A blueprint of seed desiccation sensitivity in the genome of Castanospermum australe

SummaryMost angiosperms produce seeds that are desiccated on dispersal with the ability to retain viability in storage facilities for prolonged periods. However, some species produce desiccation sensitive seeds which rapidly lose viability in storage, precluding ex situ conservation. Current consensus is that desiccation sensitive seeds either lack or do not express mechanisms necessary for the acquisition of desiccation tolerance.We sequenced the genome of Castanospermum australe, a legume species producing desiccation sensitive seeds, and characterized its seed developmental physiology and - transcriptomes.C. australe has a low rate of evolution, likely due to its perennial life-cycle and long generation times. The genome is syntenic with itself, with several orthologs of genes from desiccation tolerant legume seeds, from gamma whole-genome duplication events being retained. Changes in gene expression during development of C. australe seeds, as compared to desiccation tolerant Medicago truncatula seeds, suggest they remain metabolically active, prepared for immediate germination.Our data indicates that the phenotype of C. australe seeds arose through few changes in specific signalling pathways, precluding or bypassing activation of mechanisms necessary for acquisition of desiccation tolerance. Such changes have been perpetuated as the habitat in which dispersal occurs is favourable for prompt germination.

Assembly and comparative analyses of the mitochondrial genome of Castanospermum australe (Papilionoideae, Leguminosae)

Plant mitochondrial genomes are often difficult to assemble because of frequent recombination mediated by repeats. Only a few mitochondrial genomes have been characterised in subfamily Papilionoideae of Leguminosae. Here, we report the complete mitochondrial genome of Castanospermum australe A.Cunn. & C.Fraser, an important medicinal and ornamental species in the Aldinoid clade of Papilionoideae. By mapping paired-end reads, seven hypothetical subgenomic conformations were rejected and two hypothetical complete isometric mitochondrial genome conformations that differed by a 64-kb inversion were strongly supported. Quantitative assessment of repeat-spanning read pairs showed a major conformation (MC1) and a minor conformation (MC2). The complete mitochondrial genome of C. australe was, thus, generated as 542079bp in length, with a high depth of coverage (~389.7×). Annotation of this mitochondrial genome yielded 58 genes encoding 37 proteins, 18 tRNAs and three rRNAs, as well as 17 introns and three medium-sized repeats (133, 119 and 114bp). Comparison of 10 mitochondrial genomes from Papilionoideae demonstrated significant variation in genome size, structure, gene content and RNA editing sites. In addition, mitochondrial genes were shown to be potentially useful in resolving the deep relationships of Papilionoideae.