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Nitrogen metabolism interactions with hormonal signaling and root growth
The responsiveness of root architecture to soil water and nutrient conditions is an important part of plant stress adaptations to marginal soil conditions.
Recent studies have shown that low nitrogen promotes root elongation in maize while high nitrogen inhibits elongation.
Root re-generation as a plant defense responsePlants have evolved with mechanisms to both tolerate and actively defend themselves against attack from herbivores and pathogens. The attackers have also evolved counter-intelligence capabilities. We are studying the communication processes that coordinate these responses and hope to translate this new knowledge to creating hardier plants. The early emphasis of our program was on the role of jasmonate, a plant hormone and signaling component in the chain of metabolic defense responses linked to leaf herbivory. Past research strategies include:
Some of our early findings using 11C-methyl jasmonate (MeJA) is that the defense hormone utilizes protein transporters to actively load and move within the phloem. However, jasmonates appear to transport differently than bulk flow of photosynthate within the phloem. Comparative studies using 11C-MeJA and 11CO2 in same plants (in the same day) showed different trends in both tissue allocation and transport speeds. New developments in radiochemistry providing 11C-methyl salicylate (MeSA) a pathogen defense hormone has enabled preliminary studies of leaf allocation in tobacco. Comparison of radiographic images for 11C-photosynthate, 11C-MeJA and 11C-MeSA show similarities in transport and allocation between the defense hormones, but very different patterns relative to 11C-photosynthate movement. 
Recent interests of the group have also turned to root responses to herbivory. In a recent case study we demonstrated the potential utility of maize roots infested by western corn rootworm
as a model system to explore root re-growth responses. While corn rootworms are considered a specialist herbivore to maize, with U.S. economic crop losses averaging $1B annually, they are indeed
opportunists and will attack other grasses including bioenergy relevant crops like Miscanthus. This pest can present longer-range problems to the DOE for sustainability of a bioeconomy.
In collaboration with Drs. Erb and Roberts at the Max Planck Institute, Prof. Turlings at the FARCE Laboratory (U. Neuschatel) and Prof Hibbard at the USDA-ARS facility (U. Missouri),
our early studies demonstrated ectopic branch root patterning stimulated in response to damage by the feeding larvae. These findings correlated with increased IAA levels and with
increased expression of early auxin response genes. Furthermore, elevated levels of salicylic acid and abscisic acid were noted in these tissues relative to controls, though no change
was found in jasmonic acid. Using radiographic imaging we also observed hotspots of 11C-photosynthate, indicating sites where clusters of lateral root re-growth were stimulated in
damaged roots. The role of hormonal cross-talk in regulating root growth
Auxin is a major regulator of many developmental processes including root architecture through a complicated signal transduction network. We know that SA acts on the auxin signaling network by stabilizing the AUX/IAA repressor proteins, which inhibits auxin signaling in the process. Downstream auxin signaling is considered to feed back upstream to auxin biosynthetic genes maintaining a homeostatic state. Even so, it is not known whether SA will affect upstream auxin homeostasis, which is maintained not only by biosynthetic production, but by metabolic turnover (including catabolism and conjugation) as well as by physical transport. Our research here explores the physiological and metabolic basis for IAA and SA hormone cross-talk in root tissues. Some of recent studies have shown that treatment of roots with physiological doses of one or the other hormone will result in very different root system architecture. Future studies will focus on whether one hormone will impact the homeostatic functions of the other hormone. Aspects of this research also have relevance in plant-microorganism associations where many bacteria in these associations can actually produce auxin. Auxin regulation in the growth of large grass stemsSince stress resistance often negatively correlates with growth rates, efforts to improve hardiness of bioenergy crops will need to be balanced with an effort to maintain growth rates in stress resistant plants, guided by an understanding of how growth is controlled. Although gibberellins are key stem growth regulators that were important in the green revolution, recent research suggests that auxin also plays a key role, and represents another potential tool to manipulate stem growth (e.g., Knoller et al., 2010; Multani et al., 2003). Understanding how these multiple signals are integrated to determine plant growth rates and stem architecture will be a critical to engineering plants for more efficient food and energy production.
These static snapshots by autoradiography show that 11C-Indole-acetic acid administered to Zea mays and Nicotiana tabacum leaf tips for 1 hr was transported down through the vascular
tissue of the leaves. The mechanisms controlling whole plant source-sink relationsIt is important to understand how photosynthate is delivered to the various sink tissues within the plant. Coordinated allocation of resources amongst sink tissues, including the shoot apical meristem, nodes, stems, root apical meristems, and lateral roots is important for proper plant development, impacting biomass yield and plant hardiness. We anticipate the need to manipulate source-sink relations as a strategy to improve energy crop hardiness while maintaining productivity, which will require greater depth of knowledge of (i) the mechanisms of transport, and (ii) how source and sink functions are coordinated.
