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Priming is the process through which periodic elevations in auxin signalling prepattern future sites for lateral root formation, called prebranch sites. Thusfar is has remained a matter of debate to what extent elevations in auxin concentration and/or auxin signalling are critical for priming and prebranch site formation. Recently, we discovered a reflux-and-growth mechanism for priming generating periodic elevations in auxin concentration that subsequently dissipate. Here we reverse engineer a mechanism for prebranch site formation that translates these transient elevations into a persistent increase in auxin signalling, resolving the prior debate into a two-step process of auxin concentration mediated initial signal and auxin signalling capacity mediated memorization. A critical aspect of the prebranch site formation mechanism is its activation in response to time integrated rather than instantaneous auxin signalling. The proposed mechanism is demonstrated to be consistent with prebranch site auxin signalling dynamics, lateral inhibition and symmetry breaking mechanisms and perturbations in auxin homeostasis.

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A plants' fitness to a large extent depends on its capacity to adapt to spatio-temporally varying environmental conditions. One such environmental condition to which plants display extensive phenotypic plasticity is soil nitrate levels and patterns. In response to heterogeneous nitrate distribution, plants show a preferential foraging response, enhancing root growth in high nitrate patches and repressing root growth in low nitrate locations beyond a level that can be explained from local nitrate sensing. Although various molecular players involved in this preferential foraging behavior have been identified, how these together shape root system adaptation has remained unresolved. Here we use a simple modeling approach in which we incrementally incorporate the various known molecular pathways to investigate the combination of regulatory mechanisms that underly preferential root nitrate foraging. Our model suggests that instead of a thus far not discovered growth suppressing supply signal, growth reduction on the low nitrate side may simply arise from a reduced root foraging and increased competition for carbon. Additionally, our work suggests that the long distance CK signaling involved in root growth increase in high nitrate patches may represent a supply signal specifically functioning in modulating demand signaling strength. We illustrate how this integration of demand and supply signals prevents excessive preferential foraging under conditions in which demand is not met by sufficient supply and a more generic foraging in search of nitrate should be maintained.

No abstract