SEARCH · Search NASA

Results for “Alternative precursor”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5

All sources NASA NTRS DOE OSTI

NASA NTRS2015

Integrated Surface Power Strategy for Mars

A National Aeronautics and Space Administration (NASA) study team evaluated surface power needs for a conceptual crewed 500-day Mars mission. This study had four goals: 1. Determine estimated surface power needed to support the reference mission; 2. Explore alternatives to minimize landed power system mass; 3. Explore alternatives to minimize Mars Lander power self-sufficiency burden; and 4. Explore alternatives to minimize power system handling and surface transportation mass. The study team concluded that Mars Ascent Vehicle (MAV) oxygen propellant production drives the overall surface power needed for the reference mission. Switching to multiple, small Kilopower fission systems can potentially save four to eight metric tons of landed mass, as compared to a single, large Fission Surface Power (FSP) concept. Breaking the power system up into modular packages creates new operational opportunities, with benefits ranging from reduced lander self-sufficiency for power, to extending the exploration distance from a single landing site. Although a large FSP trades well for operational complexity, a modular approach potentially allows Program Managers more flexibility to absorb late mission changes with less schedule or mass risk, better supports small precursor missions, and allows a program to slowly build up mission capability over time. A number of Kilopower disadvantages-and mitigation strategies-were also explored.

Rucker, Michelle↗

NASA NTRS2014

Asteroid Redirect Robotic Mission: Robotic Boulder Capture Option Overview

The National Aeronautics and Space Administration (NASA) is currently studying an option for the Asteroid Redirect Robotic Mission (ARRM) that would capture a multi-ton boulder (typically 2-4 meters in size) from the surface of a large (is approximately 100+ meter) Near-Earth Asteroid (NEA) and return it to cislunar space for subsequent human and robotic exploration. This alternative mission approach, designated the Robotic Boulder Capture Option (Option B), has been investigated to determine the mission feasibility and identify potential differences from the initial ARRM concept of capturing an entire small NEA (4-10 meters in size), which has been designated the Small Asteroid Capture Option (Option A). Compared to the initial ARRM concept, Option B allows for centimeter-level characterization over an entire large NEA, the certainty of target NEA composition type, the ability to select the boulder that is captured, numerous opportunities for mission enhancements to support science objectives, additional experience operating at a low-gravity planetary body including extended surface contact, and the ability to demonstrate future planetary defense strategies on a hazardous-size NEA. Option B can leverage precursor missions and existing Agency capabilities to help ensure mission success by targeting wellcharacterized asteroids and can accommodate uncertain programmatic schedules by tailoring the return mass.

Mazanek, Daniel D.↗

NASA NTRS1994

Alternative bases in the RNA world: the prebiotic synthesis of urazole and its ribosides

Urazole is a five-membered heterocyclic compound which is isosteric with uracil's hydrogen-bonding segment. Urazole reacts spontaneoulsy with ribose (and other aldoses) to give a mixture of four ribosides: alpha and beta pyranosides and furanosides. This reaction occurs in aqueous solution at mild temperatures. Thermodynamic and kinetic parameters for the reaction of urazole with ribose were determined. In contrast, uracil is completely unreactive with ribose under these conditions. Urazole's unusual reactivity is ascribed to the hydrazine portion of the molecule. Urazole can be synthesized from biuret and hydrazine under prebiotic conditions. The prebiotic synthesis of guanazole, which is isosteric in part to diaminopyrimidine and cytosine, is accomplished from dicyandiamide and hydrazine. Kinetic parameters for both prebiotic reactions were measured. Urazole and guanazole are transparent in the UV, which would be a favorable property in the absence of an ozone layer on the early Earth. Urazole makes hydrogen bonds with adenine in DMSO similar to those of uracil, as established by H NMR. All of these properties make urazole an attractive potential precursor to uracil and guanazole a potential precursor to cytosine in the RNA or pre-RNA world.

