Desiccation tolerance: From genomics to the field

Desiccation tolerance is defined as the ability to survive the removal of all, or almost all the cellular water without irreversible damage. It confers to dried organisms the ability to survive extreme conditions of the environment and to stay alive in a suspended animation for long periods of time. The biotechnological potential of anhydrous biology has been recognized for more than 60 years. With the fast development of “omics” technologies, it is now possible to better appreciate the biotechnological promises that can be made from the understanding of desiccation tolerance. This review will discuss the impact of post-genomics tools on identifying genes or gene products, and will give a comprehensive overview of the agronomical and biotechnological applications. We propose the term desiccomics to define the approach consisting of combining “omics” approaches to address the specific issues associated with the dry state

Régulation des qualités physiologiques et sanitaires de la graine de Medicago truncatula. Rôles de MtABI5 et transmission des Xanthomonas aux semences

La maîtrise des qualités physiologique (performance germinative) et sanitaire des semences (vection d\u27agents pathogènes) constitue un verrou scientifique et technologique majeur dans le cadre d\u27une agriculture raisonnée et mondialisée. La qualité physiologique repose notamment sur la tolérance à la dessiccation, la dormance et l\u27aptitude à la conservation qui s\u27acquièrent pendant le développement de la graine. Cependant les mécanismes régulateurs de ces processus restent mal compris. Chez Medicago truncatula, par une approche transcriptomique nous avons montré que la signalisation par l\u27acide abscissique participe via le facteur de transcription ABI5 à la régulation de la survie à l\u27état sec et à la dormance. ABI5 joue un rôle prépondérant dans les phases du développement allant de la fin de la maturation et à la levée. La transmission à et par les semences est l\u27un des principaux moyens de survie et de dissémination des bactéries phytopathogènes telles que Xanthomonas. Les éléments du dialogue moléculaire qui pourrait s\u27établir entre ces bactéries et la graine sont cependant méconnus. Après avoir établi que la contamination est plus efficace en situation compatible [Xanthomonas alfalfae subsp. alfalfae] qu\u27en situation incompatible [X. campestris pv. campestris, Xcc], nous démontrons l\u27existence d\u27un dialogue moléculaire entre Xcc et la graine en développement. La réponse transcriptionelle de la graine contaminée par Xcc et la réduction de leur poids suggèrent un trade-off entre l\u27activation des défenses basales et son développement. Cette thèse apporte des éléments permettant de suggérer que les qualités sanitaire et physiologique sont liées

Foreword. Special issue on desiccation biology.

A minority of living organisms are able to dry out completely and yet remain viable, a phenomenon known as desiccation tolerance. Its occurrence is established in prokaryotes and eukaryotes across all life kingdoms, including plants and lower invertebrates. In the dry state, the metabolism of the organisms is suspended. Even more striking, desiccation tolerant organisms are often also highly resistant to other environmental stresses, surviving extremes of temperature and pressure, for example. How do organisms survive without water, the driving force for cellular organization, and why does the lack of water confer such prodigious abilities, have intrigued scientists for the past 40 years. [...

Introduction to desiccation biology: from old borders to new frontiers.

MAIN CONCLUSION: A special issue reviews the recent progress made in our understanding of desiccation tolerance across various plant and animal kingdoms. It has been known for a long time that seeds can survive near absolute protoplasmic dehydration through air drying and complete germination upon rehydration because of their desiccation tolerance. This property is present both in prokaryotes and eukaryotes across all life kingdoms. These dry organisms suspend their metabolism when dry, are extremely tolerant to acute environmental stresses and are relatively stable during long periods of desiccation. Studies aiming at understanding the mechanisms of survival in the dry state have emerged during the past 40 years, moving from in vitro to genomic models and comparative genomics, and from a view that tolerance is an all-or-nothing phenomenon to a quantitative trait. With the prospect of global climate change, understanding the mechanisms of desiccation tolerance appears to be a promising avenue as a prelude to engineering crops for improved drought tolerance. Understanding desiccation is also useful for seed banks that rely on dehydration tolerance to preserve plant genetic resources in the form of these propagules. Articles in this special issue explore the recent progress in our understanding of desiccation tolerance, including the evolutionary mechanisms that have been adopted across various plant (algae, lichens, seeds, resurrection plants) and animal model systems (Caenorhabditis elegans, brine shrimp). We propose that the term desiccation biology defines the discipline dedicated to understand the desiccation tolerance in living organisms as well as the limits and time constraints thereof

LEA polypeptide profiling of recalcitrant and orthodox legume seeds reveals ABI3-regulated LEA protein abundance linked to desiccation tolerance

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

The MtSNF4b subunit of the sucrose non-fermenting-related kinase complex connects after-ripening and constitutive defense responses in seeds of Medicago truncatula

Dormant seeds are capable of remaining alive in the hydrated state for extended periods of time without losing vigor, until environmental cues or after-ripening result in the release of dormancy. Here, we investigated the possible role of the regulatory subunit of the sucrose non-fermenting-related kinase complex, MtSNF4b, in dormancy of Medicago truncatula seeds. Expression of MtSNF4b and its involvement in a high-molecular-weight complex are found in dormant seeds, whereas imbibition of fully after-ripened, non-dormant seeds leads to dissociation of the complex. MtSNF4b is capable of complementing the yeast Δsnf4 mutant and of interacting with the MtSnRK1 α-subunit in a double hybrid system. Transcriptome analyses on freshly harvested and after-ripened RNAi Mtsnf4b and wild-type embryos implicate MtSNF4b in the defense response in hydrated dormant embryonic tissues, affecting the expression of genes encoding enzymes of flavonoid and phenylpropanoid metabolism, WRKY transcription factors and pathogenesis-related proteins. Silencing MtSNF4b also increased the speed of after-ripening during dry storage, an effect that appears to be related to a change in base water potential. No significant difference in ABA content or sensitivity was detected between mutant and wild-type seeds. Pharmacological studies using hexoses and sugar analogs revealed that mannose restored germination behavior and expression of the genes PAL, CHR and IFR in RNAi Mtsnf4b seeds towards that of the wild-type, suggesting that MtSNF4b might act upstream of sugar-sensing pathways. Overall, the results suggest that MtSNF4b participates in regulation of a constitutively activated defense response in hydrated, dormant seeds