Physiological And Genetic Characterization Of A Novel Mutant With Low Phosphorus Insensitivity In Arabidopsis

Phosphate (Pi), as an essential nutrient, plays an important role in plant growth and development. Because of the bioavailability of Pi is extremely low in many ecosystems, plants have evolved a wide range of strategies adaptive to low Pi stress. Alteration of root architecture is one of the adaptive traits to increase the ability of plants to efficiently explore and exploit the insoluble phosphorus. The physiological and genetic characterizations of the alteration of plant root architecture under Pi-starvation were extensively reported in Arabidopsis, while few of mutants insensitive to Pi-starvation have been isolated. In this study, we isolated an Arabidopsis mutant with insensitive to Pi-starvation, designated as psi-1 (Pi-starvation insensitive-1). The physiological and genetic characterizations of the mutant were investigated. The results are summarized as follows:1. psi-1 mutant showed reduced sensitivity to low Pi (15μM Pi) stress compared to the wild type (WT). The typical traits in response to Pi-starvation were not found in psi-1 mutant, including inhibition of primary root cell division and elongation and anthocyanin accumulation.2. The analysis of inorganic phosphate (Pi) and total P concentrations indicates that the psi-1 mutant has a significantly higher Pi uptake and total P accumulation in shoots under the low Pi condition compared with the WT.3. The expressions of several Pi-starvation-induced genes in Arabidopsis were tested. The result indicates that Pi-starvation signaling was systemically reduced in psi-1 mutant.4. psi-1 mutant seedlings could sustain the ability of auxin transport to the primary root tip under low Pi stress, while the sensitivity to exogenous auxin of psi-1 mutant was similar with the WT, indicating that the auxin signaling pathway may not changed in psi-1 mutant. A further experiment using WOX5::IAAH transgenic line showed that the ability of psi-1 mutant to sustain the higher auxin concentration in root tip than that in the WT may not be causable to sustain the long-root phenotype under low Pi stress.5. Under low Pi level, the primary root length of the F1 progenies from the backcross between psi-1 mutant and accession Columbia (Col-0), and from the crosses between psi-1 and accessions Shahdara (Sha) and Wassilevskaja (WS) which are extremely sensitive to low Pi stress showed the average of the two parents, indicating that PSI-1 is a partial dominant gene. The segregation among F₂ populations derived from the 3 crosses showed continual two-peak distributions with a larger portion of short and middle length lines, suggesting that long-root phenotype of psi-1 mutant is controlled by a single semi-dominant Mendelian factor.6. By using an F₂ population derived from the cross between the psi-1 mutant and Sha accession, two QTLs were detected at LPR1 and LPR2 loci associated with the response to low Pi stress on chromosome 1 and 3, respectively. By using an F₂ population derived from the cross between the psi-1 mutant and accessions Sha and WS, the primary mapping showed that PSI-1 is potentially allelic to LPR2 locus. The result suggests that the effect of the gene at LPR1 from Bay-0 for long root phenotype is conserved in Col ecotype, the natural variation in the allele at LPR2 may be crucial for the response to Pi-starvation in Arabidopsis, while the function of LPR1 is dependent on the function of PSI-1 at LPR2 locus.7. Genetic analysis showed that PSI-1 potentially interacted with SIZ1.