Non-NASA Center↗

NASA NTRS1980

An interstellar precursor mission

A mission out of the planetary system, launched about the year 2000, could provide valuable scientific data as well as test some of the technology for a later mission to another star. Primary scientific objectives for the precursor mission concern characteristics of the heliopause, the interstellar medium, stellar distances (by parallax measurements), low-energy cosmic rays, interplanetary gas distribution, and the mass of the solar system. Secondary objectives include investigation of Pluto. The mission should extend to 400-1000 AU from the sun. A heliocentric hyperbolic escape velocity of 50-100 km/sec or more is needed to attain this distance within a reasonable mission duration (20-50 years). The trajectory should be toward the incoming interstellar gas. For a year 2000 launch, a Pluto encounter and orbiter can be included. A second mission targeted parallel to the solar axis would also be worthwhile. The mission duration is 20 years, with an extended mission to a total of 50 years. A system using one or two stages of nuclear electric propulsion (NEP) was selected as a possible baseline. The most promising alternatives are ultralight solar sails or laser sailing, with the lasers in earth orbit, for example. The NEP baseline design allows the option of carrying a Pluto orbiter as a daughter spacecraft.

Jaffe, L. D.↗

NASA NTRS1993

The Fe/Mn constraint on precursors of basaltic achondrites

Most achondritic meteorites have Fe/Mn ratios that are lower than those of carbonaceous chondrites and of course are lower than the solar system abundance ratio of these elements. Models of the origin of achondritic assemblages must, therefore, account for these ratios. Fe/Mn ratios are suggested to be distinctive for samples from each achondrite parent body and for the Earth and Moon, but the correspondence between the Fe/Mn systematics of achondrites and chondritic precursors is unclear. Most models of achondrite genesis involve magmatic differentiation of chondritic precursors. The Fe/Mn difference between achondrites and chondrites is particularly significant since Fe and Mn are geochemically similar elements with similar partitioning behavior in familiar magmatic systems and are generally coupled during crystal-liquid fractionation. In contrast, however, Mn is more volatile than Fe in a nebular setting. Variation of Fe/Mn ratios based on the relative volatility of these elements in the early nebula provides a constraint for models by which the basaltic achondrites (with Fe/Mn ratios approximately = 25-50) are derived from mixtures of nebular components that were enriched in volatile components such as Mn. However, such volatile enriched components have not been identified in chondrites. When the abundance in achondrites of elements of similar volatility is examined, anomalies appear. For example, Na is massively depleted in basaltic achondrites when compared to Mn. These anomalies might be explained using current models but the alternative hypothesis, that Fe/Mn ratio is controlled not by nebular volatility constraints, but by planetary differentiation should be explored.

Delaney, Jeremy S.↗

NASA NTRS1993

The major nucleoside triphosphatase in pea (Pisum sativum L.) nuclei and in rat liver nuclei share common epitopes also present in nuclear lamins

The major nucleoside triphosphatase (NTPase) activities in mammalian and pea (Pisum sativum L.) nuclei are associated with enzymes that are very similar both biochemically and immunochemically. The major NTPase from rat liver nuclei appears to be a 46-kD enzyme that represents the N-terminal portion of lamins A and C, two lamina proteins that apparently arise from the same gene by alternate splicing. Monoclonal antibody (MAb) G2, raised to human lamin C, both immunoprecipitates the major (47 kD) NTPase in pea nuclei and recognizes it in western blot analyses. A polyclonal antibody preparation raised to the 47-kD pea NTPase (pc480) reacts with the same lamin bands that are recognized by MAb G2 in mammalian nuclei. The pc480 antibodies also bind to the same lamin-like bands in pea nuclear envelope-matrix preparations that are recognized by G2 and three other MAbs known to bind to mammalian lamins. In immunofluorescence assays, pc480 and anti-lamin antibodies stain both cytoplasmic and nuclear antigens in plant cells, with slightly enhanced staining along the periphery of the nuclei. These results indicate that the pea and rat liver NTPases are structurally similar and that, in pea nuclei as in rat liver nuclei, the major NTPase is probably derived from a lamin precursor by proteolysis.

Non-NASA Center↗

NASA NTRS2002

Modeling Grade IV Gas Emboli using a Limited Failure Population Model with Random Effects

Venous gas emboli (VGE) (gas bubbles in venous blood) are associated with an increased risk of decompression sickness (DCS) in hypobaric environments. A high grade of VGE can be a precursor to serious DCS. In this paper, we model time to Grade IV VGE considering a subset of individuals assumed to be immune from experiencing VGE. Our data contain monitoring test results from subjects undergoing up to 13 denitrogenation test procedures prior to exposure to a hypobaric environment. The onset time of Grade IV VGE is recorded as contained within certain time intervals. We fit a parametric (lognormal) mixture survival model to the interval-and right-censored data to account for the possibility of a subset of "cured" individuals who are immune to the event. Our model contains random subject effects to account for correlations between repeated measurements on a single individual. Model assessments and cross-validation indicate that this limited failure population mixture model is an improvement over a model that does not account for the potential of a fraction of cured individuals. We also evaluated some alternative mixture models. Predictions from the best fitted mixture model indicate that the actual process is reasonably approximated by a limited failure population model.

