Abstract
The C-repeat binding factor (CBF)/dehydration-responsive element binding (DREB1) proteins play a prominent role in freezing tolerance and are highly conserved in higher plants. Here we performed a genome-wide search of the CBF/DREB1 gene family in lettuce (Lactuca sativa L.) and identified 14 members of the family with one member gene containing a non-sense mutation within the AP2 DNA-binding domain. A comprehensive phylogenetic analysis of the CBF/DREB1 family members in 20 plant species from the Asterid or Rosid clade provided evidence that tandem duplication played an important role in the expansion of the CBF/DREB1 family. Expression analysis showed that twelve of the lettuce CBF genes were responsive to low temperature (4 °C), and that three and six of them could also be responsive to salt and heat stresses, respectively. Unlike Arabidopsis thaliana whose members of the CBF/DREB1 family respond only to a particular stress, lettuce CBFs provide wider protection from combinations of abiotic stresses. A global transcriptome analysis revealed distinctive temporal expression patterns among the cold-regulated genes in lettuce plants exposed to low temperature. Genes induced throughout the cold treatment are enriched in functions associated with protection from UV and high-light intensity and the genes suppressed after 7 days of cold exposure are enriched in photosynthesis-associated functions. These results provide insight into the molecular evolutionary properties of the CBF/DREB1 gene family in lettuce and a reference for genetic improvement of the lettuce response to cold acclimation.
Highlights
Freezing stress is a primary environmental factor that limits productivity and growth of plants
CBF1, CBF2, and CBF3 respond only to low temperature and are involved in cold acclimation; CBF4 is involved in drought tolerance[33]; and DDF1 and DDF2 are involved in high-salinity tolerance[34]
Arabidopsis AtCBF1–3 in clade A are in chromosome 4, whereas AtDDF1–2 in clade B are in chromosome 1; lettuce genes in clade A and clade B are located in chromosome 9 and 6, respectively; and so do genes from carrot, Medicago, sunflower, olive, and Eucalyptus (Figs. 2, S6)
Summary
Freezing stress is a primary environmental factor that limits productivity and growth of plants. CBF1, CBF2, and CBF3 respond only to low temperature and are involved in cold acclimation; CBF4 is involved in drought tolerance[33]; and DDF1 and DDF2 are involved in high-salinity tolerance[34] These results suggest that much of their functional divergence lies in different responsiveness to stress rather than protein sequences. As a first step toward the goal, we identified the CBF/DREB1 gene family in lettuce through a comprehensive phylogenetic analysis and characterized expression of the genes under various stress conditions to determine their associations with abiotic stress response. We investigated the cold-regulated (COR) genes that might play important roles in the process of cold acclimation in lettuce These results provide insight into the mechanism of freezing tolerance as well as evolution of the CBF/DREB1 genes in lettuce
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