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Plant Signal Behav. Sep 2008; 3(9): 654–656.
PMCID: PMC2634547

Biotic and abiotic stress responses through calcium-dependent protein kinase (CDPK) signaling in wheat (Triticum aestivum L.)

Abstract

Calcium-dependent protein kinases (CDPKs) sense the calcium concentration changes in plant cells and play important roles in signaling pathways for disease resistance and various stress responses as indicated by emerging evidences. Among the 20 wheat CDPK genes studied, 10 were found to respond to drought, salinity and ABA treatments. Consistent with previous observations, one CDPK gene was shown to respond to multiple abiotic stresses in wheat suggesting that CDPKs could be converging points for multiple signaling pathways. Among the 12 wheat CDPK genes that were responsive to Blumeria graminis tritici (Bgt) infection or the treatment of hydrogen peroxide (H2O2), eight also responded to abiotic stresses, suggesting a cross-talk between biotic and abiotic stress signaling pathways. Phylogenetic analysis indicated that some of these genes were closely related to CDPKs from other species, whose functions have been partially studied, suggesting similar functions wheat CDPK genes. Combining the up-to-date knowledge of CDPK functions and our observations, a model was developed to project the possible roles of wheat CDPK genes in the signaling of biotic and abiotic stress responses.

Key words: CDPK, calcium, kinase, stress response, disease resistance, signal transduction, wheat

Sessile plants have developed sophisticated signaling pathways to deal with dramatic environmental changes that may affect their normal growth, such as pathogen attack, drought, and cold. Calcium is a universal secondary messenger that responds to these stimuli. The fluctuation in cytosolic Ca2+ levels can be sensed by calcium-dependent protein kinases (CDPKs), which will modify the phosphorylation status of substrate proteins.13 Accumulating evidence indicate that CDPKs mediate biotic and abiotic stress signaling pathways.47 For example, overexpression of the rice CDPK gene OsCDPK7 provides cold, salt, and drought tolerance for the transgenic rice plants, demonstrating the potential of CDPK engineering to generate stress tolerance enhanced crops.8,9

In wheat, 10 out of 14 CDPK genes appeared to respond to abiotic stresses including drought, NaCl, as well as ABA stimulus (Fig. 1A).10 Five CDPKs (TaCPK4, 6, 9, 10 and 18) were particularly interesting since they could respond to at least two of the three treatments, among which the expression level of TaCPK9 was enhanced under all three treatments suggesting that TaCPK9 is the point where multiple signaling pathways cross. In wheat, TaCPK4 responded to both ABA treatment and NaCl stress (Fig. 1A). Interestingly, its best Arabidopsis homologs AtCPK4 and AtCPK11, as suggested by a Neighbor-Joining phylogenetic analysis (Fig. 1B), have been postulated as two important positive regulators in CDPK/calcium-mediated ABA signaling pathways.11 Such a correlation strongly supports the idea that TaCPK4 is a good candidate in wheat for ABA signaling. Figure 1A also shows that one wheat CDPK gene could respond to multiple abiotic stresses suggesting that CDPKs are converging points for multiple signaling pathways. On the other hand, multiple CDPKs were involved in single stress response. It is however not clear how these CDPKs are organized in one signaling pathway.

Figure 1
The roles of wheat CDPKs in abiotic and biotic stress responses. (A) One CDPK gene responded to multiple abiotic stresses and multiple CDPKs were required for single stress response. (B) Phylogenetic relationship of wheat CDPKs with functionally studied ...

Regarding the roles of CDPKs in defense reactions, 12 TaCPKs were found to be responsive to either Blumeria graminis tritici (Bgt) infection or H2O2 treatment. The response to H2O2 was investigated because cytosolic calcium influx and reactive oxygen species, such as H2O2 are known to be implicated in both plant innate immunity and abiotic stresses.1217 Among these CDPK genes, five responded to both treatments (Group II) whereas the ones that responded to Bgt infection (Group I) or H2O2 treatment (Group III) were four and three respectively. The differential expression patterns suggest different functional modes of these CDPK genes. Involvement of CDPK genes in plant defense response has been shown in multiple species.5,7 Recently, two barley CDPK paralogs (HvCDPK3 and HvCDPK4) were found to play antagonistic roles during the early phase of powdery mildew pathogenesis.5 The close similarity between wheat CDPK genes (TaCPK2 and TaCPK5, Fig. 1B) with these two barley genes may suggest their potential roles in wheat powdery mildew resistance. Surprisingly, we did not detect the responsiveness of TaCPK5 to wheat Bgt infection, indicating the divergence of CDPK functions in these two members of Triticeae family. Recently, one potato (Solanum tuberosum) CDPK gene StCDPK5 has been shown to be directly involved in regulating oxidative burst via phosphorylation of the NADPH oxidase StRBOHB.18 In light of the close relationship of TaCPK2 with HvCDPK5 and StCDPK5 (Fig. 1B), we speculate that TaCPK2 could be associated with both biotic and abiotic stress response signaling pathways and therefore play multiple roles in wheat.

A model was proposed in Figure 1C regarding the positions of wheat CDPK genes in signaling pathways for biotic and abiotic responses. The hypothesis depicted four different roles of wheat CDPK genes: (1) Group I genes that respond only to Bgt infection may, like potato StCDPK5, render defense response through an oxidase like NADPH oxidase that generates increased amount of H2O2;18 (2) At one aspect, Group II genes may participate in defense response in a manner similar to Group I genes; (3) On the other hand, since Group II genes also respond to H2O2 treatment directly, an auto-regulation circuit was proposed, which eventually joins the oxidase pathway; (4) Group III CDPK genes and some remaining CDPK genes are considered to be mainly involved in abiotic stress responses. The model positioned CDPKs both upstream and downstream of H2O2, presenting a complicated wiring of the signaling pathway network involving wheat CDPKs. Future biochemical, genetic, and transgenic analyses may help elucidate the genuineness of such a rather early model for the functions of wheat CDPK genes.

Acknowledgements

This work is supported in part by National HITECH Research and Development Program of China (“863” program, #2006AA10A104) and National Natural Science Foundation of China (NSFC, #30771336).

Footnotes

Previously published online as a Plant Signaling & Behavior E-publication: http://www.landesbioscience.com/journals/psb/article/5757

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