Date Published: July 12, 2017
Publisher: Public Library of Science
Author(s): Juliana Joice Pereira Lima, Julia Buitink, David Lalanne, Rubiana Falopa Rossi, Sandra Pelletier, Edvaldo Aparecido Amaral da Silva, Olivier Leprince, Belay T. Ayele.
Seed longevity, defined as the ability to remain alive during storage, is an important agronomic factor. Poor longevity negatively impacts seedling establishment and consequently crop yield. This is particularly problematic for soybean as seeds have a short lifespan. While the economic importance of soybean has fueled a large number of transcriptome studies during embryogenesis and seed filling, the mechanisms regulating seed longevity during late maturation remain poorly understood. Here, a detailed physiological and molecular characterization of late seed maturation was performed in soybean to obtain a comprehensive overview of the regulatory genes that are potentially involved in longevity. Longevity appeared at physiological maturity at the end of seed filling before maturation drying and progressively doubled until the seeds reached the dry state. The increase in longevity was associated with the expression of genes encoding protective chaperones such as heat shock proteins and the repression of nuclear and chloroplast genes involved in a range of chloroplast activities, including photosynthesis. An increase in the raffinose family oligosaccharides (RFO)/sucrose ratio together with changes in RFO metabolism genes was also associated with longevity. A gene co-expression network analysis revealed 27 transcription factors whose expression profiles were highly correlated with longevity. Eight of them were previously identified in the longevity network of Medicago truncatula, including homologues of ERF110, HSF6AB, NFXL1 and members of the DREB2 family. The network also contained several transcription factors associated with auxin and developmental cell fate during flowering, organ growth and differentiation. A transcriptional transition occurred concomitant with seed chlorophyll loss and detachment from the mother plant, suggesting the activation of a post-abscission program. This transition was enriched with AP2/EREBP and WRKY transcription factors and genes associated with growth, germination and post-transcriptional processes, suggesting that this program prepares the seed for the dry quiescent state and germination.
Soybean is one of the most important oil crop species for food, feed and a range of industrial applications. Producing highly vigorous seeds is a key lever to increase crop production. Seed longevity, defined as the ability to remain alive during storage under dry conditions, is an important agronomic factor in the preservation of seed fitness after harvest . Poor longevity leads to unexpected losses in seed viability during storage and negatively impacts seedling establishment and crop yield [1, 2]. This is particularly problematic for soybean as seeds have a short lifespan during storage, especially in humid and tropical environment [2–4]. In addition, longevity is pivotal to ensure the preservation of our genetic resources through dry seeds of crops and wild species [5, 6].
Poor longevity results in economic losses due to the impossibility of carry-over of seed lots, having lost their vigor and viability so that they are no longer marketable. Identification of the underlying regulatory factors should provide information to design marker for prebreeding aiming to improve soybean seed quality. In developing soybean seeds, physiological maturity corresponds to the stage when final seed weight is reached, germination/desiccation tolerance and seed vigor are acquired [4, 25, 27]. In this study using an indeterminate cultivar, physiological maturity corresponded to stage 7.2, in agreement with previous works . At this stage, most seeds were detached from the mother plant and had lost most of their chlorophyll. However, our physiological, sugar and transcriptome data show that the seed maturation program has not yet ended at physiological maturity. An additional period of 14 days after physiological maturity is necessary to acquire maximum longevity (Fig 2), in agreement with previous data on other genotypes [25, 26]. During this period, we detected 16,248 transcripts being differentially expressed until the developing seeds reached the dry state. Our RNAseq study complements and extends previous transcriptome characterization of soybean seed development [28–33]. These studies focused mostly from fertilization to end of seed filling whereas here we characterized the phase from end of seed filling to final maturation drying. Our RNAseq data and co-expression network analysis suggest that complex transcriptome changes occur after the so-called physiological maturity until the dry state, identifying several TFs associated with seed longevity. Several of these TFs were previously identified in a gene co-expression network associated with longevity in M. truncatula and Arabidopsis  and thus provide new resources for marker of seed development.