Molecular mechanisms and ecological drivers of seed persistence in fire-prone ecosystems

Fire-prone ecosystems are distributed across the globe, prompting many plant species to evolve persistence strategies to tolerate the extreme conditions generated by wildfire. Post-fire recruitment from seed is one such mechanism, with some seeds having dormancy-break cued to heat shock generated by the passage of fire. Temperatures that break dormancy vary across species and can reach up to 150 °C, however, the fundamental mechanisms behind how seeds tolerate this extreme heat are unknown. In this thesis, I sought to understand the molecular mechanisms and ecological drivers of seed persistence through extreme heat shock in fire-prone ecosystems. In Chapter 2, I examined whether seed fatty acid composition drove seed dormancy-breaking temperatures during fire. While I did not find evidence of this, seed fatty acid composition was shaped by the maternal environment for germination and the maintenance of dormancy. In Chapters 3-4, I examined seed gene expression of the post-fire recruiter, Acacia pycnantha, to capture a wide range of molecular mechanisms used during fire. I analysed seed gene expression immediately following fire-related heat shock up until germination to create a timeline of post-fire gene expression. After acute heat shock, I found that seeds reduced protein synthesis and delayed germination, potentially to repair seed damage. As seed moisture content increased, as would occur through post-fire rainfall, defences against pathogens were highly downregulated and developmental processes (e.g. growth) upregulated. These results highlight that seed persistence during wildfire is achieved through a series of trade-offs. As heat shock responses are genetically conserved, in Chapter 5 I assessed the ecological heat shock responses of 35 Acacia species from 12 vegetation types across Australia. Results indicated that fire return interval, fuel type and seed mass shaped the heat shock response, suggesting that the mechanisms used by species to persist during fire are driven by both species’ phylogeny and fire regime. Collectively, this research improves our understanding of seed molecular persistence strategies during fire and how they are shaped by different ecological drivers. A comprehensive understanding of species persistence mechanisms in fire-prone regions is critical to better understand how plant populations may be affected under future fire regimes.