SeedAdapt - Dimorphic fruits, seeds and seedlings as adaptation mechanisms to abiotic stress

Higher plant dispersal units - diaspores, here: fruits and seeds - support the distribution and early life history of the progeny. The aim of the SeedAdapt project is to elucidate molecular mechanisms of fruit, seed and seedling traits that evolved in annual plant species as adaptations to changing and unpredictable environments. Although these traits are cornerstones for food quality and safety as well as for the fate of ecosystems, the molecular and developmental biodiversity of mechanisms underlying the adaptation to abiotic stresses including heat and drought are only poorly understood. To provide novel insight into this important adaptation mechanism, our interdisciplinary and integrative project utilizes the distinct `dimorphic diaspore syndromes` (DDS) that develop on the same plant of annual Aethionema species (Brassicaceae) as dormancy bet-hedging strategy. The `Seed-Diaspore Syndrome (SDS)` resembles the `default pathway` of weeds like Arabidopsis: dehiscent fruits disperse seeds with non-deep dormancy, these seeds form mucilage upon imbibition, and embryos emerge with radicles first during germination. The `Fruit-Diaspore Syndrome (FDS)` constitutes a `novel pathway`: indehiscent fruits are dispersed that contain non-mucilaginous seeds with deep dormancy, and embryos emerge with cotyledons first during germination. We will compare SDS and FDS of two Aethionema arabicum accessions adapted to distinct climates, for their responses to abiotic stress during three sensitive processes: (1) reproduction, (2) germination, (3) seedling growth. For the dimorphic diaspore formation we will investigate the SDS/FDS-ratios depending on the parental environment, perform an evolutionary comparison of dispersal genes, and study syndrome- and stress-related glucosinolate patterns (diaspore herbivory). The dimorphic diaspores` hormonomes, epigenomes, and transcriptomes provide a `syndrome x stress memory` which will be quantified in a comparative manner and integrated with stress physiology modeling, seed biomechanics, and growth imaging of dimorphic seedlings. The genome sequence and an RNA-Seq bioinformatics pipeline for Ae. arabicum are available. The comparison of the `syndrome x stress memory` data will provide differentially expressed genes and distinct epigenetic markers. Their targeted analyses will be extended across several Aethionema populations/species, as well as by phylogenomics (e.g. promoter elements) and cross-species reverse-genetics in crops and models. We will conduct forward and reverse genetics of SDS and FDS to separate individual traits, including EMS-mutant screens and QTL analyses using already existing RILs, and we will establish genetic transformation and resources. We propose that investigating the regulatory basis of fruit, seed, and seedling trait diversity is ideal for integrating new technologies and complementary expertise in order to study a field with utmost importance in ecology, evolution, seed industry and crop breeding.

The formation of fruits in higher plants supports the dissemination of seeds, and seeds provide plant embryos with protection and nutrition early in life. While most plants produce uniform progeny that is optimized to the respective life style, some plant species have developed a different strategy: they form structurally and functionally different seeds and/or fruits on the same plant. This is interpreted as risk management in regions with unpredictable environmental conditions, so that at least part of the progeny has a chance to survive. Both seed types grow into indistinguishable plants that again can form two seed or fruit forms. Within the network SeedAdapt, a collaboration between seven European research groups, we have investigated this interesting phenomenon studying Aethionema arabicum, a small plant originating from the Mediterranean region and the Middle East. The species belongs to the family of crucifers (cabbage plants) and is therefore also related with thale cress, famous for genetic and molecular plant research. The network has documented that the feature of distinct propagation units appeared twice independently during evolution among closely related species. Seed and fruit types differ with respect to several anatomic, biomechanical, and physiological features. The total number of fruits and the ratio between the two types depends in part on the developmental scheme of the flower branches, but can change in response to different environmental conditions. In a large experiment, systematically testing these conditions for the consequences on fruit formation, we have generated data about plant hormones involved in the regulation as well as different activity of specific genes. In addition, we have discovered that seeds of some subspecies do not germinate as long as they are exposed to light. This was surprising, as seeds of most plants studies so far even require light for successful germination, or at least germinate equally well in light or darkness. We hypothesize that light inhibition in Aethionema is another adaptive trait, one that helps to ensure sufficiently deep anchoring of the young seedling in the soil. The ongoing analysis of the light influence on gene expression and hormone concentration is expected to provide insight into the underlying regulatory mechanism.