Arabidopsis MYB-Related HHO2 Exerts a Regulatory Influence on a Subset of Root Traits and Genes Governing Phosphate Homeostasis.

Phosphate (Pi), an essential macronutrient required for growth and development of plants, is often limiting in soils. Pi deficiency modulates the expression of Pi starvation-responsive (PSR) genes including transcription factors (TFs). Here, we elucidated the role of the MYB-related TF HYPERSENSITIVITY TO LOW PHOSPHATE-ELICITED PRIMARY ROOT SHORTENING1 HOMOLOG2 (HHO2, At1g68670) in regulating Pi acquisition and signaling in Arabidopsis thaliana HHO2 was specifically and significantly induced in different tissues in response to Pi deprivation. Transgenic seedlings expressing 35S::GFP::HHO2 confirmed the localization of HHO2 to the nucleus. Knockout mutants of HHO2 showed significant reduction in number and length of first- and higher-order lateral roots and Pi content of different tissues compared with the wild-type irrespective of the Pi regime. In contrast, HHO2-overexpressing lines exhibited augmented lateral root development, enhanced Pi uptake rate and higher Pi content in leaf compared with the wild-type. Expression levels of PSR genes involved in Pi sensing and signaling in mutants and overexpressors were differentially regulated as compared with the wild-type. Attenuation in the expression of HHO2 in the phr1 mutant suggested a likely influence of PHR1 in HHO2-mediated regulation of a subset of traits governing Pi homeostasis.

Arabidopsis Pht1;5 mobilizes phosphate between source and sink organs and influences the interaction between phosphate homeostasis and ethylene signaling.

Phosphorus (P) remobilization in plants is required for continuous growth and development. The Arabidopsis (Arabidopsis thaliana) inorganic phosphate (Pi) transporter Pht1;5 has been implicated in mobilizing stored Pi out of older leaves. In this study, we used a reverse genetics approach to study the role of Pht1;5 in Pi homeostasis. Under low-Pi conditions, Pht1;5 loss of function (pht1;5-1) resulted in reduced P allocation to shoots and elevated transcript levels for several Pi starvation-response genes. Under Pi-replete conditions, pht1;5-1 had higher shoot P content compared with the wild type but had reduced P content in roots. Constitutive overexpression of Pht1;5 had the opposite effect on P distribution: namely, lower P levels in shoots compared with the wild type but higher P content in roots. Pht1;5 overexpression also resulted in altered Pi remobilization, as evidenced by a greater than 2-fold increase in the accumulation of Pi in siliques, premature senescence, and an increase in transcript levels of genes involved in Pi scavenging. Furthermore, Pht1;5 overexpressors exhibited increased root hair formation and reduced primary root growth that could be rescued by the application of silver nitrate (ethylene perception inhibitor) or aminoethoxyvinylglycine (ethylene biosynthesis inhibitor), respectively. Together, these data indicate that Pht1;5 plays a critical role in mobilizing Pi from P source to sink organs in accordance with developmental cues and P status. The study also provides evidence for a link between Pi and ethylene signaling pathways.

Histone H2A.Z regulates the expression of several classes of phosphate starvation response genes but not as a transcriptional activator.

Phosphate (Pi) availability is a major constraint to plant growth. Consequently, plants have evolved complex adaptations to tolerate low Pi conditions. Numerous genes implicated in these adaptations have been identified, but their chromatin-level regulation has not been investigated. The nuclear actin-related protein ARP6 is conserved among all eukaryotes and is an essential component of the SWR1 chromatin remodeling complex, which regulates transcription via deposition of the H2A.Z histone variant into chromatin. Here, we demonstrate that ARP6 is required for proper H2A.Z deposition at a number of Pi starvation response (PSR) genes in Arabidopsis (Arabidopsis thaliana). The loss of H2A.Z at these target loci results in their derepression in arp6 mutants and correlates with the presence of multiple Pi-starvation-related phenotypes, including shortened primary roots and increases in the number and length of root hairs, as well as increased starch accumulation and phosphatase activity in shoots. Our data suggest a model for chromatin-level control of Pi starvation responses in which ARP6-dependent H2A.Z deposition modulates the transcription of a suite of PSR genes.

Variations in the composition of gelling agents affect morphophysiological and molecular responses to deficiencies of phosphate and other nutrients.

Low inorganic phosphate (Pi) availability triggers an array of spatiotemporal adaptive responses in Arabidopsis (Arabidopsis thaliana). There are several reports on the effects of Pi deprivation on the root system that have been attributed to different growth conditions and/or inherent genetic variability. Here we show that the gelling agents, largely treated as inert components, significantly affect morphophysiological and molecular responses of the seedlings to deficiencies of Pi and other nutrients. Inductively coupled plasma-mass spectroscopy analysis revealed variable levels of elemental contaminants not only in different types of agar but also in different batches of the same agar. Fluctuating levels of phosphorus (P) in different agar types affected the growth of the seedlings under Pi-deprivation condition. Since P interacts with other elements such as iron, potassium, and sulfur, contaminating effects of these elements in different agars were also evident in the Pi-deficiency-induced morphological and molecular responses. P by itself acted as a contaminant when studying the responses of Arabidopsis to micronutrient (iron and zinc) deficiencies. Together, these results highlighted the likelihood of erroneous interpretations that could be easily drawn from nutrition studies when different agars have been used. As an alternative, we demonstrate the efficacy of a sterile and contamination-free hydroponic system for dissecting morphophysiological and molecular responses of Arabidopsis to different nutrient deficiencies.

Differential effects of sucrose and auxin on localized phosphate deficiency-induced modulation of different traits of root system architecture in Arabidopsis.

Phosphorus, one of the essential elements for plants, is often a limiting nutrient in soils. Low phosphate (Pi) availability induces sugar-dependent systemic expression of genes and modulates the root system architecture (RSA). Here, we present the differential effects of sucrose (Suc) and auxin on the Pi deficiency responses of the primary and lateral roots of Arabidopsis (Arabidopsis thaliana). Inhibition of primary root growth and loss of meristematic activity were evident in seedlings grown under Pi deficiency with or without Suc. Although auxin supplementation also inhibited primary root growth, loss of meristematic activity was observed specifically under Pi deficiency with or without Suc. The results suggested that Suc and auxin do not influence the mechanism involved in localized Pi sensing that regulates growth of the primary root and therefore delineates it from sugar-dependent systemic Pi starvation responses. However, the interaction between Pi and Suc was evident on the development of the lateral roots and root hairs in the seedlings grown under varying levels of Pi and Suc. Although the Pi+ Suc- condition suppressed lateral root development, induction of few laterals under the Pi- Suc- condition point to increased sensitivity of the roots to auxin during Pi deprivation. This was supported by expression analyses of DR5uidA, root basipetal transport assay of auxin, and RSA of the pgp19 mutant exhibiting reduced auxin transport. A significant increase in the number of lateral roots under the Pi- Suc- condition in the chalcone synthase mutant (tt4-2) indicated a potential role for flavonoids in auxin-mediated Pi deficiency-induced modulation of RSA. The study thus demonstrated differential roles of Suc and auxin in the developmental responses of ontogenetically distinct root traits during Pi deprivation. In addition, lack of cross talk between local and systemic Pi sensing as revealed by the seedlings grown under either the Pi- Suc- condition or in the heterogeneous Pi environment highlighted the coexistence of Suc-independent and Suc-dependent regulatory mechanisms that constitute Pi starvation responses.