Plant basal resistance - Universiteit Utrecht

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General discussion
2 of this thesis shows natural variation between Arabidopsis accessions in the sensitivity of
jasmonic acid (JA)- and SA-inducible marker genes, pointing to variation in responsiveness of
relatively late-acting post-invasive defence barriers. The same set of accessions also showed
natural variation in responsiveness of chitosan-induced callose deposition, suggesting
variation in responsiveness of early-acting post-invasive defence barriers. Interestingly,
there was a negative correlation between the responsiveness of early-acting defences
(callose) and that of late-acting defences (SA-induced PR-1 gene induction). Although a
sample size of six accessions is too small to reach definite conclusions about the ecological
meaning of these findings, it is tempting to speculate that some of these accessions have
de-sensitised their early defences in order to interact with plant beneficial microbes, such
as plant-growth promoting rhizobacteria (PGPRs). In order to compensate for this immuno-
compromised defence layer, these accessions have primed later-acting defences. Further
experiments should be performed to test this hypothesis. Of the possibilities to consider, a
HapMap population of 360 Arabidopsis accessions could be screened for PGPR colonization
and PAMP-induced callose deposition in the roots (Buckler and Gore, 2007). Identification of
genes promoting PGPR colonization but suppressing PAMP-induced callose would confirm
this hypothesis.
CYP81D11: A SUPPRESSOR OF EARLY ACTING POST-INVASIVE DEFENCE?
The QTL analysis described in Chapter 2 identified two loci regulating PAMP-induced callose
deposition. One major QTL was found on chromosome III, whereas a much weaker QTL was
identified at the top of chromosome I. The direction of both QTLs show opposite effects: the
major QTL at chromosome III mediates a suppressive effect from Bur-0 parent and stimulatory
effect from Col-0 parent, whereas the weaker QTL at chromosome I mediates a stimulatory
effect from Bur-0 parent and suppressive effect from Col-0 parent. Although both QTLs still
spans a multitude of genes, it is interesting that the major locus on chromosome III maps
onto the cis-jasmone inducible gene CYP81D11 (At3g28740). This gene was previously linked
to the regulation of indirect plant defences against insects (Bruce et al., 2008; Matthes et
al., 2010). Its role in direct plant defences, however, remains unclear. To further investigate
a possible function of CYP81D11 in callose deposition, a T-DNA insertion mutant and a
CYP81D11 over-expression line in the genetic background of Col-0 (line CYP81D11OE 5-5.3;
Bruce et al., 2008; Matthes et al., 2010), were tested for flg22-induced callose deposition,
as described by Luna et al., (2011). Interestingly, this experiment revealed that CYP81D11OE
plants are severity affected in PAMP-induced callose deposition (Figure 1).