SHORT-ROOT stabilizes PHOSPHATE1 to regulate phosphate allocation in Arabidopsis.

The coordinated distribution of inorganic phosphate (Pi) between roots and shoots is an important process that plants use to maintain Pi homeostasis. SHORT-ROOT (SHR) is well characterized for its function in root radial patterning. Here we demonstrate a role of SHR in controlling Pi allocation from root to shoot by regulating PHOSPHATE1 in the root differentiation zone. We recovered a weak mutant allele of SHR in Arabidopsis that accumulates much less Pi in the shoot and shows a constitutive Pi starvation response under Pi-sufficient conditions. In addition, Pi starvation suppresses SHR protein accumulation and releases its inhibition on the HD-ZIP III transcription factor PHB. PHB accumulates and directly binds the promoter of PHOSPHATE2 to upregulate its transcription, resulting in PHOSPHATE1 degradation in the xylem-pole pericycle cells. Our findings reveal a previously unrecognized mechanism of how plants regulate Pi translocation from roots to shoots.

The chromatin remodeler BRAHMA recruits HISTONE DEACETYLASE6 to regulate root growth inhibition in response to phosphate starvation in Arabidopsis.

Plasticity in root system architecture (RSA) allows plants to adapt to changing nutritional status in the soil. Phosphorus availability is a major determinant of crop yield, and RSA remodeling is critical to increasing the efficiency of phosphorus acquisition. Although substantial progress has been made in understanding the signaling mechanism driving phosphate starvation responses in plants, whether and how epigenetic regulatory mechanisms contribute is poorly understood. Here, we report that the Switch defective/sucrose non-fermentable (SWI/SNF) ATPase BRAHMA (BRM) is involved in the local response to phosphate (Pi) starvation. The loss of BRM function induces iron (Fe) accumulation through increased LOW PHOSPHATE ROOT1 (LPR1) and LPR2 expression, reducing primary root length under Pi deficiency. We also demonstrate that BRM recruits the histone deacetylase (HDA) complex HDA6-HDC1 to facilitate histone H3 deacetylation at LPR loci, thereby negatively regulating local Pi deficiency responses. BRM is degraded under Pi deficiency conditions through the 26 S proteasome pathway, leading to increased histone H3 acetylation at the LPR loci. Collectively, our data suggest that the chromatin remodeler BRM, in concert with HDA6, negatively regulates Fe-dependent local Pi starvation responses by transcriptionally repressing the RSA-related genes LPR1 and LPR2 in Arabidopsis thaliana.

The involvement of abscisic acid-insensitive mutants in low phosphate stress responses during rhizosphere acidification, anthocyanin accumulation and Pi homeostasis in Arabidopsis.

Phosphorus is an essential plant nutrient, used in the formation of macromolecules such as nucleic acids and phospholipids. Abscisic acid (ABA) may be involved in the process of low inorganic phosphate (Pi) responses. The phenotypes of ABA-insensitive Arabidopsis mutants (abi1/2/3/4/5) under low Pi stress were investigated to identify possible low Pi response mutant genes. The results showed enhanced rhizosphere acidification in the abi1-1/abi2-1/abi5-1 mutants under low Pi stress compared with wild-type (WT) seedlings. The abi1-1/abi2-1/ abi3-1/abi5-1 mutants accumulated less anthocyanin than the WT, while the abi4-1 mutant showed greater accumulation, implicating all the ABA-insensitive mutants in anthocyanin deposition under Pi deficiency. Alterations in the Pi contents of roots or shoots were also observed in the mutants in response to both Pi sufficiency and deficiency, indicating that the mutants were involved in Pi uptake or transportation. The primary root length and root-shoot ratio of abi3-1 and abi4-1 mutants decreased compared with WT seedlings under low Pi condition. Further research showed that ABI5 could regulate PHT1;5 and WRKY42 expression by combining with ACGT cis-acting elements of the PHT1;5 and WRKY42 promoters.

Inositol Pyrophosphate InsP8 Acts as an Intracellular Phosphate Signal in Arabidopsis.

The maintenance of cellular phosphate (Pi) homeostasis is of great importance in living organisms. The SPX domain-containing protein 1 (SPX1) proteins from both Arabidopsis and rice have been proposed to act as sensors of Pi status. The molecular signal indicating the cellular Pi status and regulating Pi homeostasis in plants, however, remains to be identified, as Pi itself does not bind to the SPX domain. Here, we report the identification of the inositol pyrophosphate InsP8 as a signaling molecule that regulates Pi homeostasis in Arabidopsis. Polyacrylamide gel electrophoresis profiling of InsPs revealed that InsP8 level positively correlates with cellular Pi concentration. We demonstrated that the homologs of diphosphoinositol pentakisphosphate kinase (PPIP5K), VIH1 and VIH2, function redundantly to synthesize InsP8, and that the vih1 vih2 double mutant overaccumulates Pi. SPX1 directly interacts with PHR1, the central regulator of Pi starvation responses, to inhibit its function under Pi-replete conditions. However, this interaction is compromised in the vih1 vih2 double mutant, resulting in the constitutive induction of Pi starvation-induced genes, indicating that plant cells cannot sense cellular Pi status without InsP8. Furthermore, we showed that InsP8 could directly bind to the SPX domain of SPX1 and is essential for the interaction between SPX1 and PHR1. Collectively, our study suggests that InsP8 is the intracellular Pi signaling molecule serving as the ligand of SPX1 for controlling Pi homeostasis in plants.

miR156 modulates rhizosphere acidification in response to phosphate limitation in Arabidopsis.

