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Cold Sensitivity Gradient in Tuber-Bearing Solanum Based on Physiological and Transcript Profiles

^a Jacob Shur Research Facility, Maine Agricultural and Forest Experiment Station, Univ. of Maine, Orono, ME 04469
^b Dep. of Biological Sciences, Univ. of Maine, 5735 Hitchner Hall, Orono, ME 04469

View larger version (39K): [in this window] [in a new window]   Figure 1. Gradient of cold sensitivity in representative Solanum species based on electrolyte leakage analysis. Leakage of cellular electrolytes was measured on leaf disks of plants at the mid-vegetative stage before and after exposure to chilling (13°C, top panel) or above-freezing (2°C, bottom panel) temperatures. Electrolyte leakage index (ELI) is the cellular electrolyte leakage ratio between cold-treated and control plants. An ELI = 1 indicates no significant leakage of electrolytes due to cold, while ELI >1 indicates significant leakage due to cold. Means (±SE, n = 4) are significantly greater than 1.5 (arbitrary cutoff level) at P < 0.001 (***) or P < 0.01 (**).

View larger version (40K): [in this window] [in a new window]   Figure 2. Cold acclimation (CA) response in Solanum species. Plants at the mid-vegetative stage were exposed to an above-freezing temperature (2°C) for a maximum of 20 d after establishment at a control temperature (24°C) and then exposed to freezing (–3°C) for 30 min. Electrolyte leakage index (ELI) is the cellular electrolyte leakage ratio between cold-treated and control plants. An ELI = 1 indicates no significant leakage of electrolytes due to cold, while ELI >1 indicates significant leakage due to the treatment. Means (±SE, n = 4) are significantly different between CA and non-CA plants at P < 0.001 (***) or P < 0.01 (**).

View larger version (21K): [in this window] [in a new window]   Figure 3. Binary diagram summarizing the gradient of cold sensitivity in Solanum species. In both the chilling (13°C) and above-freezing (2°C) temperature experiments, the accessions with statistically significant increases from the electrolyte leakage index (ELI) cutoff value of 1.5 at each time point were assigned a value of 1 (black), while those within or below the cutoff, a value of 0 (white).

View larger version (16K): [in this window] [in a new window]   Figure 4. Comparative analysis of cold-induced expression of selected cold acclimation (CA) associated gene homologs (CBF1, ZAT12, COR47, and GolS3). Three test genotypes or species at the extreme ends of the cold-sensitivity gradient, i.e., Solanum commersonii (cold acclimating and freeze tolerant), S. tuberosum cv. Red Pontiac (cold sensitive), and S. trifidum (highly cold sensitive, i.e., chilling sensitive) were compared by quantitative real-time polymerase chain reaction. Relative expression was based on the average obtained from independent biological replicates (±SE, n = 3).

View larger version (26K): [in this window] [in a new window]   Figure 5. Differential responses of Solanum species to high concentrations of NaCl (200 mmol/L) and mannitol (200 mmol/L) in hydroponics. Relative ranking for osmotic or high-salt sensitivity is very similar to the cold-sensitivity ranking. Electrolyte leakage index (ELI) is equal to the ratio of the cellular electrolyte leakage percentage between experimental (treatment) and control plants. An ELI = 1 indicates no significant leakage of electrolytes due to treatment, while ELI >1 indicates significant leakage due to the treatment. Means (±SE, n = 5) are significantly greater than 1.5 (cutoff level) at P < 0.001 (***) or P < 0.01 (**).

View larger version (52K): [in this window] [in a new window]   Figure 6. Comparative analysis of the NaCl–- and mannitol-induced expression of CBF1, GolS2, and GolS3 homologs in Solanum species: C = control, M = mannitol, S = NaCl. Gene expression analysis was performed after 2 d of exposure to a high concentration of NaCl (200 mmol/L) or mannitol (200 mmol/L) in hydroponics. Expression analysis was performed by semiquantitative real-time polymerase chain reaction. Gel images were representatives of the profiles from three independent biological replicates with two technical replicates each.