- © 2015 American Society of Plant Biologists. All rights reserved.
At maturation, seeds generally become dormant and desiccation tolerant; after dormancy breaks, the seeds remain quiescent for an indefinite period and will germinate only when provided with adequate moisture. Some seeds remain in this quiescent state for thousands of years, only to germinate under the right conditions, while other seeds fail to germinate after extended storage. How do some seeds survive for so long? This question has crucial importance for agriculture and for conservation of genetic resources. Environmental conditions strongly affect seed longevity, as high temperature, high humidity, and high oxygen can decrease the longevity of seeds.
Many innate factors also affect seed longevity; for example, seeds accumulate factors that protect against damage from desiccation, such as heat shock proteins, sugars, and late embryogenesis abundant proteins. Seeds also produce factors that help maintain genome integrity and mitigate the effects of oxidative stress (reviewed in Waterworth et al., 2015). To examine the establishment of longevity during maturation, Righetti et al. (2015) characterized 104 Medicago truncatula transcriptomes collected under five different conditions to measure transcript levels of all genes expressed in during seed maturation. They constructed a coexpression network, MatNet (see figure, panel A), which has 2912 nodes, 73,6290 edges, and includes many seed-specific genes and transcriptional regulators (see figure, panel B).
Representation of Mat Net. (A) MatNet, represented as interconnected nodes. Genes with high transcript levels at the end of embryogenesis are shown in yellow-green and form a central core; genes with high transcript levels in seed filling and maturation (dark green) or late maturation (blue) form the tail of the spiral. (B) Transcriptional regulators present in MatNet (red) and seed-specific transcripts (blue). (C) Nodes related to acquisition of longevity (top) and desiccation tolerance (bottom) are shown as black dots. (Reprinted from Righetti et al. [2015], Figures 2 and 3.)
Based on their comprehensive profiling of seed physiology during maturation under different conditions, they then used a trait-based measure of significance to determine the relationship of the expressed genes to longevity and desiccation tolerance. By identifying genes that showed both strong correlation to the trait and strong coexpression, the authors defined key modules affecting longevity and desiccation tolerance (see figure, panel C). To facilitate functional validation of the longevity module and examine its conservation, the authors next compared M. truncatula seed maturation to Arabidopsis thaliana. Out of 130 Arabidopsis genes homologous to members of the M. truncatula longevity module, 113 showed connections in the Arabidopsis interaction database, indicating conservation of 87% of the module nodes between both species.
The longevity module nodes showed an overrepresentation of genes involved in defense responses, and the authors examined T-DNA knockout mutants of several such genes. For example, the wrky3 and nf-x1-like1 (nfxl1) mutants, which affect two defense-related transcription factors, showed altered transcript levels at several of the longevity module nodes, as well as decreased seed longevity. The longevity module also showed overrepresentation of genes with auxin response factor binding sites in their promoters, suggesting an important role for auxin in longevity. The authors tested the role of the central regulator ABSCISIC ACID INSENSITIVE3, which affects longevity and desiccation tolerance in both species. Examination of the transcriptomes of M. truncatula abi3 mutant seeds and ABI3-overexpressing hairy root cultures indicated that nodes in the longevity module showed ABI3-dependent and -independent regulation. However, expression of WRKY3 and NFX11 did not change in the abi3 mutants, indicating that they function independently of ABI3.
Unlike the fairy tales, falling into a “sleep like death” happens as a normal part of seed development. However, failing to wake from this sleep has serious implications for conservation and agriculture. Identification of a conserved longevity module that includes many defense-related genes indicates evolutionary interplay between defense responses and longevity. Moreover, manipulation of this module might give farmers vigorous, long-lasting seeds—no Prince Charming required.