Rhizosphere acidification is a general response to Pi deficiency, especially in dicotyledonous plants. However, the signaling pathway underlying this process is still unclear. Here, we demonstrate that miR156 is induced in the shoots and roots of wild type Arabidopsis plants during Pi starvation. The rhizosphere acidification capacity was increased in 35S:MIR156 (miR156 overexpression) plants, but was completely inhibited in 35S:MIM156 (target mimicry) plants. Both 35S:MIR156 and 35S:MIM156 plants showed altered proton efflux and H(+)-ATPase activity. In addition, significant up-regulation of H(+)-ATPase activity in 35S:MIR156 roots coupled with increased citric acid and malic acid exudates was observed. qRT-PCR results showed that most H(+)-ATPase and PPCK gene transcript levels were decreased in 35S:MIM156 plants, which may account for the decreased H(+)-ATPase activity in 35S:MIM156 plants. MiR156 also affect the root architecture system. Collectively, our results suggest that miR156 regulates the process of rhizosphere acidification in plants.

Modulation of the Phosphate-Deficient Responses by MicroRNA156 and its Targeted SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 3 in Arabidopsis.

The microRNA156 (miR156)-modulated SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) is involved in diverse biological processes that include growth, development and metabolism. Here, we report that the Arabidopsis miR156 and SPL3 as regulators play important roles in phosphate (Pi) deficiency response. MiR156 was induced during Pi starvation whereas SPL3 expression was repressed. Phenotypes of reduced rhizosphere acidification and decreased anthocyanin accumulation were observed in 35S:MIM156 (via target mimicry) transgenic plants under Pi deficiency. The content and uptake of Pi in 35S:MIM156 Arabidopsis plants were increased compared with wild-type (Col-0 ecotype) plants. 35S:rSPL3 seedlings showed similar anthocyanin accumulation and Pi content phenotypes to those of 35S:MIM156 plants. Chromatin immunoprecipitation and an electrophoretic mobility shift assay indicated that the SPL3 protein directly bound to GTAC motifs in the PLDZ2, Pht1;5 and miR399f promoters. The expression of several Pi starvation-induced genes was increased in 35S:MIM156 and 35S:rSPL3 plants, including high-affinity Pi transporters, Mt4/TPS1-like genes and phosphatases. Collectively, our results suggest that the miR156-SPL3-Pht1;5 (-PLDZ2 and -miR399f) pathways constitute a component of the Pi deficiency-induced regulatory mechanism of Arabidopsis.

[NO may function in the downstream of H2O2 in ABA-induced stomatal closure in Vicia faba L].

It is usually suggested that either H(2)O(2) or NO function as a signal molecule in mediating the ABA-induced stomatal closure of guard cells, but there has been no report on the relationship between H(2)O(2) and NO in ABA signal transduction pathway. Here, using stomatal analysis and laser scanning cofocal microscope techniques, we show firstly that NO functions as a downstream intermediate of H(2)O(2) signaling to mediate ABA-induced stomatal closure in Vicia faba L. Sodium nitroprusside (SNP, a NO donor) and H(2)O(2) can mimic the effects of ABA on stomatal closure. Carboxy-PTIO (c-PTIO, a specific scavenger of NO) partly reverse the stomatal closure induced by ABA or H(2)O(2), while catalase (CAT), a H(2)O(2) scavenger, failed to reverse the NO-induced aperture reduction in Vicia faba guard cells. Monitoring the changes in both NO and H(2)O(2) generation in guard cells by using fluorescent probe of NO or H(2)O(2), DAF-2DA or H2DCFDA, respectively, we found that the generating rate of H(2)O(2) in guard cells was faster than that of NO after being treated with ABA 10 micromol/L. CAT almost completely inhibited the increase in DAF fluorescence induced by ABA. Similar to ABA, exogenous H(2)O(2) provoked the production of NO. c-PTIO slightly enhanced the fluorescent intensity of DCF stimulated by ABA, while exogenous SNP did not increase DCF fluorescence in guard cells. Taken together, these results suggest that H(2)O(2) could probably act as upstream component of NO signaling and NO negatively regulate H(2)O(2) generation during ABA-induced stomatal closure in guard cells.