Thompson, Laura A.↗

NASA NTRS1992

Database of proposed payloads and instruments for SEI missions

A database of all payloads and instruments proposed for lunar and Mars missions was compiled by the author for the Exploration Programs Office at NASA's Johnson Sapce Center. The database is an outgrowth of the document produced by C. J. Budney et al. at the Jet Propulsion Laboratory in 1991. The present database consists not only of payloads proposed for human exploratory missions of the Moon and Mars, but also experiments selected or proposed for robotic precursor missions such as Lunar Scout, Mars Observer, and MESUR. The database consists of two parts: a written payload description and a matrix that provides a breakdown of payload components. Each payload description consists of the following information: (1) the rationale for why the instrument or payload package is being proposed for operation on the Moon or Mars; (2) a description of how the instrument works; (3) a breakdown of the payload, providing detailed information about the mass, volume, power requirements, and data rates for the constituent pieces of the experiment; (4) estimates of the power consumption and data rate; (5) how the data will be returned to Earth and distributed to the scientific community; (6) any constraints on the location or conditions under which the instrument can or cannot operate; (7) what type of crew interaction (if any) is needed; (8) how the payload is to be delivered to the lunar or martian surface (along with alternative delivery options); (9) how long the instrument or payload package will take to set up; (10) what type of maintenance needs are anticipated for the experiment; (11) stage of development for the instrument and environmental conditions under which the instrument has been tested; (12) an interface required by the instrument with the lander, a rover, an outpost, etc.; (13) information about how often the experiment will need to be resupplied with parts or consumables, if it is to be resupplied; (14) the name and affiliation of a contact person for the experiment; and (15) references where further information about the experiment can be found.

Barlow, N. G.↗

NASA NTRS1998

Guidance and Control Concepts for the X-33 Technology Demonstrator

The X-33 technology demonstrator is a suborbital precursor to the Reusable Launch Vehicle (RLV) with first flight planned for summer of 1999. The flight test program will include about 15 flights originating from Edwards Air Force Base, California, each with widely varying flight profiles in order to test new thermal protection system (TPS) materials, structures, and linear aerospike engines. The first flights will be relatively short range flights with about a 300 nmi range, maximum Mach number of 7, maximum altitude of 190,000 feet, whereas the latter flights will cover about 800 nmi range, with max altitude of about 260,000 feet and max Mach of about 15. The guidance algorithms must be flexible enough to accommodate these various profiles and to adapt to severe off-nominal dispersions, such as early engine failure (partial or total) where possibly more than half the thrust is lost. An onboard real-time performance monitor will be used to assess the viability of the nominal landing site as well as alternate landing sites that would potentially be used in extreme off-nominal conditions. During ascent, a single entry guidance-related parameter, which is easy to calculate, is used to assess the viability of the nominal landing site as well as alternate landing sites. Real-time adjustment of the stored ascent attitude profile will be performed, as required, to maximize the probability of making it to the nominal landing site. Numerical results are given for various engine-out cases to illustrate the adaptability of the performance monitor.

Dukeman, Gregory A.↗

NASA NTRS2004

Theoretical Investigation of OCN(-) Charge Transfer Complexes in Condensed Phase Media: Spectroscopic Properties in Amorphous Ice

Density functional theory (DFT) calculations of cyanate (OCN(-)) charge-transfer complexes were performed to model the "XCN" feature observed in interstellar icy grain mantles. OCN(-) charge-transfer complexes were formed from precursor combinations of HNCO or HOCN with either NH3 or H2O. Three different solvation strategies for realistically modeling the ice matrix environment were explored, including (1) continuum solvation, (2) pure DFT cluster calculations, and (3) an ONIOM DFT/PM3 cluster calculation. The model complexes were evaluated by their ability to reproduce seven spectroscopic measurements associated with XCN: the band origin of the OCN(-) asymmetric stretching mode, shifts in that frequency due to isotopic substitutions of C, N, O, and H, plus two weak features. The continuum solvent field method produced results consistent with some of the experimental data but failed to account for other behavior due to its limited capacity to describe molecular interactions with solvent. DFT cluster calculations successfully reproduced the available spectroscopic measurements very well. In particular, the deuterium shift showed excellent agreement in complexes where OCN(-) was fully solvated. Detailed studies of representative complexes including from two to twelve water molecules allowed the exploration of various possible solvation structures and provided insights into solvation trends. Moreover, complexes arising from cyanic or isocyanic acid in pure water suggested an alternative mechanism for the formation of OCN(-) charge-transfer complexes without the need for a strong base such as NH3 to be present. An extended ONIOM (B3LYP/PM3) cluster calculation was also performed to assess the impact of a more realistic environment on HNCO dissociation in pure water.

