Carrión
University of Puerto Rico, Cayey campus; North Carolina State University
IAA, also known as auxin, is a phytohormone that control numerous processes in plant system,
such as growth, development and regulation. Several studies has concluded that auxin
biosynthesis is controlled by tryptophan pathway. However it's complete pathway and transport
is not fully understood. For plants to develop it is needed a fine balance of auxin; too much our
too little is bad for the plant development. By understanding how the plant synthesized auxin, we
can get a better understanding of how plants regulate this fine balance. This study aims to
understand auxin synthesis by identifying the missing genes involved in this pathway using
Arabidopsis as plant model.
Introduction: Several studies have indicated
thatauxinbiosynthesis is controlled by the
Arabidopsis thaliana is a small tryptophan independent pathway and
flowering plant related to the mustard tryptophan dependent pathway, however
family. This type of family is the most how this pathways work remain poorly
widely distributed; it has approximately 340 understood. Several pathways has been
genera and 3,350 species. Arabidopsisis postulated for Trp-dependent pathway; the
highly distributed around the world, it can indole-3- acetamide, the indole-3-pyruvic
be found in central Asia, Mediterranean acid, the trypthamine, and the indole-3-
regions and North and South America. This acetaldoxime pathway. The Trp-
plant is highly used as a model organism for independent pathways was postulated in
the study of plant biology, since is the first 1991 but how this pathway designs IAA is
plant to have it entire genome sequenced. not well understood. By understanding how
Studying Arabidobsis thaliana can help to a plants synthesis this hormone, we could
better understanding of plants biological understand better how plants regulate it fine
systems. balance.
Indole acetic acid, also known as
auxin was one of the first hormones to be II. Pathways of auxin biosynthesis
discovered. It is a phytohormone that
controls numerous processes in plant
systems, such as regulation, growth and A) Tryptophan dependent pathway
development. Indole acetic acid molecules
can inhibit or stimulate the expression of 1. The indole-3-acetamide pathway
certain genes. Auxin can control plant (IAM)
development at all levels; that’s why it is
important for plants to maintain a balance of The indole-3-acetamine
IAA, too much or too little can be fatal for pathway is a bacteria-specific
the plant pathway. Agrobacterium rhizogenes,

2. a bacterium that give rise to hairy into IAD in plants since the genes or
root disease in dicot plants, is enzymes has not been found. The
thought to induce IAA synthesis. IPA pathway also has it doubts since
Agrobacterium rhizogenes produce IPA has only been detected in
hairy roots were IAA is synthesis Arabidopsis seedlings.
from Trp. Tryptophan is converted
into indole-3-acetamide (IAM) by
Trpmonooxygenase enzyme. 3. The tryptamine pathway (TAM)
Afterwards, indole-3-acetamide
hydrolase encoded by AMl1 gene During the tryptamine pathway,
converts IAM into IAA. Indole-3- Trp is converted into TAM by
acetamide is found in numerous tryptophan decarboxylase (TDC).
plants indicating that IAM- TDC genes also contribute to indole
dependent pathway is functional in alkaloid and serotonin biosynthesis.
monocots or dicot plants. TAM is oxidated by the
amonooxygenase-like enzyme
Trp. encoded in the YUCCA gene and
• Trp monooxygenase
converted into N-hydroxytryptamine.
IAM The convertion of TAM into N-
• indole-3-acetimide hydrolase - AML1
hydroxytryptamine by the YUCCA
IAA gene has been questioned. A study
conducted by Trivendale et al.
indicated that YUCCA gene does not
Figure 1.Shows the tract for play a role in the conversion of TAM
auxin biosynthesis in the into N-hydroxytryptamine (Mano
indoleindole-3-acetamide and Nemoto, 2011; Trivendale et al.,
pathway. 2010). This
discrepancyletthescientific world
doubting thetryptamine pathway as
2. The indole-3-pyruvic acid intermediate in IAA biosynthesis.
pathway (IPA)
4. The indole 3-aetaldoxime
The indole-3-pyruvic acid pathway (IAOX)
pathway is important for IAA
synthesis not only in plants but also In the indole-3-aetaldoxime
for microorganisms. During the pathway, Trp is oxidized into IAOX
indole-3-pyruvic acid pathway, Trp by two P450 monooxygenase
is converted into IPA. Arabidopsis enzymes: CYP79B2 and CYP79B3.
seedlings contain the TAA1 gene that IAOX also contributes in the
encodes an aminotransferase that is synthesis of indoleglucosinolates and
used to converts Trp into IPA. IPA is the alkaloid camalexin. IAOX is then
then converted into indole-3- converted into IAN by another
acetaldehyde (IAD). Finally the monooxygenase enzyme:
product of IAD oxidation, cause by CYP71A13. According to a study
indole-3-acetaldehyde oxidase, is conducted by Nafisiet. al,
IAA. What is still unknown in this CYP71A13 catalyses the
pathway is whether IPA is converted

