The Arabidopsis downy mildew resistance gene RPP5 shares similarity to the toll and interleukin-1 receptors with N and L6.

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Abstract
Absorption microscopy is a promising alternative to fluorescence microscopy for single-molecule imaging. So far, molecular absorption has been probed optically via the attenuation of a probing laser or via photothermal effects. The sensitivity of optical probing is not only restricted by background scattering but it is fundamentally limited by laser shot noise, which minimizes the achievable single-molecule signal-to-noise ratio. Here, we present nanomechanical photothermal microscopy, which overcomes the scattering and shot-noise limit by detecting the photothermal heating of the sample directly with a temperature-sensitive substrate. We use nanomechanical silicon nitride drums, whose resonant frequency detunes with local heating. Individual Au nanoparticles with diameters from 10 to 200 nm and single molecules (Atto 633) are scanned with a heating laser with a peak irradiance of 354 ± 45 µW/µm2 using 50× long-working-distance objective. With a stress-optimized drum we reach a sensitivity of 16 fW/Hz1/2 at room temperature, resulting in a single-molecule signal-to-noise ratio of >70. The high sensitivity combined with the inherent wavelength independence of the nanomechanical sensor presents a competitive alternative to established tools for the analysis and localization of nonfluorescent single molecules and nanoparticles. Significance Absorption microscopy is a promising technique that can detect single nonfluorescent molecules. However, fundamental limitations of existing optical absorption methods result in noisy detection signals for single molecules, which has hindered many anticipated applications. A promising method is to optically measure the photothermal heating of single molecules. In this paper, we present a photothermal microscopy technique where we detect the photothermal heating of single molecules mechanically with a temperature-sensitive nanomechanical drum. With our method, we achieve an unprecedented optical absorption sensitivity, enabling the detection of single molecules with large signal-to-noise ratios. This enables interesting applications such as the accurate localization of naturally occurring marker molecules or the identification of single molecules by measuring their absorption spectrum.