Differential UVR8 signal across the stem controls UV-B-induced inflorescence phototropism

Vanhaelewyn, Lucas and Viczián, András and Prinsen, Els and Bernula, Péter and Serrano, Alejandro Miguel and Nagy, Ferenc (2019) Differential UVR8 signal across the stem controls UV-B-induced inflorescence phototropism. PLANT CELL, 31. pp. 2070-2088. ISSN 1040-4651 (print); 1532-298X (online)

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Abstract

In the course of evolution, plants have developed mechanisms to orient their organs towards the incoming light. At the seedling stage, positive phototropism is mainly regulated by phototropin photoreceptors in blue and ultraviolet wavelengths. Contrasting with this, we report that UV RESISTANCE LOCUS 8 (UVR8) serves as predominant photoreceptor of UV-B-induced phototropic responses in inflorescence stems. We examined the molecular mechanisms behind this response and our findings support the Blaauw theory (1919), suggesting rapid differential growth through unilateral photomorphogenic growth inhibition. UVR8-dependent UV-B perception occurs mainly in the epidermis and cortex, but deeper tissues such as endodermis can also contribute. Within stems, a spatial difference of UVR8 signal causes transcript and protein increase of transcription factors ELONGATED HYPOCOTYL 5 (HY5) and its homologue HYH at the UV-B-exposed side. The irradiated side shows 1) strong activation of flavonoid synthesis genes and flavonoid accumulation, 2) Increased GA2oxidase expression, diminished gibberellin1 levels and accumulation of DELLA protein REPRESSOR OF GA1 (RGA) and, 3) increased expression of the auxin transport regulator, PINOID, contributing to diminished auxin signalling. Together the data provide a mechanism of phototropin-independent inflorescence phototropism through multiple, locally UVR8-regulated hormone pathways.

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