The chemical compound ‘Heatin’ stimulates hypocotyl elongation and interferes with the Arabidopsis NIT1‐subfamily of nitrilases
Plant J. 2021 Mar 26 Epub ahead of print
Woude LV, Piotrowski M, Klaasse G, Paulus JK, Krahn D, Ninck S, Kaschani F, Kaiser M, Novák O, Ljung K, Bulder S, van Verk M, Snoek BL, Fiers M, Martin NI, van der Hoorn RAL, Robert S, Smeekens S, van Zanten M

Abstract
Temperature passively affects biological processes involved in plant growth. It is therefore challenging to study the dedicated temperature signalling pathways that orchestrate thermomorphogenesis, a suite of elongation growth-based adaptations that enhance leaf cooling capacity. We screened a chemical library for compounds that restored hypocotyl elongation in the pif4-2 deficient mutant background at warm temperature conditions in Arabidopsis thaliana to identify modulators of thermomorphogenesis. The small aromatic compound 'Heatin', containing 1-iminomethyl-2-naphthol as pharmacophore, was selected as enhancer of elongation growth. We show that ARABIDOPSIS ALDEHYDE OXIDASES redundantly contribute to Heatin-mediated hypocotyl elongation. Following a chemical proteomics approach, the members of the NITRILASE1-subfamily of auxin biosynthesis enzymes were identified among the molecular targets of Heatin. Our data reveal that nitrilases are involved in promotion of hypocotyl elongation in response to high temperature and Heatin-mediated hypocotyl elongation requires the NITRILASE1-subfamily members NIT1 and NIT2. Heatin inhibits NIT1-subfamily enzymatic activity in vitro and Heatin application accordingly results in accumulation of NIT1-subfamily substrate indole-3-acetonitrile (IAN) in vivo. However, also levels of the NIT1-subfamily product, bioactive auxin (Indole-3-acetic acid; IAA), were significantly increased. Possibly, Heatin's stimulation of hypocotyl elongation might be independent of its observed interaction with NITRILASE1-subfamily members. But nitrilases may contribute to the Heatin response by stimulating IAA biosynthesis in an indirect way. Heatin and its functional analogues present novel chemical entities for studying auxin biology.

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