Chen DL, CA Delatorre, A Bakker, Abel S (2000) Conditional identification
of phosphate starvation-response mutants in Arabidopsis thaliana.
Planta 211:13-22 [PDF]
Abel S, T Nürnberger, V Ahnert,
GJ Krauss, Glund K (2000) Induction of an extracellular cyclic nucleotide
as an accessory
ribonucleolytic activity during phosphate starvation of cultured tomato
Plant Physiol 122:543-532 [PDF]
Our research on phosphate sensing has been funded by the U.S. Department of Energy.
Mechanisms of Phosphate Sensing
Inorganic phosphate (Pi) plays a central role in metabolism as reactant and effector molecule at the nexus of photosynthesis, energy conservation and carbon assimilation. Consequently, Pi availability has a profound and direct impact on plant performance and productivity. To cope with inadequate phosphate (Pi) availability, a common situation in many ecosystems, plants activate a set of adaptive responses that reprioritize internal Pi use and maximize external Pi acquisition. Such countermeasures include adjustment of metabolism to maintain intracellular Pi homeostasis and remodeling of root system architecture to enhance soil exploration. While the physiological responses to Pi limitation are relatively well understood, the sensory mechanisms that monitor environmental Pi status and interpret the nutritional signal in Pi rescue efforts are largely unknown (Ticconi and Abel, 2004; Abel et al., 2002). A better understanding of the limiting factors in plant phosphorus nutrition and how plants sense and respond to Pi deficiency will provide the knowledge and biotechnological tools to ultimately develop strategies for more efficient phosphorus use in agriculture. We are particularly interested in the elucidation of the sensory mechanisms and signaling networks that govern metabolic as well as developmental plant responses to Pi limitation. We have taken a molecular genetics approach to dissect Pi sensing in Arabidopsis thaliana. Our research uncovered a Pi-sensitive checkpoint in root development regulating meristem activity in response to local Pi availability (Ticconi et al., 2004).
Screen for Conditional Phosphate Response Mutants
To dissect Pi sensing in plants, we devised novel genetic screens for isolating insensitive and constitutive Pi-deficiency response (or pdr) mutations in Arabidopsis (Chen et al., 2000; Ticconi et al., 2001). Both strategies are based on the ability of wild-type plants to grow on media containing nucleic acids as the only source of phosphorus. We demonstrated that wild-type plants induce and secrete a set of nucleolytic enzymes in Pi-limiting conditions, which hydrolyze exogenous DNA or RNA to Pi and nucleosides. Biochemical redundancy of sequentially acting nucleolytic enzymes is thought to favor identification of mutations in regulatory rather than structural genes (Chen et al., 2000; Abel et al., 2000). We have focused on pdr mutant isolation and characterization, as well as on PDR gene cloning and functional analysis of the gene products in Pi sensing. Two classes of conditional pdr mutations have attracted most of our attention. Class I mutants, typified by pdr1, impair metabolic adjustments to limiting Pi by disrupting global changes in Pi-responsive gene expression. Class II mutants, typified by pdr2 and similar pdr mutants, specifically interfere with Pi-responsive regulation of root cell division and meristem activity, leading to a dramatically altered root system architecture in limiting Pi. While the core cell cycle machinery in plants is relatively well understood, very little is known about how environmental cues regulate cell division. We are primarily focusing our efforts on the analysis of class II mutations as those have the intriguing potential to provide first insight into the regulation of meristem activity and root system architecture by nutrient availability.
Mineral Crosstalk in Phosphate Sensing
General responses to Pi limitation such as changes in root system architecture, Pi homeostasis and accumulation of starch and anthocyanins are altered in pdr1, whereas overall Pi uptake is not impaired. We have demonstrated by enzyme activity assays, RNA blot analyses and GeneChip expression profiling that the conditional short root phenotype of pdr1 on nucleic-acid containing (-Pi) media is caused by an inability to sufficiently induce multiple genes regulated by external Pi availability. Surprisingly, the single recessive pdr1 mutation not only reduces the sensitivity of Pi deficiency responses, but also enhances the sensitivity to nitrate and other nitrogen sources in the medium. Thus, two factors contribute to the conditional short root phenotype of pdr1: a decreased sensitivity to low Pi and an increased sensitivity to high nitrate and other nitrogen compounds. We hypothesize that PDR1 encodes a regulatory component upstream of Pi starvation-inducible gene expression, which may further play a role in sensing external nitrogen status. Cloning and molecular characterization of PDR1 will illuminate the emerging connection between phosphorus and nitrogen sensing.
3. Phosphate Sensing in
the Arabidopsis Root Meristem
Class II mutations, typified by pdr2 and other mutations (pdr3-pdr5), point to a Pi-sensitive checkpoint in root development that monitors environmental Pi status, maintains and fine-tunes root meristem activity, and adjusts root system architecture to maximize soil exploration. The pdr2-like mutations specifically interfere with Pi-responsive regulation of cell division and root meristem activity, which result in a dramatically truncated root system when Pi is limiting (Ticconi et al., 2004). Loss of PDR2 causes unrestrained meristem consumption in low Pi. A failure of root meristem maintenance in pdr2 can be caused by a loss of specification/activity of the quiescent center (QC), which maintains the identity of the surrounding stem cells, or by a loss of division potential and/or accelerated differentiation of stem cell descendents. To distinguish between these two scenarios, we monitored the expression of several QC marker genes (e.g., QC25::GUS) and the differentiation status of columella stem cells (iodine staining of starch grains) in wild-type and pdr2 roots during transfer from high to low Pi medium. Our data clearly demonstrate that limiting Pi does not impair QC viability in pdr2, even after meristem consumption is evident, but causes rapid (1-2 days after transfer) differentiation of columella stem cells and of other stem cells in the root meristem. Thus, PDR2 is required for root meristem maintenance independently of QC activity in limiting Pi. This suggests that external Pi modulates the transition of cell division to cell differentiation in root meristems via PDR2. We have recently identified PDR2 and PDR3. Interestingly, our data indicate that both gene products function in the secretory pathway and affect the activity of the root stem cell population in a Pi-dependent manner, possibly via polypeptide-mediated cell-cell communication. While the core cell cycle machinery in plants is well understood, very little is known about how the environment regulates cell division and differentiation and thus meristem activity and maintenance. PDR2 and PDR3 provide an excellent impetus for exploring the importance of cell-cell communication in Pi sensing by root meristems and the largely uncharted area of plant cell cycle control by environmental cues.