Genetic analysis of environmental control of inflorescence development in perennial Arabis alpina

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Seasonal changes and daylength determine the appropriate timing of flowering in plants, to ensure the subsequent optimal production of seeds. In crucifers such as Arabis alpina and Brassica napus, the formation of flowers takes place in a two-step procedure, whereby buds are formed in one year and their outgrowth occurs in the following spring. However, the genetic mechanisms that underlie the seasonal regulation of bud arrest and inflorescence growth have not been examined in perennial plants. In this project, two different approaches will be used to analyse these mechanisms. First, a forward-genetic, QTL analysis will be applied to a Norwegian population of Arabis alpina that shows natural variation for flowering time and contains early- and late-flowering individuals. In early-, and late-flowering plants, floral buds are formed without vernalization at an approximately similar time; however, the floral buds of late-flowering plants arrest and generate mature inflorescences much more slowly. Vernalization of both genotypes overcomes the floral bud-arrest phenotype of the late-flowering parent, and both types flower at approximately the same time soon after vernalization. This suggests that these two accessions differ genetically at a checkpoint that arrests floral bud opening of the late type and that this can be overcome by vernalization. The host group has identified a QTL that contributes to the floral bud-arrest phenotype. The QTL has been localised to a 700-kb genomic segment of chromosome 8 in near-isogenic lines in which the locus segregates and the region has been assembled from PacBio reads. The causal gene responsible for the environmentally determined bud-arrest phenotype will be identified by further rounds of recombination mapping and its function will be analysed by expression analysis and transgenesis. Second, the Arabidopsis thaliana FT gene promotes flowering in long days and is proposed to regulate inflorescence development and flower-bud opening in Arabidopsis lyrata. However, the contribution of florigen-encoding FT genes to floral induction of perennial Brassicaceae such as Arabis alpina has not been analysed, mainly because AaFT null alleles are not available. Therefore, a reverse-genetic approach using CRISPR-Cas9 will be employed to generate knockout mutants of four AaFT genes with which to study the individual and combined contributions of the genes to floral induction, floral development and inflorescence development in different photoperiods and following vernalization. The two proposed approaches will not only identify the casual gene responsible for the bud-arrest phenotype in late-flowering Arabis alpina and contribute to current models for how floral buds formed during vernalization in autumn only produce open flowers and inflorescences in spring, but will allow comparative analysis of the function of florigen genes in related annual and perennial species.