Abstract
The timing of the transition from growth to reproduction is essential for the regulation of the seaweed life cycle. Variable environmental conditions can stress seaweeds and promote trade-offs between their growth and reproduction. Here, we demonstrate that reproductive responses can be induced by environmental stresses in the gametophytic thalli of the marine red alga Pyropia yezoensis. Excision of explants accelerated release of asexual monospores and sexual carpospores. The algal sensitivity to wounding was enhanced by a 3-day dark treatment prior to the excision of the explants. By contrast, heat stress at 25°C stimulated the production of a callus, a three-dimensional aggregation of randomly divided cells with multiple cell layers. This callus produced new gametophytic thalli with a normal shape; therefore, callus formation is thought to be one of the asexual reproductive strategies used by the alga to increase the number of thalli under heat stress conditions. Our results demonstrate that wounding and heat stress can reset the timing of reproduction and that gametophytic thalli therefore use a variety of distinct reproductive strategies under different stress conditions. These findings provide insights into the induction of reproduction by environmental stresses as a life cycle trade-off in seaweeds.
Highlights
Plants and seaweeds control the timing of their growth and reproduction in response to environmental stresses, trading off these phases to optimize their life cycle progression and best adapt to the variable natural conditions they experience (Liu et al, 2017; Mohring et al, 2013; Karasov et al, 2017; Shaar-Moshe et al, 2019)
We demonstrated that spore release and the gametogenesis of male and female gametes was enhanced by wounding the gametophytic thalli of P. yezoensis and that the formation of calli in these tissues could be induced by heat stress
Since wounding promoted gametogenesis (Figure 1C), we hypothesized that the carpospores were produced by the fertilization of the male and female gametes, we previously showed that oxidative stress-dependent diploid apogamy produced carpospores without gametogenesis in P. yezoensis (Takahashi and Mikami, 2017; Mikami, 2019)
Summary
Plants and seaweeds control the timing of their growth and reproduction in response to environmental stresses, trading off these phases to optimize their life cycle progression and best adapt to the variable natural conditions they experience (Liu et al, 2017; Mohring et al, 2013; Karasov et al, 2017; Shaar-Moshe et al, 2019). The Bangiales, including the marine red algal genera Pyropia and Porphyra, undergo a haploiddiploid heteromorphic life cycle, in which the haploid leafy gametophyte (thallus) and the diploid filamentous sporophyte (conchocelis) occur in a mutually exclusive manner (Blouin et al, 2011; Mikami et al, 2012; Takahashi and Mikami, 2017; Adams et al, 2018). Since we have observed that the stimulation of gametogenesis by environmental cues lowers the growth rate of the thalli by reducing the number of vegetative cells, sexual reproduction is thought to represent a life cycle trade-off in gametophytic thalli
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