Park, Jin-Young↗

NASA NTRS2020

The Flexible Lunar Architecture for Exploration (FLARE): Designed for the Artemis-3 Moon 2024 Mission and Beyond

The Flexible Lunar Architecture for Exploration (FLARE) is a concept to deliver four crew to the lunar surface for 7 to 14 days and then return them safely to Earth by 2024. This meets NASA’s internal 2024 lunar landing deadline directed by President Trump (Trump, 2017) and the “5-year” goal set forth by Vice President Pence (Pence, 2019). FLARE is an alternative to NASA’s Human Landing System reference architecture from the Design Analysis Cycle (DAC) #2 (NASA, 2019b). The minimum FLARE concept uses one Space Launch System launch, one Orion, one European Service Module (ESM), and one human lander to deliver four crew to the Moon for a minimum surface duration of 7 days and return them to Earth. FLARE adds a new capability, called the SpaceTug, based upon the mature and successful United Launch Alliance “Common” Centaur Upper Stage vehicle, with modifications. In FLARE, the SpaceTug provides propulsion needed to return the Orion+ESM from the Moon to Earth. The SpaceTug also provides propulsion to deliver the human lander Descent Element (DE) and Ascent Element (AE) separately to lunar orbit. The Orion+ESM then completes a rendezvous with the mated DE+AE in lunar orbit. FLARE also offers optional phases to the Moon 2024 mission. The SpaceTug can also deliver components of the planned Gateway - including the Power and Propulsion Element and the Habitation and Logistics Outpost - to lunar orbit; however, the planned FLARE destination is a Low Lunar Frozen Polar Orbit unlike the NASA DAC2 plan for a Near Rectilinear Halo Orbit. FLARE also provides an option to deliver precursor equipment - including a habitation module, crew mobility devices and an In-Situ Resource Utilization demonstration - to the lunar surface for enhanced crew exploration and science with the extended 14-day surface mission.

Commercial Launch Vehicles (CLV)↗

NASA NTRS2017

Revised Models of Interstellar Nitrogen Isotopic Fractionation

Nitrogen-bearing molecules in cold molecular clouds exhibit a range of isotopic fractionation ratios and these molecules may be the precursors of 15N enrichments found in comets and meteorites. Chemical model calculations indicate that atom–molecular ion and ion–molecule reactions could account for most of the fractionation patterns observed. However, recent quantum-chemical computations demonstrate that several of the key processes are unlikely to occur in dense clouds. Related model calculations of dense cloud chemistry show that the revised 15N enrichments fail to match observed values. We have investigated the effects of these reaction rate modifications on the chemical model of Wirström et al. (2012) for which there are significant physical and chemical differences with respect to other models. We have included 15N fractionation of CN in neutral–neutral reactions and also updated rate coefficients for key reactions in the nitrogen chemistry. We find that the revised fractionation rates have the effect of suppressing 15N enrichment in ammonia at all times, while the depletion is even more pronounced, reaching 14N/15N ratios of >2000. Taking the updated nitrogen chemistry into account, no significant enrichment occurs in HCN or HNC, contrary to observational evidence in dark clouds and comets, although the 14N/15N ratio can still be below 100 in CN itself. However, such low CN abundances are predicted that the updated model falls short of explaining the bulk 15N enhancements observed in primitive materials. It is clear that alternative fractionating reactions are necessary to reproduce observations, so further laboratory and theoretical studies are urgently needed.

