Seed longevity and genome damage

Abstract

Seeds are the mode of propagation for most plant species and form the basis of both agriculture and ecosystems. Desiccation tolerant seeds, representative of most crop species, can survive maturation drying to become metabolically quiescent. The desiccated state prolongs embryo viability and provides protection from adverse environmental conditions, including seasonal periods of drought and freezing often encountered in temperate regions. However, the capacity of the seed to germinate declines over time and culminates in the loss of seed viability. The relationship between environmental conditions (temperature and humidity) and the rate of seed deterioration (ageing) is well defined, but less is known about the biochemical and genetic factors that determine seed longevity. This review will highlight recent advances in our knowledge that provide insight into the cellular stresses and protective mechanisms that promote seed survival, with a focus on the roles of DNA repair and response mechanisms. Collectively, these pathways function to maintain the germination potential of seeds. Understanding the molecular basis of seed longevity provides important new genetic targets for the production of crops with enhanced resilience to changing climates and knowledge important for the preservation of plant germplasm in seedbanks.

Keywords: DNA repair; Seed; genome stability; germination; mutation; recombination.

Figure 1. Critical stages in the life of a seed

The key stages from seed maturation to seedling establishment have been the subject of recent reviews: the acquisition of desiccation tolerance (I) is followed by a developmental programme that extends longevity during storage (II) [13]. Maturation drying (III) decreases seed moisture content to ∼10% fresh weight and solidifies the cytoplasm into an intracellular glass [3,13]. Storage of seeds under ideal conditions of low temperature and low humidity extends survival whereas suboptimal environmental conditions result in seed ageing (IV) [3,30,32,150]. Imbibition (water uptake) initiates metabolism and cellular repair (V) which is followed by germination (VI) in non-dormant seeds [14,151,152]. The impact of seed ageing extends into post-germinative growth (VII) [29,122]. Seed imbibition is reversible (VIII): seeds in the soil undergo hydration-desiccation cycles. Commercial seed priming technologies hydrate seeds, followed by a dry back, to facilitate cellular repair and improve the vigour of germination and seedling growth [128,129]. Desiccation tolerance is lost as seeds progress to germination but can be re-established by treatment with ABA or PEG (IX) allowing survival after re-drying [13].

Figure 2. Oxidation products in seeds

Examples of cellular macromolecular adducts produced by reactive oxygen species. (A) Lipids are oxidised to form lipid peroxides and lipid hydroperoxides (A) directly by reactive oxygen species (ROS) or through reactions with other metabolites [153]. (B) Amino acid side chains (e.g. arginine) are oxidised to form carbonyl groups on proteins [154] (C) Oxidation of methionine results in production of methionine sulfone. Progressive oxidation of cysteine forms sulfenic, sulfinic and sulfonic acids. Sulfenic acid can undergo further reactions to form disulphide bonds and intermolecular disulphide bonds with glutathione (gluthionylation) and other proteins [59]. (D) Oxidation of base guanine to form 8-oxoguanine (8-oxoG) is the major oxidative damage product in DNA (8-oxo-2′-deoxyguanosine) [155] and a similar product (8-hydroxyguanosine [8-OHG]) is a prevalent result of RNA oxidation [156].

Figure 3. DNA damage and repair activities in seeds

DNA damage results in single and double stranded DNA breaks, base loss and damage to damage to the sugar-phosphate backbone. This requires the activities of the major DNA repair pathways, all of which influence germination. BER: Base Excision Repair; NER: Nucleotide Excision Repair; NHEJ: Non-Homologous End Joining; HR: Homologous recombination. Alternative end-joining (alt-EJ) pathways operate in plants, including DNA polymerase theta (POLQ) mediated end-joining (TMEJ) [157], although functions in seeds are not well characterised. However, recently a ku70 polq double mutant was reported to have reduced germination [158].

Figure 4. DNA damage responses in plants

The DNA damage signalling kinases ATAXIA TELANGIECTASIA MUTATED (ATM) and ATM AND RAD3-RELATED (ATR) orchestrate plant cellular responses to DNA damage, with major roles played by the transcription factor suppressor of γ1 (SOG1). (A) Post-translational modification of proteins includes acetylation (Ac) of histones and phosphorylation (P) of hundreds of proteins including the DNA damage signalling factors HISTONE H2AX (H2AX) and SOG1 [96,159–161]. (B) DNA damage results in arrest of the cell cycle at the transitions between G1- and S-phase, G2- and M-phase and within S-phase (intra-S) [112]. (C) The DNA damage response (DDR) in seeds results in the transcriptional regulation of hundreds of genes in the first few hours of imbibition and delays both DNA replication and germination [113,162]. (D) DNA damage can lead to the switch from the mitotic cell cycle to endocycles or programmed cell death in meristem cells, revealed by propidium iodide staining of non-viable stem cell initials (coloured red) [115,116].