Investigations into response of potato to cadmium with special emphasis on genotypic and somaclonal variations.
Thesis DisciplinePlant Biotechnology
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Tuber crops such as potato (Solanum tuberosum L.) can take up high levels of soil cadmium (Cd) which can be accumulated in their tubers. Thus, they can act as vehicles for transporting Cd to human body which can seriously threaten our health due to its high toxicity. In some circumstances, consumption of potato can contribute to more than 50 percent of human dietary Cd intake. In the present research two approaches were used to probe the potential for genetic improvement to contribute towards the goal of “minimisation of Cd level in potato” which is a novel food safety strategy: (1) assess the natural occurrence of variation in Cd accumulating potential among different potato cultivars already in cultivation in New Zealand, and (2) as a proof-of-concept study to generate potato plants with improved Cd resistance from a model experimental potato cultivar Iwa based on plant cell culture-selection (in vitro breeding approach).
In the first approach, 10 New Zealand cultivars, namely Red Rascal (RR), Russet Burbank (RB), Fianna (F), Agria (A), Laura (L), Purple Heart (PH), Purple Passion (PP), Yukon Gold (YG), Moonlight (M) and Summer Delight (SD) were chosen randomly among over 30 cultivars grown for seed production in a block situated approximately one kilometre east of Lincoln, Canterbury within the 2011-2012 growing season. The tubers as well as the soil samples immediately surrounding the tubers were harvested and prepared for analytical analyses using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The results showed that the soil samples were low in Cd level (0.06 mg kg-1), compared with the national soil average (0.35 mg kg-1), while the tubers varied widely in Cd content from 0.04 to 0.34 mg kg-1 among different cultivars. Therefore, SD with the lowest mean Cd content (0.05 mg kg-1) and an Enrichment Factor (EF) of just below one showed promise as a potential cadmium safe cultivar (CSC). There might be of concern if L, YG, PP with the highest mean Cd contents (0.18-0.21 mg kg-1) are grown in soils with higher Cd levels.
Potato tissue culture required for the second approach based on somaclonal variation was established from the model experimental genotype Iwa in the lab. Leaf and internode explants were used as the starting plant materials to initiate two morphologically distinct calli (type-A and -B). Upon morphological assessment and analysis of antioxidative enzymes such as peroxidase, it was revealed that they exhibited differential Cd sensitivity. The more Cd-resistant callus type (type-B) was chosen for in vitro selection using 18 different Cd treatments varying in Cd exposure timing and duration. Following shoot and root regeneration from these calli, 18 different new Iwa plant lines were obtained. After at least three months of sub-culturing of all 18 plant lines on Cd-free media, in vitro screening of the lines was carried out to identify the most promising plant lines as far as Cd resistance was concerned. After two rounds of in vitro growth screening under a low and high Cd levels, two lines including line 9 (L9) and line 11 (L11) were found to exhibit enhanced Cd resistance compared to control Iwa plants. Further studies of L9 and L11 compared to control Iwa plants including biochemical analysis of reactive oxygen species such as hydrogen peroxide, and transmission electron microscopic studies uncovered that L11 was more resistant to Cd than L9 and control plants. L11 plantlets had about 20 and 10 percent less H2O2 level than control and L9 plantlets, while antioxidative activities were four and two times higher in L11 compared with control and L9, respectively. Moreover, L11 seemed to exhibit a high rate of Cd compartmentalisation in the vacuoles and Cd binding to the cell walls in the roots, suggesting a potential to exclude or limit Cd translocation to other parts of the plant.