astrochemistry – molecular processes – comets: gen↗

NASA NTRS2017

Revised Models of Interstellar Nitrogen Isotopic Fractionation

astrochemistry – molecular processes – comets: gen↗

NASA NTRS2017

Revised Models of Interstellar Nitrogen Isotopic Fractionation

Nitrogen-bearing molecules in cold molecular clouds exhibit a range of isotopic fractionation ratios and these molecules may be the precursors of 15N enrichments found in comets and meteorites. Chemical model calculations indicate that atom-molecular ion and ion-molecule reactions could account for most of the fractionation patterns observed. However, recent quantum-chemical computations demonstrate that several of the key processes are unlikely to occur in dense clouds. Related model calculations of dense cloud chemistry show that the revised 15N enrichments fail to match observed values. We have investigated the effects of these reaction rate modifications on the chemical model of Wirstrom et al. (2012) for which there are significant physical and chemical differences with respect to other models. We have included 15N fractionation of CN in neutral-neutral reactions and also updated rate coefficients for key reactions in the nitrogen chemistry. We find that the revised fractionation rates have the effect of suppressing 15N enrichment in ammonia at all times, while the depletion is even more pronounced, reaching 14N/15N ratios of >2000. Taking the updated nitrogen chemistry into account, no significant enrichment occurs in HCN or HNC, contrary to observational evidence in dark clouds and comets, although the 14N/15N ratio can still be below 100 in CN itself. However, such low CN abundances are predicted that the updated model falls short of explaining the bulk 15N enhancements observed in primitive materials. It is clear that alternative fractionating reactions are necessary to reproduce observations, so further laboratory and theoretical studies are urgently needed.

Wirstrom, E. S.↗

NASA NTRS2017

Revised Models of Interstellar Nitrogen Isotopic Fractionation

Nitrogen-bearing molecules in cold molecular clouds exhibit a range of isotopic fractionation ratios and these molecules may be the precursors of 15N enrichments found in comets and meteorites. Chemical model calculations indicate that atom-molecular ion and ion-molecule reactions could account for most of the fractionation patterns observed. However, recent quantum-chemical computations demonstrate that several of the key processes are unlikely to occur in dense clouds. Related model calculations of dense cloud chemistry show that the revised 15N enrichments fail to match observed values. We have investigated the effects of these reaction rate modfications on the chemical model of Wirstrom et al. (2012) for which there are significant physical and chemical differences with respect to other models. We have included 15N fractionation of CN in neutral-neutral reactions and also updated rate coefficients for key reactions in the nitrogen chemistry. We find that the revised fractionation rates have the effect of suppressing 15N enrichment in ammonia at all times, while the depletion is even more pronounced, reaching 14N/15N ratios of > 2000. Taking the updated nitrogen chemistry into account, no significant enrichment occurs in HCN or HNC, contrary to observational evidence in dark clouds and comets, although the 14N/15N ratio can still be below 100 in CN itself. However, such low CN abundances are predicted that the updated model falls short of explaining the bulk 15N enhancements observed in primitive materials. It is clear that alternative fractionating reactions are necessary to reproduce observations, so further laboratory and theoretical studies are urgently needed.

astrochemistry – molecular processes – comets: gen↗

NASA NTRS2017

Revised Models of Interstellar Nitrogen Isotopic Fractionation

Nitrogen-bearing molecules in cold molecular clouds exhibit a range of isotopic fractionation ratios and these molecules may be the precursors of (15)N enrichments found in comets and meteorites. Chemical model calculations indicate that atom-molecular ion and ion-molecule reactions could account for most of the fractionation patterns observed. However, recent quantum-chemical computations demonstrate that several of the key processes are unlikely to occur in dense clouds. Related model calculations of dense loud chemistry show that the revised (15)N enrichments fail to match observed values. We have investigated the effects of these reaction rate modifications on the chemical model of Wirstrom et al. (2012) for which there are significant physical and chemical differences with respect to other models. We have included (15)N fractionation of CN in neutral-neutral reactions and also updated rate coefficients for key reactions in the nitrogen chemistry. We find that the revised fractionation rates have the effect of suppressing (15)N enrichment in ammonia at all times, while the depletion is even more pronounced, reaching (14)N/(15)N ratios of > 2000. Taking the updated nitrogen chemistry into account, no sigificant enrichment occurs in HCN or HNC, contrary to observational evidence in dark clouds and comets, although the (14)N/(15)N ratio can still be below 100 in CN itself. However, such low CN abundances are predicted that the updated model falls short of explaining the bulk (15)N enhancements observed in primitive materials. It is clear that alternative fractionating reactions are necessary to reproduce observations, so further laboratory and theoretical studies are urgently needed.

