Combining a modelling with a genetic approach in establishing associations between genetic and physiological effects in relation to phosphorus uptake

The Pup1 locus confers tolerance to phosphorus (P) deficiency in rice (Oryza sativa L.). Transferring the Pup1 locus to an intolerant genotype increased P uptake by a factor 3 to 4. Lines with the Pup1 locus maintained higher root growth rates under P deficiency, but only as they started to diverge from intolerant lines in P uptake. It was thus not possible to determine if differences in root growth preceded and caused differences in P uptake or whether high root growth was the result of higher external P uptake efficiency (P influx per root size). The purpose of this paper is to review experimental evidence on the effect of Pup1 in light of recent results in modelling cause-and-effect relations between root growth, external efficiency and P uptake. Model simulations suggested that only very small changes in factors enhancing root growth were needed to explain the effect of Pup1 on P uptake. A 22% increase in root fineness or in internal P utilization efficiency (root dry matter per root P) was sufficient to triple P uptake . External root efficiency had to increase by 33 to account for the effect of Pup1. However, the most noticeable effect of increases in external efficiency was a subsequent stimulation of root growth that contributed eight times more to final P uptake compared to the change in external efficiency. Comparisons of model simulations with empirical data suggested that measured differences in external efficiency between Nipponbare and NIL-Pup1 were sufficiently large to account for the increase in P uptake. A segregation analysis using several pairs of contrasting NILs (at the Pup1 locus) further confirmed this as Pup1 co-segregated with external efficiency but not with seedling root growth or internal efficiency.