Asynchrony of ovule primordia initiation in Arabidopsis

Plant ovule initiation determines the maximum of ovule number and has a great impact on the seed number per fruit. The detailed processes of ovule initiation have not been accurately described although two connected processes, gynoecium and ovule development, have been investigated. Here we report that ovules initiate asynchronously. The first group of ovule primordia grows out, the placenta elongate, the boundaries of existing ovules enlarge and new group of primordia initiates from the boundaries. The expression pattern of different marker genes during ovule development illustrates that this asynchronicity continues throughout whole ovule development. PIN-FORMED1 polar distribution and auxin response maxima correlate to ovule primordia initiation asynchronous. We established computational modeling to show how auxin dynamics influence ovule primordia initiation. Brassinosteroid signaling positively regulates ovule number by promoting placentae size and ovule primordia initiation through strengthening auxin response. Transcriptomic analysis demonstrates numerous known regulators of ovule development and hormones signaling, and many new genes are identified to involve in ovule development. Taken together, our results illustrate the ovule primordia initiate asynchronously and the hormone signal involve in the asynchrony.