Homeostasis of oligogalacturonides (OGs) and their activity as damage associated molecular patterns (DAMPs)
Author(s)
Verrascina, Ilaria
Date Issued
June 6, 2017
Type
Doctoral Thesis
Abstract
Cell walls are important features of plant cells that perform
a number of essential functions, both during plant
development and growth and in plant-microbe interactions.
Recognition of endogenous molecules released during the
infection process and acting as “damage-associated
molecular patterns” (DAMPs) is a key feature of immunity
in plants. Oligogalacturonides (OGs), generated from
disruption of homogalacturonan by pathogen pectic
enzymes act as DAMPs and their perception leads to a
response cascade that leads to intracellular Ca2+release,
oxidative burst activation and transcriptional
reprogramming. Recently transgenic Arabidopsis plants
expressing a chimeric protein, named “OG-machine”
(OGM), constituted by a fungal PG (FpPG) and a
polygalacturonase-inhibiting protein (PGIP) from common
bean (PvPGIP2) have been generated. OGM-expressing
plants accumulate OGs in their tissue and exhibit enhanced
resistance to a variety of pathogens, thereby providing direct
evidence for the function of OGs as in vivo elicitors of the
plant defense responses (DAMPs).
Besides inducing immunity, OGs negatively affect plant
growth and development, likely due to an auxin-antagonistic
activity. This effect reflects the typical trade-off that exists
between defence and growth and entails that the recognition
of DAMPs poses the intrinsic risk of activating an
exaggerated response that reduces or completely arrests
plant growth. Thus, regulatory homeostatic mechanisms that
prevent the deleterious effects of DAMP hyperaccumulation
are is essential for an optimal immune
response. Starting from transgenic plants that express, in a
β-estradiol-inducible manner, the OGM, we looked for such
a regulatory mechanism in Arabidopsis thaliana by
searching for elicitor-inactive OGs that may derive from
active OGs through an enzymatic modification. In the
presence of β-estradiol, these plants contain increased levels
of elicitor-active OGs compared to wild-type plants and a
large amount of modified OG-like fragments.
Mass spectrometry analyses showed that the oligomers
indeed corresponded to modified OGs and are characterized
by a galactaric acid residue at the reducing end, leading to
the conclusion they are oxidized OGs (oxOGs). oxOGs
were tested for their ability to induce defense responses and
antagonize auxin responses. In all experiments, they were
inactive as compared to the corresponding typical OGs.
The enzyme with OG oxidizing activity was identified as
Isoform 2 of Reticuline oxidase-like protein and was named
OGOX1. OGOX1 produces elicitor-inactive oxidized OGs
and H2O2 and is encoded by the gene At4g20830. This
belongs to the superfamily of genes encoding the FADbinding
berberine-bridge enzyme-like proteins (BBEls).
In order to analyze the role of OGOX1, I have obtained an
ogox1 null insertional mutant and transgenic plants
overexpressing OGOX1 under the CaMV 35S promoter
(OGOX1-OE#1.9 and #11.8). Since additional T-DNA
insertional mutants of OGOX1 were not available,
transgenic plants (amiR-OO1) expressing a β-estradiolinducible
artificial microRNA (amiR) against the OGOX1
transcript (amiR-OO1 lines #2.5 and #3.4) were also
generated. The characterization of these plants suggests that
OGOX are responsible of an homeostatic mechanism
involved in both development and immunity. In the last part
of my work the characterization of closely related
Arabidopsis BBlEs has revealed the presence of at least
three additional enzymes with OG oxidizing activity. Thus,
OGOX is a functional and redundant key element for
regulating OG homeostasis and likely avoiding an
exaggerated activation of plant defences.
Additional information
Dottorato di ricerca in Scienze delle produzioni vegetali e animali
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