Low fluence UV-B as a positive regulator of photosynthesis in Arabidopsis thaliana : a thesis presented in partial fulfilment of the requirements for the degree of Doctor in Philosophy in Agriculture and Horticulture at Massey University, Palmerston North, New Zealand

Abstract

UV-B radiation can induce a wide range of developmental responses in plants, and magnitudes of UV-B exposure can also vary greatly. Historically, research into the effects of UV-B radiation on photosynthetic processes has often utilised high fluence rates of UV-B, which have been frequently shown to impede photosynthetic performance and induce photosystem damage. More recently, a number of studies have focused on the impact of low fluence UV-B exposure, and have found that such treatments can be beneficial to photosynthesis by upregulating photosynthetic performance. The aim of this PhD was to understand the consequences of low fluence UV-B exposure on net photosynthetic rate and underlying mechanistic responses. We characterised the photosynthetic response to 0.5 μmol m⁻² s⁻¹ of UV-B and established that net photosynthetic rate increased by 12% in wild type Arabidopsis plants at 24hrs of UV-B exposure. Through analysis of knockout lines for the UV-B photoreceptor UVR8, we determined that the photosynthesis phenotype is dependent on the presence of UVR8. To determine how low fluence UV-B exposure mediates the increase in photosynthetic rate, transcriptomic analysis via RNA-seq was undertaken. Our analysis showed that UV-B exposure results in the upregulation of photosynthesis-associated genes during the initial exposure period. The most highly upregulated genes were related to chloroplast biogenesis and synthesis of photosynthetic proteins within the chloroplast, as well as chloroplastic oxidoreductase activity. We further investigated three of these candidates: RBF1, TOC33 and TFP, and found that each of those genes plays a role in the UV-B mediated increase of photosynthetic rate at 24hrs and that the upregulation of these genes in response to UV-B exposure is regulated by UVR8. Taken together, we describe here for the first time, that low fluence UV-B increases net photosynthetic rate through UVR8-mediated upregulation of key genes, resulting in increased synthesis of chloroplastic photosynthesis-associated proteins and chloroplastic oxidoreductase activity. This further extends our knowledge of UV-B plant-response and offers further potential for exploitation of UV-B photomorphogenesis in agriculture.