Integrating PI signals into a greater signalling network: a switch between JA and SA

Having addressed the detailed mode of action of inositol polyphosphates in JA-Ile perception by COI1, the question remains how this information must be interpreted in a greater view of plant signalling.

Analysis of jasmonates in SA-deficient Arabidopsis lines revealed that wound-induced oxylipin formation was reduced or delayed, with JA-Ile levels showing the most pronounced reduction compared to wild type induction (Fig. 23). This may indicate that JA and SA signals act synergistically during wound responses, a concept in contrast to results observed upon pathogen infection. Spoel et al (2003) reported strongly elevated JA accumulation in NahG

84 plants after inoculation with P. syringae, while in wild type plants only minor increases of JA were detectable. Whereas JA and SA seem to antagonize one another during responses to pathogen infection, responses to wounding may require both JA and SA, with SA acting upstream of JA signals. Synergistic effects were reported upon JA and SA application at low concentrations (Loake & Grant, 2007; Mur et al, 2006).

To elucidate the relation between PI and SA signals during wound responses, wound-induced InsP3 levels were determined in sid2 and NahG plants (Fig. 24) and, reciprocally, wound-induced SA-levels were determined in InsP 5-ptase and ipk1-1 plants (Fig. 25). InsP3

accumulation was reduced in both sid2 and NahG, indicating that SA signals are relevant for PI signal induction upon wounding. This observation is in line with reports on Arabidopsis cell suspension cultures, in which PtdInsP(4,5)P2 formation driven by increased PI4-kinase activity was inducible by SA treatment (Krinke et al, 2007b). However, InsP3 accumulation was not SA-inducible in the cell culture, which may specify PI lipids as a factor being induced by SA alone.

The wound stimulus may then induce further metabolism of PI lipids to form InsP3 and other soluble inositol polyphosphates. As InsP3 accumulation also failed in JA-deficient dde2-2 plants (Mosblech et al, 2008), it is conceivable that both SA and JA are required in combination for PI induction upon wounding.

In a reciprocal experiment, when InsP 5-ptase and ipk1-1 plants were tested for their wound-induced SA contents (Fig. 25), PI signals seemed to be required for SA formation. InsP 5-ptase plants with reduced PI signals displayed strongly reduced SA levels, whereas ipk1-1 plants exhibited SA amounts strongly elevated over those of wild type controls (Fig. 25). This pattern indicates that PI signalling components other than InsP6, which accumulate in ipk1-1 plants, may boost SA accumulation upon wounding.

The results allow speculation on a self-reinforcing loop of JA, SA and PI signals in the Arabidopsis wound response. A possible network is depicted as a simplified model in Fig. 26.

Upon wounding, in the first place JA-Ile is rapidly formed (Fig. 26, point 1; Glauser et al, 2008) and required for induction of PI signals, as indicated by JA-deficient dde2-2 plants, which do not accumulate InsP3 upon wounding (Fig. 26, point 2; Mosblech et al, 2008). PI signals in turn promote SA formation (Fig. 26, point 3). This notion is supported by the finding that InsP 5-ptase plants exhibited reduced SA-levels, whereas ipk1-1 plants exhibit elevated SA levels (Fig. 25). SA accumulation then further induces JA-Ile signals or promotes the maintenance of the JA-Ile signal (Fig. 26, point 4), as SA-deficient mutants contained reduced levels of JA-Ile upon wounding (Fig. 23). Furthermore, the reduced wound induction of InsP3 signals in SA-deficient mutants (Fig. 24) suggests that SA is required for the induction of the PI pathway or

85 Fig. 26: Simplified model of JA/SA/PI action in Arabidopsis wound response. Black arrows indicate synergistic effects, dashed line indicates antagonistic effect.

Explanation of numbers 1-6 see text.

to maintain PI signalling (Fig. 26, point 4). Data represented in this thesis suggest that JA-Ile can best be perceived with InsP5 present in the COI1 protein. To allocate sufficient amounts of InsP5, SA may activate a PI4-kinase and the PI pathway is put into operation (Fig. 26, point 4;

Krinke et al, 2007b). With InsP5 bound in the COI1 protein (Fig. 26, point 5), JA responses may proceed and induce appropriate gene expression as well as a repression of JA-Ile signals (Fig. 26, point 6), as coi1 mutants and also InsP 5-ptase plants exhibit increased wound-induced accumulation of JA-Ile. In this model, the role of SA would be to potentiate the wound-induced JA signal in close interaction with PI signals. A process like this would offer various points to fine tune the signal composition in order to achieve a specific and targeted defence response.

The model presented in Fig. 26 is based on a limited volume of data and must remain largely speculative. As potato plants with altered inositol polyphosphate metabolism and deficient in InsP6 formation exhibited reduced resistance to viruses, bacteria and fungal pathogens (Murphy et al, 2008), it may be speculated that InsP6 is also involved in SA-mediated defence, whereas InsP5 supports JA-mediated defence. The balance between InsP5 and InsP6 might display a switch to push the defence reaction in one or the other direction, and possibly there is a link to auxin signalling via TIR1. JA and auxin signals are both perceived via similar

Wounding

JA-Ile

PI pathway

InsP

SA

JA responses 1

2

3

4 5

COI1

6

InsP

86 machineries, including F-box protein receptors that are part of SCF E3 ubiquitin ligase complexes. Importantly, the auxin receptor TIR1 carries an InsP6 as cofactor, whereas the JA receptor COI1 requires an InsP5 cofactor, as demonstrated in this thesis. Truman et al (2010) have proposed a model, in which JA signals induced by pathogen infection are followed by a phase of auxin signals, which in turn initiate SA signalling. In such a model, the transition from JA to auxin signals could be supported by a change in InsP5/InsP6 balance, which would influence the sensitivity of either the JA or the auxin receptor. In fact, this suggestion does not interfere with the proposed scheme presented in Fig. 26, but would rather complete the suggested model.

Future analysis may fill some of the informational gaps and reveal even more links between signalling pathways that should be viewed as an integrated and interdependent signalling network of exceeding complexity.