Characterization of low phosphorus insensitive mutants reveals a crosstalk between low phosphorus-induced determinate root development and the activation of genes involved in the adaptation of Arabidopsis to phosphorus deficiency

Abstract

Low phosphorus (P) availability is one of the most limiting factors for plant productivity in many natural and agricultural ecosystems. Plants display a wide range of adaptive responses to cope with low P stress, which generally serve to enhance P availability in the soil and to increase its uptake by roots. In Arabidopsis (Arabidopsis thaliana), primary root growth inhibition and increased lateral root formation have been reported to occur in response to P limitation. To gain knowledge of the genetic mechanisms that regulate root architectural responses to P availability, we designed a screen for identifying Arabidopsis mutants that fail to arrest primary root growth when grown under low P conditions. Eleven low phosphorus insensitive (lpi) mutants that define at least four different complementation groups involved in primary root growth responses to P availability were identified. The lpi mutants do not show the typical determinate developmental program induced by P stress in the primary root. Other root developmental aspects of the low P rescue system, including increased root hair elongation and anthocyanin accumulation, remained unaltered in lpi mutants. In addition to the insensitivity of primary root growth inhibition, when subjected to P deprivation, lpi mutants show a reduced induction in the expression of several genes involved in the P starvation rescue system (PHOSPHATE TRANSPORTER 1 and 2, PURPLE ACID PHOSPHATASE 1, ACID PHOSPHATASE 5, and INDUCED BY PHOSPHATE STARVATION 1). Our results provide genetic support for the role of P as an important signal for postembryonic root development and root meristem maintenance and show a crosstalk in developmental and biochemical responses to P deprivation.

Figure 1.

Genetic screen and phenotypic characterization of lpi mutants. A, Photograph of an agar plate with low P medium showing a putative lpi mutant (arrow). B, Five 14-d-old wild-type (Col-0) and lpi1 seedlings growing side by side on media containing high (left plate) or low (right plate) P. C, Col-0 (WT), lpr1, and lpi2 plants grown in soil.

Figure 2.

Specificity of the lpi phenotype. Wild-type, lpi1, and lpi2 seedlings were grown for 12 d on vertically oriented agar plates of the indicated media composition. Mean values (±se) of primary root length (A), lateral root number (B), and lateral root density (C) are presented (n = 30 seedlings). Different letters indicate that the means differ significantly (P < 0.05).

Figure 3.

The effect of phosphate availability on cell parameters of primary roots. Wild-type, lpi1, and lpi2 seedlings were grown for 12 d on vertically oriented agar plates with high (1 mm P) or low (1 μm) P content. Data are presented for epidermal cell length (A), the number of cells in the elongation zone (B), and the number of cells in the root meristem (C). Values shown represent the means of 15 seedlings ± se.

Figure 4.

CycB1;1:uidA expression in transgenic wild-type and lpi seedlings. Overnight GUS staining of CycB1;1:uidA primary roots in wild-type, lpi1, and lpi2 seedlings grown for 12 d in medium with high (A, C, and E) or low (B, D, and F) P content. Photographs are representative individuals of at least 20 stained plants. Root hair formation on the meristematic zone (B, arrow).

Figure 5.

The effect of P availability on several rescue system responses. Wild-type and lpi seedlings were grown for 12 (A and B) and 18 (C and D) d on the surface of agar plates containing high or low P, and several rescue system responses were evaluated. Mean root hair length (A), root hair density indicating the number of root hairs per cm in the primary root (B), anthocyanin content (C), and root/shoot ratio (D) are shown. Values shown are the means ± se of 20 seedlings (A and B), three repetitions (C), and five groups of 40 seedlings (D). Letters represent statistically different (P < 0.05) means.

Figure 6.

AtPT1:uidA and AtPT2:uidA expression in transgenic wild-type and lpi seedlings. Overnight GUS staining of AtPT1:uidA and AtPT2:uidA root system and its primary root tips in wild-type, lpi1, and lpi2 seedlings grown for 12 d in medium with high (A–D, I–L, and Q–T) or low (E–H, M–P, and U–X) P content. Photographs are representative individuals of at least 20 plants stained.

Figure 7.

Northern analysis of AtPT1, AtPAP1, At4, AtPT2, AtACP5, and AtIPS1 gene expression in the shoots and roots of Arabidopsis. Wild-type, lpi1, and lpi2 seedlings were grown for 14 d on vertically oriented agar plates with high (1 mm P) or low (1 μm) P content. Total RNA was prepared from shoots and roots, separated by electrophoresis, blotted, and hybridized with gene-specific probes (A). The values of normalized signal intensities of shoot (left graphic) and root (right graphic; B). Signal intensity was quantified using ImageJ 1.33u software and normalized to the corresponding tubulin signal.