UV‐B Irradiation Results in Changes in Elongation, Cell Cycle, and Decreased Endoreduplication Mediated by miR5642

UV-B as a component of natural solar radiation can induce damage as well as morphological development in plants. The UV-B response from germination and early development in seedlings is still largely unknown, with most studies focused on older, light-exposed seedlings. We used fluence response curves measuring hypocotyl length after UV-B exposure coupled with RNA-seq and sRNA-seq evaluation of the early seedling response in the model organism Arabidopsis thaliana. We identified miR5642 as a potential novel key regulator of UV-B responses. MiR5642 is a non-canonical miRNA predicted to target previously known and unknown components involved in hypocotyl growth inhibition. These include (i) SMAX1, a signal transmitter for seedling germination and growth, (ii) ZAT1, an uncharacterized transcription factor, and (iii) membrane pores and transporters (VHA-E1, VHA-E3, EPSIN-LIKE, PIP1.4) implicated in cell elongation. In addition, HY5 and HYH, two homologous and redundant transcription factors involved in seedling photomorphogenesis, may interact with these newly identified components. Interestingly, UV-B-induced DNA photodimer formation seems to be the direct trigger leading to inhibition of hypocotyl growth through a combination of cellular decisions including cell cycle arrest, reduced endoreduplication, and reduced cell elongation, and this inhibition appears to be modulated by miR5642 target genes.

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