UV-B Irradiation Results in Inhibition of Hypocotyl Elongation, Cell Cycle Arrest, and Decreased Endoreduplication Mediated by miR5642

Phytochrome-interacting factor PIF3 integrates phytochrome B and UV-B signaling pathways to regulate gibberellin- and auxin-dependent growth in cucumber hypocotyls

In Arabidopsis, the photoreceptors phytochrome B (PhyB) and UV-B resistance 8 (UVR8) mediate light responses that play a major role in regulating photomorphogenic hypocotyl growth, but how they crosstalk to coordinate this process is not well understood. Here we report map-based cloning and functional characterization of an ultraviolet (UV)-B-insensitive, long-hypocotyl mutant, lh1, and a wild-type-like mutant, lh2, in cucumber (Cucumis sativus), which show defective CsPhyB and GA oxidase2 (CsGA20ox-2), a key gibberellic acid (GA) biosynthesis enzyme, respectively. The lh2 mutation was epistatic to lh1 and partly suppressed the long-hypocotyl phenotype in the lh1lh2 double mutant. We identified phytochrome interacting factor (PIF) CsPIF3 as playing a critical role in integrating the red/far-red and UV-B light responses for hypocotyl growth. We show that two modules, CsPhyB-CsPIF3-CsGA20ox-2-DELLA and CsPIF3-auxin response factor 18 (CsARF18), mediate CsPhyB-regulated hypocotyl elongation through GA and auxin pathways, respectively, in which CsPIF3 binds to the G/E-box motifs in the promoters of CsGA20ox-2 and CsARF18 to regulate their expression. We also identified a new physical interaction between CsPIF3 and CsUVR8 mediating CsPhyB-dependent, UV-B-induced hypocotyl growth inhibition. Our work suggests that hypocotyl growth in cucumber involves a complex interplay of multiple photoreceptor- and phytohormone-mediated signaling pathways that show both conservation with and divergence from those in Arabidopsis.

Ultraviolet-B Radiation Represses Primary Root Elongation by Inhibiting Cell Proliferation in the Meristematic Zone of Arabidopsis Seedlings

In Arabidopsis thaliana plants, exposure to UV-B induces an inhibition of primary root elongation. Different mutants have been isolated that are deficient in this response; however, little is known about the cellular and molecular mechanisms that regulate inhibition of root elongation in seedlings exposed to UV-B. In this work, we investigated the effect UV-B irradiation of different organs on primary root elongation. Our results demonstrate that irradiation of the leaves and shoots only induce a partial inhibition of primary root elongation, while when only roots are exposed to this radiation, primary root inhibition is similar as that measured when the complete seedling is irradiated. The consequences of exposure at different root developmental stages and times after the end of the treatment was also studied. We here show that inhibition of primary root elongation is a consequence of a decrease in cell proliferation in the meristematic zone of the primary roots, while the elongation zone size is not affected by the treatment. The decrease in cell number after UV-B exposure is partially compensated by an increase in cell length in the root meristem; however, this compensation is not enough to maintain the meristem size. We also here demonstrate that, similarly as what occurs in developing leaves, GROWTH REGULATING FACTOR 3 (GRF3) transcription factor regulates cell proliferation in UV-B irradiated roots; however, and in contrast to what occurs in the leaves, this response does not depend on the presence of MITOGEN ACTIVATED PROTEIN KINASE 3 (MPK3). Inhibition of primary root elongation by UV-B under our experimental conditions is also independent of the UV-B photoreceptor UV RESISTANT LOCUS 8 (UVR8) or ATAXIA TELANGIECTASIA MUTATED (ATM); but a deficiency in ATM AND RAD3-RELATED (ATR) expression increases UV-B sensitivity in the roots. Finally, our data demonstrate that UV-B affects primary root growth in various Arabidopsis accessions, showing different sensitivities to this radiation.