| dc.description.abstract | As sessile organisms, plants are continually exposed to signals from
environmentally adverse conditions and internal developmental cues. Gathering this
information, processing it, and generating a proper physiological response is
determinant in plant growth and fitness. Synchronized responses are mediated by
phytohormones through the whole plant, as well as small peptide hormones over shorter
distances via cell-cell communication. Phytohormones are able to diffuse through the
plasma membrane and are perceived by their corresponding intracellular receptors,
while peptide hormones are not permeable and interact with receptors localized on the
plasma membrane to trigger intracellular downstream transduction cascades. In
addition, nutrients and especially sugars are also known to be strong signalling
molecules engaged in regulation of many aspects of plant developmental and
physiological processes.
The work presented in this thesis has been divided into three parts: 1)
Identification and expression analysis of the INFLORESCENCE DEFICIENT IN
ABSCISSION (IDA)/IDA-LIKE (IDL) and PAMP-INDUCED SECRETED PEPTIDE
(PIP)/PIP-LIKE (PIPL) gene families in Arabidopsis, 2) Functional characterization of
three PIP peptides and their role in regulation of plant immunity, and 3) Functional
analysis of an LRR-RLK and its role in sensing of extracellular sugars status and
regulation of growth in Arabidopsis.
In part one, a survey of the Arabidopsis genome for genes encoding peptides
with similarity to IDA and IDA-LIKE1-5 identified 14 new genes, including three new
IDA-LIKE members and eleven genes constituting a new family termed PAMPINDUCED
SECRETED PEPTIDEs (PIP)/PIP-LIKE (PIPL). All encoded peptides
harbor an N-terminal signal peptide for the secretory pathway and a C-terminal
conserved SGPS motif that is believed to function as part of final active peptide.
Transcriptional analysis suggests that the new identified genes can be divided into two
groups; genes that are involved in growth and development and stress inducible genes.
Exogenous application of C-terminal region of PIP-LIKE 3 followed by microarray
analysis revealed differential expression of genes involved in defence responses and cell
wall modification.
In the second part, we focused on three members of PIP/PIPL family: PIP1, PIP2
and PIP3. In silico analysis showed a high degree of co-regulation between these genes.
Expression of PIP genes was induced strongly by flg22 and Botrytis cinerea treatments,
indicating a possible role in regulation of plant immunity. Transcriptional profiling of
seedlings treated with synthetic peptides encompassing the conserved C-terminal region
of PIP1 and PIP2 showed that many genes involved in JA and SA biosynthesis and
signalling as well as redox homeostasis related genes were differentially regulated.
However, a set of starvation condition marker genes such DINs and bZIP11 were also
among the regulated genes. Phenotype assays using pathogens with different life styles
(Pst DC3000 and B. cinerea) showed that transgenic plants over-expressing PIP3 are
more susceptible to both pathogens compared to wt and pip3 plants. Gene expression
analysis upon B. cinerea infection showed that genes involved in JA and SA
biosynthesis and signalling in PIP3 over-expression background are up-regulated
compared to wt and pip3 plants. PIP1 and PIP3 over-expression plants also showed
altered phenotype when grown on high concentrations of sucrose, indicating a possible
role in nutrient sensing and partitioning. Based on our data, we conclude that PIP3 plays
a role in growth and defence trade-off, probably through modulation of nutrient
partitioning towards growth and attenuation of immunity.
In the last part, we have identified and functionally characterized an LRR-RLK,
hereafter termed as Sugar Responsive RLK1 (SRR1) with a potential role as
extracellular sugar sensor. Phenotype assays of insertion mutant lines showed that plants
lacking this gene suffer from growth retardation when grown on sugar-free medium.
Transcriptome analyses of plants in presence and absence of exogenous sugar in the
growth medium revealed a massive transcriptional reprogramming of srr1 plants in the
sugar-free medium compared to wt. Functional categorization of differentially regulated
genes revealed that anabolic biological processes were repressed while catabolic
pathways were induced upon starvation in the srr1 background. This global switch
seems to be essential when plants enter starvation condition in order to sustain a
functional metabolism. A comparative analysis of SRR1 regulated pathways to SnRK1
and TOR-kinase mediated signalling pathways exhibited a striking similarity,
suggesting a role for SRR1 in sensing of the main cellular metabolite status. SRR1 overexpression
plants showed increased tolerance to high concentrations of sucrose, further
supporting this hypothesis. pSRR1:GUS studies and qRT-PCR analysis revealed that
SRR1 is induced by exogenous sugars, with strongest effect of sucrose. Measurements
of major sugars showed that starch production is impaired in srr1 plants. Based on our
observations, we conclude that SRR1 is involved in sensing of extracellular sugars and
connects sugar availability to growth and development, probably through regulation of
main metabolite sensory machinery. | nb_NO |