3. convertionof IAOX into IAN in in it synthesis. Analyzing this genes
camalexin biosynthesis (Mano and expression changes in response to this
Nemoto, 2011; Nafisiet. al, 2007). environmental signal has been used to
Finally IAN is converted into IAA
monitor auxin activity in plants. By studying
by enzymatic complex NIT1 and
NIT2 encoded in Arabidopsis NIT these plant responses, auxin levels and
genes. IAOX pathway is not a analyzing the gene expression changes or
common in plants since it has only the mutants background, can help to a better
been identified in Arabidopsis and understanding of how plants regulates this
Brassica, also some genes or fine balance of auxin biosynthesis.
enzymes have not been yet
identified.
IV. Relationship between Ethylene and
B) Tryptophan independent pathway Auxin
In 1991, another pathway for
auxin biosynthesis was postulated. Plant hormones can regulate plant
This pathway proposed that indole-3- differentiation, growth and development;
glycerol phosphate or indole were they can also affect gene expression.
the precursors for this pathway. Ethylene, also known as ACC, is one of the
Indole-3- glycerol was converted first five hormones to be discovered and
into indole by TSA1 gene and IAA studied; is a gaseous hormone that can
was synthesized. However this
stimulate plant differentiation, flowering
pathway was not really understood
since some of its data and gene are opening, fruit ripening, etc. Ethylene is
missing. Tryptophan independent synthesized by an amino acid methionine
pathway is not one of the most that is later converted into S-adenosyl-L-
trusted pathways for auxin methionine, commonly refer as SAM. SAM
biosynthesis. is then converted to 1-aminocyclopropane-1-
carboxylic-acid by the Aminocyclopropane-
1-carboxylic acid synthase (ACC synthase).
III. Regulation of Auxin Biosynthesis Auxin stimulates the production of ethylene
by activating this step (Alonso et al., 2007;
As time had elapsed, plants had Abel et al. 1995). This state there is an
developed and evolve new techniques and interaction between this two fine plants
responses in order to survive. Auxin levels hormone. Preview studies has shown that
have evolved and change in response to the ethylene and auxin can interact with each
changing environment and conditions. other in order to control hypocotyl
Auxin biosynthesis is influenced by elongation and root inhibition (Alonso et al.,
environmental and developmental signals. 2007;Vandenbussche et al.,2003). It was
Auxin can be monitored by it response to concluded that ethylene and auxin can
this environmental and developmental reciprocally regulate each other biosynthesis
signals. They can also be monitored thanks and pathways.
to the identification of some genes involved

4. V. Discussion
http://www.arabidopsis.org/about/inde
x.jsp
Auxinbiosynthesis has been studied
for years; however it is not fully understood.
Various experiments hadbeenperformed
withArabidopsis mutant auxin defective
backgrounds in order to identify genes
involved in this pathway that will help
understandit to a greater extend. By isolating
mutants that can overcome or be more
sensitive to auxin deficiency, the
identification of new factors involved in the
auxin biosynthetic pathway can be
discovered. Many factors and
auxinbiosynthetic genes has been discovered
and their function in plant has been studied;
nevertheless there still many information
that has not been discovered or identified. It
is important to elucidate
References:
Alonso et al. 2007. Multilevel
Interaction between Ethylene and
Auxin in Arabidopsis Roots.
Price, Robert et al., 1994.
Arabidopsis. United States of
America: Cold Spring Harbor
Laboratory Press.
Mano, Yoshihiro. Nemoto,
Keiichirou. 2012. The pathway of
auxin biosynthesis in plants.
TAIR. [internet] [2008] USA:
Arabidopsis Information Resource
(TAIR). Available: