Abstract
In contrast to orthodox seeds that acquire desiccation tolerance during maturation, recalcitrant seeds are unable to survive drying. These desiccation-sensitive seeds constitute an interesting model for comparative analysis with phylogenetically close species that are desiccation tolerant. Considering the importance of LEA (late embryogenesis abundant) proteins as protective molecules both in drought and in desiccation tolerance, the heat-stable proteome was characterized in cotyledons of the legume Castanospermum australe and it was compared with that of the orthodox model legume Medicago truncatula. RNA sequencing identified transcripts of 16 homologues out of 17 LEA genes for which polypeptides are detected in M. truncatula seeds. It is shown that for 12 LEA genes, polypeptides were either absent or strongly reduced in C. australe cotyledons compared with M. truncatula seeds. Instead, osmotically responsive, non-seed-specific dehydrins accumulated to high levels in the recalcitrant cotyledons compared with orthodox seeds. Next, M. truncatula mutants of the ABSCISIC ACID INSENSITIVE3 (ABI3) gene were characterized. Mature Mtabi3 seeds were found to be desiccation sensitive when dried below a critical water content of 0.4g H2O g DW–1. Characterization of the LEA proteome of the Mtabi3 seeds revealed a subset of LEA proteins with severely reduced abundance that were also found to be reduced or absent in C. australe cotyledons. Transcripts of these genes were indeed shown to be ABI3 responsive. The results highlight those LEA proteins that are critical to desiccation tolerance and suggest that comparable regulatory pathways responsible for their accumulation are missing in both desiccation-sensitive genotypes, revealing new insights into the mechanistic basis of the recalcitrant trait in seeds.
Highlights
Global agriculture and the conservation of plant biodiversity of time in dedicated national and international storage facilirely on seeds and their ability to be stored for long periods ties (Li and Pritchard, 2009; Walters et al, 2013)
This work shows that C. australe and M. truncatula, both from the Papilionaceae subfamily of Fabaceae, are phylogenetically close enough to allow for a detailed sequence comparison of late embryogenesis abundant (LEA) accumulation in relation to desiccation tolerance
Of a normalized sequencing library identified contigs with high similarity for 16 of the 17 M. truncatula LEA genes (Table 2) for which protein accumulation was shown in M. truncatula (Chatelain et al, 2012)
Summary
Global agriculture and the conservation of plant biodiversity of time in dedicated national and international storage facilirely on seeds and their ability to be stored for long periods ties (Li and Pritchard, 2009; Walters et al, 2013). Orthodox seeds undergo maturation drying and are shed from the parent plant at low moisture contents During maturation, they acquire desiccation tolerance, allowing them to be dried to moisture contents in the range of 1–5% without irreversible damage. They acquire desiccation tolerance, allowing them to be dried to moisture contents in the range of 1–5% without irreversible damage Because of this ability, seeds can be stored for long periods in cold and dry vaults. Recalcitrant seeds, on the other hand, do not undergo maturation drying, and are shed at relatively high moisture contents Such seeds are highly susceptible to desiccation injury, and are not storable under conditions suitable for orthodox seeds (reviewed in Farnsworth, 2000; Berjak and Pammenter, 2008; Li and Pritchard, 2009). The mechanisms by which recalcitrant seeds lose viability during drying and/or storage are not well understood, which poses a challenge to determine appropriate measures to better conserve these species
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