Two-tier morpho-chemical defence tactic in Aethionema via fruit morph plasticity and glucosinolates allocation in diaspores.

Fruit dimorphism and the production of glucosinolates (GSLs) are two specific life history traits found in the members of Brassicales, which aid to optimize seed dispersal and defence against antagonists, respectively. We hypothesized that the bipartite dispersal strategy demands a tight control over the production of fruit morphs with expectedly differential allocation of defensive anticipins (GSLs). In dimorphic Aethionema, herbivory by Plutella xylostella at a young stage triggered the production of more dehiscent (seeds released from fruit) than indehiscent fruit morphs (seeds enclosed within persistent pericarp) on the same plant upon maturity. Total GSL concentrations were highest in the mature seeds of dehiscent fruits from Aethionema arabicum and Aethionema saxatile among the different ontogenetic stages of the diaspores. Multivariate analyses of GSL profiles indicated significantly higher concentrations of specific indole GSLs in the diaspores, which require optimal defence after dispersal (i.e., seeds of dehiscent and fruit/pericarp of indehiscent fruit). Bioassays with a potentially coinhabitant fungus, Aspergillus quadrilineatus, support the distinct defensive potential of the diaspores corresponding to their GSL allocation. These findings indicate a two-tier morpho-chemical defence tactic of Aethionema via better protected fruit morphs and strategic provision of GSLs that optimize protection to the progeny for survival in nature.

Developmental Control and Plasticity of Fruit and Seed Dimorphism in Aethionema arabicum.

Understanding how plants cope with changing habitats is a timely and important topic in plant research. Phenotypic plasticity describes the capability of a genotype to produce different phenotypes when exposed to different environmental conditions. In contrast, the constant production of a set of distinct phenotypes by one genotype mediates bet hedging, a strategy that reduces the temporal variance in fitness at the expense of a lowered arithmetic mean fitness. Both phenomena are thought to represent important adaptation strategies to unstable environments. However, little is known about the underlying mechanisms of these phenomena, partly due to the lack of suitable model systems. We used phylogenetic and comparative analyses of fruit and seed anatomy, biomechanics, physiology, and environmental responses to study fruit and seed heteromorphism, a typical morphological basis of a bet-hedging strategy of plants, in the annual Brassicaceae species Aethionema arabicum Our results indicate that heteromorphism evolved twice within the Aethionemeae, including once for the monophyletic annual Aethionema clade. The dimorphism of Ae. arabicum is associated with several anatomic, biomechanical, gene expression, and physiological differences between the fruit and seed morphs. However, fruit ratios and numbers change in response to different environmental conditions. Therefore, the life-history strategy of Ae. arabicum appears to be a blend of bet hedging and plasticity. Together with the available genomic resources, our results pave the way to use this species in future studies intended to unravel the molecular control of heteromorphism and plasticity.