Molecular analysis of potato (Solanum tuberosum) responses to increased temperatures

Language
en
Document Type
Doctoral Thesis
Issue Date
2019-02-11
Issue Year
2019
Authors
Hastilestari, Bernadetta Rina
Editor
Abstract

Potato (Solanum tuberosum L.) is one of the important crop plants feeding many people worldwide. Increasing temperature is among the most critical factors responsible for decreasing potato tuber yields. Therefore, this study aimed at getting a deeper insight into the molecular and physiological responses of potato to heat stress and at a better understanding of signals between source and the sink organs. Therefore, a set-up was developed to apply heat stress either to below-ground organs (heat plate), whole plants (heat) or above-ground organs (cold plate) using the heat stress-sensitive potato cultivar Agria. The effects of the different stress treatments on transcriptome and metabolome were investigated. Principal component analysis (PCA) of these data revealed that gene expression and metabolite contents were most severely affected when entire plants were subjected to elevated temperatures; e.g., both source capacity and sink strength were affected. Potato plants were taller and exhibited reduced tuber yields in response to heat stress. The increase in plant height was accompanied by a shift in assimilate partitioning toward the shoots causing reduced assimilate translocation to tubers. There was a decline in the assimilation rate depending on stress level leading to a decreased amount of starch in leaves, in particular under heat conditions. Accordingly, expression of photosynthesis-related genes, particularly of PSII genes, was downregulated. Additionally, transcripts encoding heat stress proteins (HSP) were upregulated most likely to protect cells from the negative effects of increased temperature. Moreover, expression of tuberization-related transcripts was altered, especially of the master regulator, StSP6A. Its expression was downregulated by all stress conditions whereas its inhibitor, StSP5G was upregulated. Interestingly, expression of StFKF1 was downregulated in the leaves in heat plate and heat conditions only, while its expression increased in cold plate conditions compared to control. This suggests that this gene might regulate the source capacity by integrating sink-derived signals. Therefore, the role of FKF1 in potato was further analyzed.

In tubers, transcripts related to heat-stress were clearly enriched under all stress conditions. In addition, tuber yield and starch contents were decreased by heating the root space (heat plate and heat conditions). The decline in starch content was in agreement with the lower sucrose content in tubers indicating limited translocation of assimilates from the leaves. The lower tuber starch content was mainly brought about a lower activity and transcript amount of sucrose synthase (Susy) which is a marker of sink strength. However, cooling the root space (cold plate) alleviated the decline in Susy activity and transcript amount leading to increased starch contents. The role of FKF1 in potato plants was further investigated by engineering transgenic plants with increased (OE-FKF1) or decreased expression (RNAi lines) of the gene. The OE lines displayed a dwarf phenotype with small leaves, short internodes and exhibited a later senescence as compared to wild-type controls. Nevertheless, these dwarf plants initiated tuberization earlier, but the tubers did not develop further. The dwarf phenotype of OE lines was not the effect of tissue culture propagation as a similar phenotype was observed when plants were grown from tubers. In contrast, the RNAi plants had similar phenotype, time of tuberization and tuber yield as wild-type plants. The earlier tuberization of OE-FKF1 lines was accompanied by high expression of StSP6A at an early time point (3 weeks). In contrast, during development (7 weeks) StSP6A levels decreased, while the StSP5G expression increased relative to wild-type plants. The expression of StSP6A in RNAi lines was similar to the wild-type, where is progressively increased during development. Under heat stress, carbon accumulation shifted to the shoots especially in OE lines which gained more biomass in shoots rather than in tubers. Pearson correlation analysis of transcriptome data revealed that StFKF1 expression correlated with another circadian clock gene (StELF4) which might be involved in mediating heat stress response. In conclusion, StFKF1 appears to be involved in the control of StSP6A and tuberization during early time point of plant development, but the mechanism needs to be unraveled and it might increase the source capacity in potato plants grown under heat stress.

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