Auxin as a conserved regulator: insights into male cone development in the genus Cycas

Abstract A comprehensive understanding of seed plant reproduction requires extending molecular and hormonal research beyond angiosperm models to include gymnosperms. Cycads belongs to one of the ancientest lineages of spermatophytes, making it a critical system for understanding the evolution of plant reproduction. While phytohormones like auxin are known to be essential regulators of male reproductive development in angiosperms, their role in gymnosperms remains largely unknown. Here, we report the first spatiotemporal mapping of IAA (indol-3-acetic acid) distribution in the male cones of six Cycas species. Studying hormonal regulation in non-model organisms presents technical challenges. To overcome this, we employed a tailored approach combining high-resolution histology with IAA-immunolocalization assays to map the phytohormone distribution. Critically, we demonstrate the presence of IAA within the developing microsporangia, with a sharp concentration peak observed in the more advanced stage. Specifically, a strong IAA peak was observed in the sporophytic tissues of the microsporangial wall, notably the degenerating tapetum, and within the developing pollen grains. This spatial and temporal pattern strongly implicates IAA in regulating tapetum breakdown and pollen production in Cycas , reinforcing the hypothesis of auxin conserved critical role in male fertility across all seed plants. Additionally, our work highlights the efficacy of immunolocalization for dissecting hormonal mechanisms in non-model systems.