Potato is the third most important food crop in the world after rice and wheat. Because of its widely distributed cultivation and high yields, it is considered a critical species in terms of food security in face of a growing world population. However, potato is particularly vulnerable to high temperature during various stages of its life cycle. Elevated temperatures strongly suppress tuberisation, negatively affect storage and shelf life of tubers and reduce fitness of seed potatoes. Breeding new heat-stress tolerant cultivars is an urgent need for sustainable increases in potato production, given the negative impact of the rises in temperature due to global warming.
In this proposal, an integrated approach will be used by combining physiology, genetics, genomics, metabolomics and natural variation studies to analyze the impact of elevated temperatures on (1) sink-source relations of potato plants, (2) potato tuber development, (3) starch accumulation and tuber quality and (4) tuber dormancy. To achieve these aims both unbiased and targeted approaches will be employed. The unbiased approaches include the elucidation of phenotypic, biochemical and molecular responses to varying environmental conditions of selected potato genotypes (diploid populations, and a panel of tetraploid varieties and GMPs). Environmental conditions will include elevated and ambient temperatures in combination with different day lengths and light intensities. The plants will be phenotyped with respect to assimilate allocation, tuberisation, tuber yield, quality and dormancy. The genetic approach aims at identifying polymorphisms of candidate genes from diploid populations exhibiting a wide response to elevated temperatures. This will lead to the identification of genes and allelic variants that confer heat tolerance. The targeted approach is based on recent breakthroughs of the partners, which show that two linked regulators (StCDF and StSP6A) play a central role in the initiation of tuberisation. The role of these regulators will be investigated in more detail in order to identify key components of the multiple signal transduction pathway(s). In addition levels of phytohormones known to regulate tuber initiation and dormancy will be manipulated and their impact on heat tolerance will be investigated.
A moderate rise in temperature has profound effects on tuberisation, assimilate partitioning, biomass allocation and tuber quality of potato plants. Photoperiod, light intensity, and high temperature have variable effects depending on the genotype, ranging from a reduction in tuber number and weight, to a complete inhibition of tuberisation. Furthermore, these tubers often display a variety of physiological disorders including reduced starch accumulation, skin russeting and secondary growth, associated with dramatic yield losses. In a recent major breakthrough, the StSP6A gene has identified that encodes the mobile signal for tuberisation (Navarro et al. 2011). Furthermore, constitutive or heat-inducible overexpression of this gene overcomes high-temperature inhibition of tuberisation. Thus, temperature is likely to affect the StSP6A pathway. A key regulatory gene that controls StSP6A expression has been identified by Kloostermann et al. (2013). The DOF-transcription factor StCDF_V controls StSP6A gene expression indirectly and allelic variants of this factor give rise to the genotypic variation in day length requirement for tuber development (Kloosterman et al. 2013). Building on our recent findings about the roles of StSP6A and StCDF_V in tuber induction, we propose to unravel the molecular details underlying the negative effects of elevated temperatures on tuberisation and sink-to-source relations in potato plants to provide strategies for protecting tuber yield and quality under heat stress. Our specific objectives will be to investigate:
1. The effect of elevated day and/or night temperatures on assimilate allocation and tuber development. The contribution of above- and below-ground temperatures will be tested by separately controlling air and soil temperatures.
2. The interaction between photoperiod, light intensities and heat stress on assimilate allocation, tuberisation, dormancy and sprouting.
3. The molecular basis for the drastic changes in biomass allocation and the down-regulation of sucrose and starch metabolism in developing tubers at elevated temperature.
4. How the hormonal regulation of tuber dormancy is influenced by elevated temperatures.
5. The molecular basis of genotypic differences in response to elevated temperatures using potato germplasm collections and diploid populations.
6. The effects of StSP6A/STCDF_V overexpression on assimilate allocation, tuberisation onset, tuber quality and dormancy.
7. The molecular basis of StSP6A action by analysing the StSP6A interactome in sub-apical stolon meristem cells and tissue-specific modulation of StSP6A expression.
8. The molecular basis of StCDF_V regulation and the identification of downstream partners.