Sensing and Signaling of Phosphate Starvation: From Local to Long Distance

Phosphorus (P) is an essential nutrient, but low concentrations of phosphate (Pi), the predominant form in which it is acquired, in the soil often limits plant growth and reproduction. To adapt to low Pi availability, plants have developed intricate regulatory mechanisms that integrate the environmental stimuli with internal cues in order to exploit the use of P. These mechanisms include sensing external and internal Pi concentrations along with co-ordination between local and long-distance signaling pathways. The downstream actions governed by these signaling pathways include local responses for remodeling the root system architecture and systemic responses for modulating the activities of Pi uptake, remobilization and recycling. As an initially acquired molecule, Pi is considered to be a primary signal that directly regulates Pi starvation responses and sets in motion the generation of subsequent signals, such as hormones, sugars, P-containing metabolites, peptides and mobile RNAs. In this review, we summarize recent progress in understanding the regulatory pathways mediated by these signaling molecules that underlie both local and systemic responses to Pi deprivation, and discuss the potential cross-talk among these signaling pathways.

The Impact of Phosphorus on Plant Immunity

Phosphorus (P) is the second most essential macronutrient in terms of limiting plant growth. The genes involved in P acquisition, transport, storage, utilization and respective regulation have been extensively studied. In addition, significant attention has been given to the crosstalk between P and other environmental stresses. In this review, we summarize recent discoveries pertaining to the emerging function of P in plant immunity. The roles of external soil P availability, internal cellular P in plants, P starvation signaling machinery and phosphate transporters in biotic interactions are discussed. We also highlight the impact of several phytohormones on the signaling convergence between cellular P and immune responses. This information may serve as a foundation for dissecting the molecular interaction between nutrient responses and plant immunity.

Identification, structure analysis, and transcript profiling of purple acid phosphatases under Pi deficiency in tomato (Solanum lycopersicum L.) and its wild relatives

Purple acid phosphatases (PAPs), a family of metallo-phosphoesterase enzymes, are involved in phosphorus nutrition in plants. In this study, we report that the tomato genome encodes 25 PAP members. Physio-biochemical analyses revealed relatively lower total root-associated acid phosphatase activity in the seedlings of Solanum pimpinellifolium than their cultivated tomato seedlings under Pi deficiency. Scrutiny of their transcript abundance shows that most of PAPs are activated, although to varying levels, under Pi deficiency in tomato. Further investigation demonstrates that the magnitude of induction of phosphate starvation inducible root-associated PAP homologs remains lower in the Pi-starved S. pimpinellifolium seedlings, hence, accounting for the lower acid phosphatase activity in this wild relative. Examination of their amino acid sequences revealed significant variation in their substrate-specificity defining residues. Among all members, only SlPAP15 possesses the critical lysine residue (R337) and atypical REKA motif in its C-terminal region. Homology modeling and docking studies revealed that ADP and ATP are preferred substrates of SlPAP15. We also identified other amino acid residues present in the vicinity of the active site, possibly facilitating such physical interactions. Altogether, the results presented here will help in the functional characterization of these genes in the tomato in the future.

BOTRYTIS-INDUCED KINASE1, a plasma membrane-localized receptor-like protein kinase, is a negative regulator of phosphate homeostasis in Arabidopsis thaliana

BACKGROUND

Plants have evolved complex coordinated regulatory networks to cope with deficiency of phosphate (Pi) in their growth environment; however, the detailed molecular mechanisms that regulate Pi sensing and signaling pathways are not fully understood yet. We report here that the involvement of Arabidopsis BIK1, a plasma membrane-localized receptor-like protein kinase that plays critical role in immunity, in Pi starvation response.

RESULTS

qRT-PCR analysis revealed that expression of BIK1 was induced by Pi starvation and GUS staining indicated that the BIK1 promoter activity was detected in root, stem and leaf tissues of plants grown in Pi starvation condition, demonstrating that BIK1 is responsive to Pi starvation stress. The bik1 plants accumulated higher Pi content in root and leaf tissues and exhibited altered root architecture such as shorter primary roots, longer and more root hairs and lateral roots, as compared with those in the wild type plants, when grown under Pi sufficient and deficient conditions. Increased anthocyanin content and acid phosphatase activity, reduced accumulation of reactive oxygen species and downregulated expression of Pi starvation-induced genes including PHR1, WRKY75, AT4, PHT1;2 and PHT1;4 were observed in bik1 plants grown under Pi deficient condition. Furthermore, the expression of PHO2 was downregulated while the expression of miRNA399a and miRNA399d, which target to PHO2, was upregulated in bik1 plants, compared to the wild type plants, when grown under Pi deficient condition.

CONCLUSION

Our results demonstrate that BIK1 is a Pi starvation-responsive gene that functions as a negative regulator of Pi homeostasis in Arabidopsis.

