Characterization of ABA sensitivity in mutants with altered abiotic stress tolerance in common wheat

ABA, a phytohormone, regulates many agronomically important aspects of plant growth and development, including seed maturation, dormancy and stress tolerance (Finkelstein et al. 2002). During vegetative growth, endogenous ABA levels increase upon conditions of water stress, and ABA acts as an essential mediator in triggering the plant response to these adverse environmental stresses (Finkelstein et al. 2002). Under low temperature conditions, although a nonABA-regulated pathway acts as a major stress signal transduction mechanism (Shinozaki and YamaguchiShinozaki 2000), cold/freezing and ABA regulatory pathways are not completely independent. In Arabidopsis thaliana, ABA-insensitive abi1 mutant reduces the accumulation levels of drought/cold-induced and ABA-regulated RAB18 transcripts, which resulted in its decreased freezing tolerance (Mäntylä et al. 1995). firery1, which is an ABA-hypersensitive mutant of Arabidopsis, showed higher freezing tolerance than wild-type without cold acclimation (Xiong et al. 2001). In wheat, ABA-responsive Wdreb2, Wlip19 and Wabi5 encoding transcription factors are involved in cold acclimation and development of freezing tolerance through regulation of their downstream Cor (coldresponsive)/Lea (late-embryogenesis abundant) gene expression (Kobayashi et al. 2007, 2008a, b). A mutant line EH47-1 was originally derived from ethylmethane sulfonate (EMS)-treated seeds of the common wheat cultivar ‘Kitakei-1354 (Kitakei)’ and selected as an ABA-insensitive mutant during seed maturation (Kawakami et al. 1997). EH47-1 was lesssensitive to exogenous ABA at the seedling stage, but the transcripts of ABA-responsive genes are more abundantly accumulated by ABA treatment than in Kitakei (Kobayashi et al. 2006). EH47-1 showed a significantly higher freezing tolerance than Kitakei at least in the seedlings without cold acclimation, although this mutation did not impair the cold acclimation ability per se of the mutant. EH47-1 mutant allele has no influences on the expression levels of cold-responsive transcription factors and Cor/Lea genes under low temperature conditions. Other mutant lines, ‘Mutant ABA 27 (ABA27)’ and ‘Mutant ABA 90 (ABA90)’, were generated from the common wheat cultivar ‘Chihoku-komugi (Chihoku)’, and the ABA sensitivities of ABA27 and ABA90 are respectively higher and lower than that of Chihoku based on the inhibition rate of seedling growth by exogenous ABA (Kobayashi et al. 2008c; Kobayashi and Takumi 2007). ABA27 showed significantly increased freezing tolerance in seedlings with and without cold acclimation and activated gene expression of Wabi5 and Cor/Lea such as Wrab15 and Wrab18 under normal temperature condition and early cold acclimation period (Kobayashi et al. 2008c). On the other hand, ABA90 showed lower freezing tolerance than Chihoku after cold acclimation, and slight differences were observed in gene expression at some time points of cold acclimation (Kobayashi and Takumi 2007). These results suggest that ABA sensitivity is associated with both determinations of the basal level of freezing tolerance and enhancement of freezing tolerance via cold acclimation including the activation of Cor/Lea genes in wheat. To obtain further information on the role of ABA in cold acclimation and freezing tolerance in wheat, we analyzed other Chihoku-derived mutant lines, ‘Mutant ABA 31 (ABA31)’ and ‘Mutant ABA 59 (ABA59)’, besides ABA27 and ABA90. In addition to these mutants, ‘Mutant ABA 122 (ABA122)’ and ‘Mutant ABA 126 (ABA126)’ derived from common wheat cultivar ‘Horoshiri-komugi (Horoshiri)’ were also characterized.