A NOVEL MODEL TO STUDY LIGHT-REGULATED SEED GERMINATION

Besides in photosynthesis, light has an important role for plant development, including seed germination. Seeds are classified into three categories based on their response to white light during germination: (I) seeds that require light to germinate; (II) seeds that germinate with or without light, and (III) seeds whose germination is inhibited by light. The seeds of the widely used laboratory plant Arabidopsis thaliana belong to the first category; they need a minimum of light to germinate, and the molecular mechanism of light-induced germination was intensively studied. In contrast, we know very little about the light inhibition of germination. A remarkable natural variation observed in Aethionema arabicum (Brassicaceae), a relative of Arabidopsis, will allow filling this gap. Seeds of one accessions originating from Turkey (TUR) are light-insensitive (category II), whereas seeds of another accession from Cyprus (CYP) do not germinate in light (category III), although the two accessions are genetically very similar. Interestingly, the germination of CYP seeds is not just temporarily inhibited by light: extended light exposure results in a memory effect that prevents germination for a long period even if seeds are transferred back to the favorable dark condition (secondary dormancy). In the frame of the NKFIH-FWF Joint Research Project, we would like to understand the photobiological aspects and the molecular mechanism of light-inhibited and light-insensitive germination. Moreover, we aim to investigate the light-induced secondary seed dormancy and its occurrence in the Aethionemeae genus as adaptation to the local environment. We hypothesize that the natural variation of seed response to light between Aethionema arabicum accessions has a genetic basis and represents an adaptive trait. We will investigate the genetic and epigenetic variation among light-inhibited and light-insensitive seeds to identify key regulators responsible for the differences. We also aim to investigate whether light-induced secondary seed dormancy is mediated and stabilized by epigenetic changes at the level of chromatin, and how it correlates with specific and genome-wide gene activity. The application proposes pioneering research on a yet under-investigated effect of light on seed germination, an essential part of plant development. Beside of the potential to identify novel key regulators, analysis of epigenetic parameters before, during and after secondary seed dormancy will offer valuable information for seed biology also in other plants and in other contexts. The research will be conducted by Zsuzsanna Merai, a senior post-doc with background in plant development, photobiology and epigenetic research, in the laboratory of Ortrun Mittelsten Scheid at the Gregor Mendel Institute of Molecular Plant Biology (Vienna, Austria). The international joint project will provide strong synergism with the well-recognized expertise in photobiology and physiology of the Hungarian co-applicant, Lszl Kozma-Bognr, group leader of the Plant Photo- and Chronobiology group of the Biological Research Centre (Szeged, Hungary).

Besides photosynthesis, light has an important role for plant development, including seed germination. Seeds are classified into three categories based on their response to white light during germination: seeds that require light to germinate; seeds that germinate with or without light, and seeds whose germination is inhibited by light. The seeds of the widely used laboratory plant Arabidopsis thaliana belong to the first category; they need a minimum of light to germinate, and the molecular mechanism of this light-requiring germination was intensively studied. In contrast, very little is known about the light inhibition of germination. A remarkable natural variation observed in Aethionema arabicum (Brassicaceae), a relative of Arabidopsis, allowed filling this gap. Seeds of one accession originating from Turkey (TUR) are light-neutral , whereas seeds of another accession from Cyprus (CYP) do not germinate in light , although the two accessions are genetically very similar. In an FWF project led by Dr. Zsuzsanna Mérai at the Gregor Mendel Institute of Molecular Plant Biology, Ae. arabicum was implemented as a novel plant model to study alternative light regulation of seed germination. While the phytohormones that regulate the germination are the same in all categories, the expression of genes encoding enzymes for hormone synthesis and degradation undergo converse changes upon light illumination, resulting in antipodal hormone regulation compared to Arabidopsis. These findings illustrate that similar modular components of a pathway in light-inhibited, light-neutral, and light-requiring germination among the Brassicaceae have been rewired during evolution to produce divergent pathways, likely as adaptive traits (Mérai et al., 2019). The photoreceptors involved in the light-inhibited germination in Ae. arabicum were unknown. A screen in a newly generated mutant seed collection identified koy-1, a mutant that lost light inhibition of germination due to a deletion in the promoter of HEME OXYGENASE 1, the gene for a key enzyme in the biosynthesis of the phytochrome chromophore. Analysis of koy-1 mutant revealed that very low light fluence stimulates germination, while high irradiance of red and far-red light is inhibitory, indicating a dual role of phytochromes in light-regulated seed germination (Mérai et al., 2023). Interestingly, the germination of CYP seeds is not just temporarily inhibited by light: extended light exposure results in secondary dormancy, a "memory" effect that prevents germination for a long period even if seeds are transferred back to the favorable dark condition. This reflects adaptation to a reliable seasonal parameter: it was demonstrated that increasing day-length like in spring induces this secondary seed dormancy, repressing germination in summer when seedling establishment in Mediterranean climate is riskier (Mérai et al., 2024). These discoveries have important implications for understanding seed germination biology, and they open a potential path to improve germination adaptation in other species, including crop plants.