Integrative analysis of the metabolome and transcriptome reveal the phosphate deficiency response pathways of alfalfa

Understanding the mechanisms underlying the responses to inorganic phosphate (Pi) deficiency in alfalfa will help enhance Pi acquisition efficiency and the sustainable use of phosphorous resources. Integrated global metabolomic and transcriptomic analyses of mid-vegetative alfalfa seedlings under 12-day Pi deficiency were conducted. Limited seedling growth were found, including 13.24%, 16.85% and 33.36% decreases in height, root length and photosynthesis, and a 24.10% increase in root-to-shoot ratio on day 12. A total of 322 and 448 differentially abundant metabolites and 1199 and 1061 differentially expressed genes were identified in roots and shoots. Increased (>3.68-fold) inorganic phosphate transporter 1;4 and SPX proteins levels in the roots (>2.15-fold) and shoots (>2.50-fold) were related to Pi absorption and translocation. The levels of phospholipids and Pi-binding carbohydrates and nucleosides were decreased, while those of phosphatases and pyrophosphatases in whole seedlings were induced under reduced Pi. In addition, nitrogen assimilation was affected by inhibiting high-affinity nitrate transporters (NRT2.1 and NRT3.1), and nitrate reductase. Increased delphinidin-3-glucoside might contribute to the gray-green leaves induced by Pi limitation. Stress-induced MYB, WRKY and ERF transcription factors were identified. The responses of alfalfa to Pi deficiency were summarized as local systemic signaling pathways, including root growth, stress-related responses consisting of enzymatic and nonenzymatic systems, and hormone signaling and systemic signaling pathways including Pi recycling and Pi sensing in the whole plant, as well as Pi recovery, and nitrate and metal absorption in the roots. This study provides important information on the molecular mechanism of the response to Pi deficiency in alfalfa.

Host SPX-PHR regulatory circuit: the molecular dynamo steering mycorrhization in plants

We highlight the newly emerged roles of plant SPX-PHR proteins beyond phosphate starvation responses in controlling arbuscular mycorrhizal colonization success in roots.

Transcription factors PHR1 and PHR1-like 1 regulate ABA-mediated inhibition of seed germination and stomatal opening in Arabidopsis

Low phosphate (LP) availability significantly impacts crop yield and quality. PHOSPHATE STARVATION RESPONSE1 (PHR1) along with PHR1-like 1 (PHL1) act as a key transcriptional regulator in a plant's adaptive response to LP conditions. Abscisic acid (ABA) plays an important role in how plants respond to environmental stresses like salinity and drought. However, the involvement of PHR1 and PHL1 in ABA response and signalling mechanisms remains to be fully understood. Our findings reveal that PHR1 and PHR1/PHL1 knockout mutations enhance the responsiveness of seed germination, early seedling growth, and stomatal opening to ABA in Arabidopsis. Furthermore, these mutations increase sensitivity to combined LP and ABA stress. In contrast, overexpression of PHR1 or PHL1 reduces this sensitivity in Arabidopsis. Knockout mutations of PHR1 and PHR1/PHL1 also increase sensitivity to salt and osmotic stresses, as well as to combined LP and salinity/osmotic stress, while overexpression of PHR1 or PHL1 reduces their sensitivity in seed germination and early seedling development. Knockout mutations of SPX1 and SPX2, negative regulators of PHR1 and PHL1, decrease sensitivity to ABA and salt/osmotic stresses in Arabidopsis. A group of genes related to ABA metabolism and signalling is significantly affected by the knockout or overexpression of PHR1 and PHL1, with a large proportion of these genes containing PHR1 binding site (P1BS) in their promoters. Moreover, the ABA-sensitive phenotype of phr1 or phr1 phl1 mutants can be rescued by PHR1 homologs from chlorophyte algae, liverwort and rice, suggesting their conserved roles in ABA signalling. These results indicate that PHR1 and its homologs negatively regulate plant responses to ABA in seed germination and stomatal aperture. This study provides new insights into the interplay between Pi homeostasis, abiotic stress and ABA signaling. Moderately increasing the expression of PHR1 or its homologs in crops could be a potential strategy to enhance plant resistance to combined LP and osmotic stress.

Modulation of plant immunity and biotic interactions under phosphate deficiency

Phosphorus (P) is an essential macronutrient for plant life and growth. P is primarily acquired in the form of inorganic phosphate (Pi) from soil. To cope with Pi deficiency, plants have evolved an elaborate system to improve Pi acquisition and utilization through an array of developmental and physiological changes, termed Pi starvation response (PSR). Plants also assemble and manage mutualistic microbes to enhance Pi uptake, through integrating PSR and immunity signaling. A trade-off between plant growth and defense favors the notion that plants lower a cellular state of immunity to accommodate host-beneficial microbes for nutrition and growth at the cost of infection risk. However, the existing data indicate that plants selectively activate defense responses against pathogens, but do not or less against non-pathogens, even under nutrient deficiency. In this review, we highlight recent advances in the principles and mechanisms with which plants balance immunity and growth-related processes to optimize their adaptation to Pi deficiency.