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Final Report Summary - EMPHOS (Dissecting the regulation of lateral root formation by phosphate)

The degree of root branching determines the efficiency of acquisition of nutrients by plants. Phosphate is an essential nutrient that is present in soils at low concentration with poor mobility and therefore represents a limiting factor for plant growth. The exponential demand of phosphate as a result of world population increase (50% by 2050) will lead to a world shortage of phosphate in a near future. Moreover, the increase in phosphate feeding for crop production has dramatic effects on the environment, polluting groundwater and triggering eutrophication, which perturbs ecosystems. Recent work has demonstrated that part of growth alteration affecting root system under phosphate starvation did not result from metabolic limitation but was triggered by signalling pathways. Understanding such mechanisms is of fundamental interest and could be used in the future to engineer crops more adapted to low-phosphate soils.
Since phosphate is an immobile resource, root system architecture adaptation in response to phosphate concentration is critical for the fitness of the plant (Fitter et al., 2002). As a result of phosphate deprivation, primary root growth is repressed and lateral root formation and growth is enhanced as shown in Fig. 1 (Williamson et al., 2001). This response allows the root system to explore the upper layer of the soil where phosphate is more abundant, a mechanism described as "topsoil foraging" (Lynch and Brown, 2001).
The aim of the EmPhos project was to understand the molecular mechanisms of the induction lateral root formation by phosphate deficiency. This project covered the costs of lab research consumables and travel expense. The main researcher’s salary was funded thanks to an EMBO “Long-Term Fellowship” (Dr. Benjamin PERET). The results presented here were used as a basis to obtain a follow-up 3-year grant from the French funding body Agence Nationale de la Recherche (ANR) in the “Retour Post-Doctorant 2011” scheme.

I. Uncoupling primary root inhibition and lateral root induction during root responses to phosphate deficiency

It has been suggested that primary root inhibition triggers a remodelling of auxin fluxes that would be the cause of lateral root induction during phosphate starvation. Using several complementary approaches, we demonstrated that this is not the case.
a) Primary and lateral root responses respond differently to the environment
It has been demonstrated that iron is necessary to trigger primary root growth inhibition. We investigated whether it is the case for lateral root induction. Interestingly, we found that whether iron is present or not in the medium lateral root induction is still observed. However, at low iron concentration, primary root growth is no longer inhibited. Therefore, we identified that at low iron conditions, phosphate deficiency still induces LR formation even though there is no primary root growth arrest.
The observation that primary and lateral root response can be uncoupled demonstrates that these two developmental mechanisms are, at least partly, distinct.
b) Different sites of phosphate perception
We previously demonstrated that plants can perceive phosphate concentration in the environment (rather than its own internal phosphate concentration). We applied a drop of a highly concentrated phosphate solution on the plant leaf and we demonstrated that this was restoring the plant internal phosphate content even if the plant grew on a phosphate-deprived medium. In this condition where plants have high internal phosphate but are situated in a phosphate-deprived medium, they still exhibit a primary root arrest. This demonstrate that plant perceive the medium Pi concentration.
However, a similar experimental setup gave opposite results in term of lateral root induction. Plants containing high phosphate levels behaved the same whether the medium was containing Pi or not (i.e. no lateral root induction).
This result identifies two different compartments for phosphate perception during primary root and lateral root response. Therefore this confirms that these two developmental responses are distinct.

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