Molecular comparison of orthodox Medicago truncatula seeds and recalcitrant Castanospermum australe seeds: a new model to unravel desiccation tolerance acquisition
Desiccation tolerance (DT) is defined as the ability to survive in the dry state and resume metabolic activity upon imbibition. Unlike orthodox seeds, that acquire DT during their maturation, recalcitrant seeds do not survive desiccation. A comparative analysis of orthodox and recalcitrant seeds development constitutes an interesting model to highlight mechanisms that are involved in DT. Using proteomic and transcriptomic analyses, the development of the recalcitrant Castanospermum australe seeds was characterized and compared to development of orthodox Medicago truncatula seeds, species from the same sub-family of Fabaceae. Our results show that most LEA (Late Embryogenesis Abundant) proteins are absent or faintly accumulated in mature seeds of C. australe compared to M. truncatula. The LEA protein profile is similar to seeds of the Mtabi3 mutant that is deficient in the expression of the major regulator of orthodox seed maturation ABI3 (ABscissic acid Insensitive 3). Transcriptome analysis reveals a strong repression of CaABI3 and its putative targets at the end of C. australe seed development while the transcript levels of these target genes remain high until the end of maturation of M. truncatula seeds. Two genes coding for ABI3 were cloned in C. australe: CaABI3 and CaABI3-like. Ectopic expression in M. truncatula roots demonstrated that only CaABI3-like is able to activate the same targets as MtABI3. Moreover, CaABI3 does not complement the abi3-5 mutant of Arabidopsis thaliana. This analysis strengthens the implication of ABI3 in the desiccation sensitivity of recalcitrant seeds. In addition, mature C. australe seeds display certain characteristics of a seed that is preparing for germination and express many stress response genes.