Abstract
Phototropism is a response to the direction of light that guides growth orientation and determines the shape of plants to optimize photosynthetic activity. The phototropic response is present not only in terrestrial plants but also in water-living algae. However, knowledge about phototropism in Bangiophycean seaweeds is limited. Here, we examined the phototropic response of the red alga Pyropia yezoensis to elucidate the regulatory mechanism of phototropism in Bangiophyceae. When leafy gametophytes and filamentous sporophytes of P. yezoensis were cultured under directional light, phototropism was observed in the gametophytes. Conchosporangia on the sporophytes also exhibited phototropism. Phototropism was positive in the majority of gametophytes and conchosporangia but in some cases was negative. In addition, a strong phototropic response occurred under white light, whereas blue and red light elicited minor and no responses, respectively. This observation is in contrast with the phototropic response in terrestrial plants and several algae, in which blue light is responsible for positive phototropism. Surprisingly, the genome of P. yezoensis has no homologues of the photoreceptors for blue and red light, revealing differences in the regulation of phototropism between terrestrial plants and P. yezoensis . Studies on the phototropism in P. yezoensis could shed light on the evolutional divergence of phototropic responses in plants.
Highlights
Phototropism is defined as the response of plants to directional light that directs growth orientation to optimize photosynthetic activity and energy production [1] [2]
Because changing the direction of light and exposure to blue or red light did not cause phototropic growth, we concluded that P. yezoensis sporophytes lack the capacity for phototropic response
When exposed to blue light from the top, positive and negative responses were observed in 39.6% and 27.3% of conchosporangia respectively, whereas exposure to unilateral red light did not cause a phototropic response (Table 1 and Figure 3(a))
Summary
Phototropism is defined as the response of plants to directional light that directs growth orientation to optimize photosynthetic activity and energy production [1] [2]. Mikami discovery of the pivotal role of auxin in phototropism [3] [4], the Cholodny-Went hypothesis has been generally accepted. This hypothesis posits the lateral movement of auxin from the illuminated to the shaded side where it promotes cell elongation and curvature of the coleoptile towards the light. Photoperception and activation of phototropins have been extensively analyzed, and several phototropinsignaling components such as nonphototropic hypocotyl 3 (NPH3) and phytochrome kinase substrate 1 (PKS1) have been identified [12] [13]. All classes of photoreceptors known in A. thaliana play a role in the early phase of phototropism
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