Title:
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The onset and long-term maintenance of defence priming
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In their struggle for life, plants can employ sophisticated strategies to defend
themselves against potentially harmful pathogens and insects. One mechanism by which
plants can increase their level of resistance is by intensifying the responsiveness of their
immune system upon recognition of selected signals from their environment. This so-called
priming of defence can provide long-lasting resistance, which is based on a faster and/or
stronger defence reaction upon pathogen or pest attack (Figure 1). Priming can target various
layers of induced defence that are active during different stages of the plant-attacker
interaction. Although it has been known for several years that priming can enhance the
effectiveness of different plant defence responses, the underpinning molecular mechanisms
have remained poorly understood.
The main aim of the work described in this thesis was to explore the mechanisms
controlling the establishment and long-term maintenance of defence priming. Chapter
describes the early signalling events leading to broad-spectrum defence priming after
treatment with the chemical agent BABA, whilst the subsequent experimental chapters focus
on long-term maintenance of defence priming (Figure 1).
Chapter 2 describes the isolation and characterization of the Impaired in J1.ABA induced
Immunity (ibi 1) mutant, which was selected for loss of BABA-induced resistance
against the oomycete Hyaloperonospora arabidopsidis. Map-based cloning of fBll revealed
that this gene encodes a pathogen-inducible aspartyl tRNA synthetase, which mediates
esterification of L-aspartic acid to its cognate tRNA. The results in Chapter 2 suggest that
IBI I exhibits an additional non-canonical function as a receptor of BABA and a native
regulator of basal resistance. Importantly, t.he work described in this Chapter provides
evidence that the stress response triggered by relatively high concentrations of BABA is
under separate genetic control than BABA-induced priming of broad-spectrum defence. This
outcome provides exciting scope to exploit IBI I-dependent resistance in crops, particularly
against plant diseases that are difficult to control by fungicides or single resistance genes.
Recently, studies have emerged suggesting that long-lasting priming phenomena are
controlled by epigenetic regulatory mechanisms. Priming of defence genes has been
associated with modifications of histone proteins at defence-related gene promoters, which
may facilitate access of the transcriptional machinery to gene promoters. Chapter 3 focuses
on the durability of BABA-induced resistance in Arabidopsis. Treatment of 5 day-old
Arabidopsis seedlings resulted in protection against H arabidopsidis and Pseudomonas
syringae that lasted up to 4 weeks after BABA treatment. This long-lasting component is
under the control of the NON-EXPRESS OR OF PR GENES 1 (NPRl) protein and is
associated with priming of SA-inducible genes. Analysis of mutants in chromatin
remodelling processes revealed that long-lasting induced resistance by BABA requires
regulation by post-translational modification of Histone 3 Lysine 9 (H3K9) and H2A.Z
occupancy.
Conclusive evidence for an epigenetic basis of defence priming came very recently
from independent laboratories across the world. Chapter 4 describes one of these studies and
shows that defence priming can be transmitted to following generations from diseased
Arabidopsis plants after fitness-reducing levels of disease by the bacterial pathogen P.
syringae pv. tomato DC3000 (PstDC3000). Compared to progeny from control-inoculated
plants (Cl), progeny from diseased plants (PI) expressed significantly higher levels of basal
resistance to the oomycete pathogen H arabidopsidis and PstDC3000. In addition, the
findings in this chapter demonstrate that PI plants are primed to respond to exogenously
applied SA, but are repressed in their responsiveness of lA-inducible genes, which correlated
with enhanced susceptibility to the necrotrophic fungus Alternaria brassicicola. Interestingly,
the transgenerational SAR phenotype of PI plants is associated with permissive and
repressive chromatin modifications at SA- and lA-inducible gene promoters, respectively.
Finally, phenotypic analysis of the drmldrm2cmt3 mutant, which is impaired in non-CpG
methylation, suggests an important role for this plant-specific form of DNA methylation in
the transmission oftransgenerational SAR.
Chapter 5 serves as an Addendum to the work described in the previous chapter, and
describes further analysis of mutants in RNA-directed DNA methylation (RdDM) for their
ability to express transgenerational SAR upon repeated infection by PstDC3000. The results
indicate that transgenerational SAR is regulated by the RdDM pathway and likely transmitted
through hypomethylation of genomic DNA at CpHpG sites.
How all the various molecular and epigenetic mechanisms of priming relate to each
other remains unknown and will require further research, which is a critical first step towards
large-scale exploitation of the phenomenon in sustainable agriculture. In Chapter 6, I describe
how long-lasting induced resistance can be applied to protect the crop Solanum lycopersicum
(tomato). The costs (plant growth reduction) and benefits (effectiveness and durability of the
induced resistance) of different resistance-inducing treatments have been studied. Seed
treatment with jasmonic acid (lA) or f3-aminobutyric acid (BABA) rendered long-lasting
protection that was not associated with major costs. However, the consistency of the induced
resistance was rather low and was cultivar-dependent. Also, the incubation of seeds for
prolonged periods with lA provided long-lasting protection against the necrotrophic pathogen
B. cinerea with no adverse effects on seed germination or plant growth. Treatment of tomato
seedlings with lA and BABA resulted in a more pronounced resistance response that lasted
up to 5 and 6 weeks after the treatment, respectively. However, the disease protection upon
these application methods was associated with residual fitness costs. Finally, the possibility
of transgenerational induced resistance in tomato was investigated. The results in Chapter 3
set the basis to integrate long-lasting induced resistance in conventional strategies of tomato
protection (Chapter 6).
Food security is one of the most challenging issues faced by humanity in this century,
and is likely to become further aggravated by climate change that can render agricultural
lands less suitable for crop production. Consequently, there is a pressing need to improve the
efficiency of sustainable food production, including intensification of durable crop protection
strategies. Integration of long-lasting induced resistance into existing disease management
schemes would allow lower energy costs to reach similar levels of disease protection. My
Ph.D project has uncovered different regulatory mechanisms of long-lasting indu~ed
resistance based on priming of defence. Future research will be necessary to narrow down the
mechanisms by which genetics and epigenetics mediate priming of defence. I hope that these
new insights will help to optimise the efficiency of robust and durable induced resistance in
plants.
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