Additionally, we have made several observations in grasses that suggest that vascular transport from source to sink tissues is not through one continuous pipeline, but may involve a
series of checkpoints and transfers between discrete pipelines. These include slower transport speeds of 11C-photosynthate in stems and roots than in leaves (A), non-uniformity in
transport in sorghum leaf sheaths (B), likely related to the collar structure where the leaf blade and leaf sheath meet (D), and high 11C-photosynthate in nodes (C), where the vasculature
of the leaf sheaths insert into the stem, suggesting that sugars are withdrawn from the phloem at the nodes and re-loaded, perhaps into different phloem vessels for further re-distribution.
We hypothesize that sites of apparent discontinuity could represent control points for both resource and signal molecule distribution, employing mechanisms such as those found in transfer
cells to determine how much of each hormone and photosynthate is re-directed to the various sink tissues. Functional genomics of sugar content in sweet sorghum stems
Last Modified: February 14, 2013 |
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These studies imply a link between
nitrogen status and auxin homeostasis and signaling, where root growth inhibition by high nitrate was found closely related to a reduction in IAA levels in roots.
Because alteration of plant nutrient levels can have undesirable effects, we are using a molecular approach to “fool” the plant into perceiving a low nitrogen status.
Our collaborators at U. Arizona (E. Herman and M. Schmidt) have reduced free asparagine in Nicotiana tabacum by overexpressing asparaginase (ASPG), the key enzyme
that catalyzes the degradation of L-asparagine to L-aspartic acid and ammonia. We showed that new carbon utilization in carbohydrate formation (measured by C-11 fixed as 11CO2)
was unaffected by ASPG overexpression, but that old carbon likely from protein turnover was reprogrammed into greater carbohydrate accumulation and increased root biomass.
This growth behavior was mimicked in subsequent studies by growing plants under elevated IAA. Further, sites of high metabolic activity correlated with sites of high
auxin signaling as reflected by the high uptake of the [11C]IAA tracer in lateral root primordia and root meristem . We also established using 11C-precursors of auxin that the roots
themselves were a major source of the auxin stimulating lateral root re-growth. This was further substantiated by metabolic assays which showed increased concentrations of the auxin
biosynthetic precursor tryptophan in damaged root tissues. However, our studies show a disparity between auxin biosynthetic rates and free pools of auxin across zones of roots.
We believe the up-regulation of homeostatic processes align with regions of high auxin signaling, but not necessarily with the free pools of the hormone.
Our work here is to better understand how root
architecture, root function, and auxin homeostasis are interwoven, but from the perspective that auxin signaling does not occur in isolation, but in concert with other plant hormones.
For example, there is evidence that salicylic acid (SA), widely known as a pathogen response signal, can be involved in growth and development in response to pathogens and abiotic stressors.
There is also evidence linking auxin signaling with plant defense as some of our later work on root worm herbivory demonstrates.
Most studies of auxin have focused on small plants or plant parts (e.g., coleoptiles), which have provided invaluable insights to signal transduction mechanisms, but polar transport is
extremely slow relative to the potential size of large plants. Furthermore, the widely utilized methods of measuring auxin transport rely on a static snapshot of radio-labeled auxin
distribution after a period of incubation, which can leave much ambiguity in interpretation. With PET isotopes, we can now conduct dynamic imaging of [11C]auxin transport, and high
sensitivity assays of auxin biosynthesis, which will provide a more complete perspective on where auxin is synthesized and how it is transported in large grasses.
The pressure gradient that drives phloem sap by bulk flow from photosynthetic source tissues to sink tissues (Munch, 1930) is generated by loading of osmolytes, mainly sucrose, into the
phloem in the source, and unloading of sucrose in the sinks. Thus, phloem functionality in most plants should be vulnerable to changes in sucrose loading and unloading. Ongoing studies
of sucrose transporter mutants are providing insights to the mechanisms that maintain sap flow through the phloem to move resources such as carbohydrates and some signaling molecules
throughout the whole plant.
Enhancing the production and conversion of plant feedstocks to utilizable sources of energy will decrease our dependence on fossil fuels while reducing greenhouse gas emissions.
Bioenergy C4 grasses, such as sorghum, will be essential components in the U.S. energy portfolio. Sweet sorghum combines rapid growth, high biomass, and wide adaptability to avoid
usage of prime lands needed for food crops, with a chemical composition well suited to biofuels production. Sweet sorghum is similar to grain sorghum in growth phenotype, except for
reduced grain, taller stature, and hyperaccumulation of sucrose in stem tissues, which provides a rich second generation feedstock for energy-efficient conversion to ethanol. We
hypothesize that sucrose accumulation can be further improved if we understand the mechanisms regulating carbon allocation specifically to stems. Our approach includes a combination
of molecular genetics using an existing sweet x grain sorghum RIL population, genomics targeting sucrose transporters, biochemical phenotyping, and detailed physiological and radio-tracer
studies to identify bioenergy-relevant genes and to understand their functions in carbon partitioning in sweet sorghum. The knowledge gained and new genetic resources developed from our
efforts will make significant contributions to strategies for improving fermentable sugar content of sweet sorghum, as well as other bioenergy grasses, for sustainable biofuels production.
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