Ultraviolet‐A1 radiation induced a more favorable light‐intercepting leaf‐area display than blue light and promoted plant growth

Plants adjust their morphology in response to light environment by sensing an array of light cues. Though the wavelengths of ultraviolet-A1 radiation (UV-A1, 350-400 nm) are close to blue light (B, 400-500 nm) and share same flavoprotein photoreceptors, it remains poorly understood how plant responses to UV-A1 radiation could differ from those to B. We initially grown tomato plants under monochromatic red light (R, 660 nm) as control, subsequently transferred them to four dichromatic light treatments containing ~20 µmol m-2 s-1 of UV-A1 radiation, peaking at 370 nm (UV-A370 ) or 400 nm (V400 ), or B (450 nm, at ~20 or 1.5 µmol m-2 s-1 ), with same total photon irradiance (~200 μmol m-2 s-1 ). We show that UV-A370 radiation was the most effective in inducing light-intercepting leaf-area display formation, resulting in larger leaf area and more shoot biomass, while it triggered weaker and later transcriptome-wide responses than B. Mechanistically, UV-A370 -promoted leaf-area display response was apparent in less than 12 h and appeared as very weakly related to transcriptome level regulation, which likely depended on the auxin transportation and cell wall acidification. This study revealed wavelength-specific responses within UV-A/blue region challenging usual assumptions that the role of UV-A1 radiation function similarly as blue light in mediating plant processes.