Astrochemistry↗

NASA NTRS2020

A Flexible Lunar Architecture for Exploration (FLARE) Supporting NASA’s Artemis Program

The Flexible Lunar Architecture for Exploration (FLARE) is a concept to deliver four crew to the lunar surface for a minimum of seven days and then return them safely to Earth. FLARE can be implemented whenever the component vehicles are operational. FLARE was developed as an alternative to NASA’s Human Landing System (HLS) reference architecture from the Design Analysis Cycle (DAC) #2 created in 2019. The DAC2 guidelines required utilization of the Gateway vehicle in a Near- Rectilinear Halo Orbit (NRHO). Instead, FLARE chooses a Low Lunar Frozen Polar Orbit (LLFPO) for lunar rendezvous of components, and an optional Gateway vehicle. The LLFPO provides a stable orbit that overflies the south pole every 2 h, ensuring easy access to the lunar surface for surface aborts with a much lower propellant requirement than NRHO. The minimum FLARE concept uses one Space Launch System (SLS) launch, one Orion, one European Service Module (ESM), and one human lander (launched on commercial vehicle(s)). FLARE adds the SpaceTug, based upon the mature and successful ULA “Common” Centaur Upper Stage vehicle, with modifications to create an Earth-Moon transfer vehicle. In the FLARE baseline mission, the SpaceTug provides propulsion needed to return the Orion + ESM from LLFPO to Earth. The SpaceTug also provides propulsion to deliver the separate human lander components – the Descent Element (DE) and the Ascent Element (AE) - from Low Earth Orbit (LEO) to LLFPO. The SLS Block 1 then launches the Orion + ESM and completes a rendezvous with the mated DE + AE components in LLFPO. FLARE offers optional phases beyond the baseline mission. The SpaceTug can deliver components of the planned Gateway, including the Power and Propulsion Element (PPE) and the Habitable and Logistics Outpost (HALO), to LLFPO. FLARE provides an option to deliver precursor equipment to the lunar surface to enhance and extend the human mission. With these components, including an inflatable habitation module and airlock, individual crew mobility vehicle(s), an In-Situ Resource Utilization (ISRU) demonstration, and science and technology experiments, the crew can explore and conduct science on the lunar surface for up to 14 days.

Moon 2024↗

NASA NTRS2002

Transcriptional control of monolignol biosynthesis in Pinus taeda: factors affecting monolignol ratios and carbon allocation in phenylpropanoid metabolism

Transcriptional profiling of the phenylpropanoid pathway in Pinus taeda cell suspension cultures was carried out using quantitative real time PCR analyses of all known genes involved in the biosynthesis of the two monolignols, p-coumaryl and coniferyl alcohols (lignin/lignan precursors). When the cells were transferred to a medium containing 8% sucrose and 20 mm potassium iodide, the monolignol/phenylpropanoid pathway was induced, and transcript levels for phenylalanine ammonia lyase, cinnamate 4-hydroxylase, p-coumarate 3-hydroxylase, 4-coumarate:CoA ligase, caffeoyl-CoA O-methyltransferase, cinnamoyl-CoA reductase, and cinnamyl alcohol dehydrogenase were coordinately up-regulated. Provision of increasing levels of exogenously supplied Phe to saturating levels (40 mm) to the induction medium resulted in further up-regulation of their transcript levels in the P. taeda cell cultures; this in turn was accompanied by considerable increases in both p-coumaryl and coniferyl alcohol formation and excretion. By contrast, transcript levels for both cinnamate 4-hydroxylase and p-coumarate 3-hydroxylase were only slightly up-regulated. These data, when considered together with metabolic profiling results and genetic manipulation of various plant species, reveal that carbon allocation to the pathway and its differential distribution into the two monolignols is controlled by Phe supply and differential modulation of cinnamate 4-hydroxylase and p-coumarate 3-hydroxylase activities, respectively. The coordinated up-regulation of phenylalanine ammonia lyase, 4-coumarate:CoA ligase, caffeoyl-CoA O-methyltransferase, cinnamoyl-CoA reductase and cinnamyl alcohol dehydrogenase in the presence of increasing concentrations of Phe also indicates that these steps are not truly rate-limiting, because they are modulated according to metabolic demand. Finally, the transcript profile of a putative acid/ester O-methyltransferase, proposed as an alternative catalyst for O-methylation leading to coniferyl alcohol, was not up-regulated under any of the conditions employed, suggesting that it is not, in fact, involved in monolignol biosynthesis.

NASA Discipline Plant Biology↗