387 / 2019-02-19 03:31:51

The brassinosteroid signaling kinase BSK3 modulates root system architectural adaptations under nitrogen deficiency

root system architecture, nitrogen deficiency, brassinosteroids, BSK3, natural variation

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Nutrient availability in soils has a strong impact on root development and morphology. When grown under nutrient-limiting conditions, certain root architectural modifications improve nutrient acquisition. In nitrogen (N)-limiting soils, plant roots adapt by forming longer primary or seminal and lateral roots to better exploit a larger soil volume for nutrients. So far, it has remained unclear how this N foraging response is regulated. In an attempt to understand underlying molecular mechanisms, we exploited the natural variation and phenotyped root lengths Arabidopsis accessions. Using genome-wide association studies, we mapped BSK3, coding for a brassinosteriod (BR) signaling kinase that contributes to natural variation in primary root length under mild N deficiency. Sequence comparisons and subsequent transgenic complementation experiments revealed that a proline to leucine substitution in the kinase domain of BSK3 determines BR sensitivity and root responsiveness to mild N deficiency. Furthermore, we observed that N deficiency, specifically upregulates the expression of the BR co-receptor BAK1 rather than affecting BSK3 gene expression. In accordance, low N-induced root elongation was severely impaired in bak1 plants, indicating that N deficiency activates the BR signaling pathway through the BR co-receptor BAK1 to stimulate root elongation. Taken together, our work uncovers a novel role of brassinosteroid signaling in determining root foraging under mild N deficiency.