Analysis of cold acclimation ability and drought tolerance of Petunia spp

Freezing tolerance is a dynamic characteristic, and many plant species increase in freezing tolerance following exposure to low non-freezing temperatures, a process referred to as cold acclimation. The genus Petunia is composed of diverse species, with the capacity for cold acclimation already known to exist in at least one species, P. hybrida . In this study, P. exserta, P. integrifolia, and two accessions of P. axillaris were also found to cold acclimate. All Petunia species had similar basal freezing tolerance of EL50 = -2°C, but freezing tolerance varied significantly among species following cold acclimation. Petunia axillaris (accession 28548) showed the greatest acclimated freezing tolerance with an EL50 temperature of -8°C, compared to only -5°C for P. exserta. Temperature, but not photoperiod, was critical for induction of cold acclimation in P. hybrida. Cold acclimation of Arabidopsis is largely controlled by genetic factors in the CBF cold-response pathway. High levels of constitutive heterologous AtCBF3 expression in P. hybrida 'Mitchell' resulted in increased basal freezing tolerance of one transgenic line by ca. 2.5°C, while expression of LeCBF1 (a CBF homolog from tomato) had no effect on freezing tolerance. However, high expression of AtCBF3 resulted in phenotypic changes including delayed flowering. Heterologous CBF expression did not enhance drought tolerance. Expression of putative endogenous CBF transcription factors, petCBF1-4, was induced at 3°C and two putative downstream genes of the petunia CBF pathway whose expression was induced by cold and CBF overexpression were identified.