Abstract
Phototropins are light-activated protein kinases, which contribute to photosynthesis optimization both through enhancement of photon absorption when light is limiting and avoidance responses in high light. This duality is in part endowed by the presence of phototropins with different photosensitivity (phot1 and phot2). Here we show that phot1, which senses low light to promote positive phototropism (growth towards the light), also limits the response in high light. This response depends in part on phot1-mediated phosphorylation of Phytochrome Kinase Substrate 4 (PKS4). This light-regulated phosphorylation switch changes PKS4 from a phototropism enhancer in low light to a factor limiting the process in high light. In such conditions phot1 and PKS4 phosphorylation prevent phototropic responses to shallow light gradients and limit phototropism in a natural high light environment. Hence, by modifying PKS4 activity in high light the phot1-PKS4 regulon enables appropriate physiological adaptations over a range of light intensities.
Highlights
Phototropins are light-activated protein kinases, which contribute to photosynthesis optimization both through enhancement of photon absorption when light is limiting and avoidance responses in high light
Phytochrome Kinase Substrate 4 (PKS4) exists in two isoforms that can be distinguished by their mobility on SDS-PAGE gels: the faster migrating PKS4D and PKS4L a light-induced phot1-mediated slower migrating phosphorylated isoform[19]
We found that plants expressing S299A failed to accumulate PKS4L in the light (Fig. 1c)
Summary
Phototropins are light-activated protein kinases, which contribute to photosynthesis optimization both through enhancement of photon absorption when light is limiting and avoidance responses in high light. We show that phot[1], which senses low light to promote positive phototropism (growth towards the light), limits the response in high light This response depends in part on phot1-mediated phosphorylation of Phytochrome Kinase Substrate 4 (PKS4). This light-regulated phosphorylation switch changes PKS4 from a phototropism enhancer in low light to a factor limiting the process in high light. Phot[1] controls phototropism towards low fluence rates while at higher light intensity both phototropins mediate the response[7] This physiological adaptation allows optimal organ positioning through directional growth to maximize photon capture by leaves when light is limiting[8]. Phot[1] and PKS4, two elements that are essential to respond to low light, have a dual role and limit the response to high light
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