Auxin‐dependent regulation of cell division rates governs root thermomorphogenesis

Roots are highly plastic organs enabling plants to adapt to a changing below‐ground environment. In addition to abiotic factors like nutrients or mechanical resistance, plant roots also respond to temperature variation. Below the heat stress threshold, Arabidopsis thaliana seedlings react to elevated temperature by promoting primary root growth, possibly to reach deeper soil regions with potentially better water saturation. While above‐ground thermomorphogenesis is enabled by thermo‐sensitive cell elongation, it was unknown how temperature modulates root growth. We here show that roots are able to sense and respond to elevated temperature independently of shoot‐derived signals. This response is mediated by a yet unknown root thermosensor that employs auxin as a messenger to relay temperature signals to the cell cycle. Growth promotion is achieved primarily by increasing cell division rates in the root apical meristem, depending on de novo local auxin biosynthesis and temperature‐sensitive organization of the polar auxin transport system. Hence, the primary cellular target of elevated ambient temperature differs fundamentally between root and shoot tissues, while the messenger auxin remains the same. Synopsis Plant root growth is influenced by temperature variation. This article shows that elevated temperatures directly promote primary root growth through an auxin‐mediated response that increases cell division rates in the root apical meristem. Plant roots respond to warmer temperatures by promoting primary root growth. Roots are able to sense and respond to elevated temperature independently of shoot‐derived signals through an as yet unknown root thermosensor. Growth promotion in roots is primarily achieved by increasing cell division rates in the root apical meristem. Auxin mediates a temperature‐dependent increase in cell division rate depending on de novo local auxin biosynthesis and temperature‐sensitive organization of the polar auxin transport system. Graphical Abstract A direct temperature‐sensing mechanism in the root acts on cell proliferation in the root apical meristem to control root growth.