		Louis Irving's page
contact to : l.j.irving@biochem.tohoku.ac.jp

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2008 – Present JSPS Post-doctoral Fellow, Graduate School of Agricultural Science, Tohoku University, Japan

2005 – 2007 Massey University Post-doctoral Fellow, Institute of Natural Resources, Massey University, New Zealand

2001 – 2004 PhD Student, School of Biological Sciences (Plant and Soil Science), University of Aberdeen, UK

1997 – 2001 BSc Biology (Hons), Department of Biological Sciences, Dundee University, UK


My research falls into 5 main themes;
Regulation of Rubisco protein concentration and turnover
Plant responses to stress
Transport processes in vascular plants
Plant Parasitism
Mathematical modeling of plant systems – seed germination

Regulation of Rubisco protein concentration and turnover

Rubisco is the enzyme responsible for CO2 fixation in photosynthesis. In cereal leaves Rubisco comprises approximately a quarter to a third of leaf nitrogen, and is encoded by the nuclear rbcS and chloroplast rbcL genes. Rubisco is primarily synthesized during leaf expansion, reaching its maximal concentration just before full leaf expansion, then falling through leaf senescence prior to death.

While basic patterns of synthesis and degradation have been known for some time, the mechanisms, regulation and plasticity of responses are not well understood. My current work aims to explore the effects of genetics and nitrogen supply on rbc gene expression, and the effects of shading on protein degradation rates.

Relevant papers;

“A dynamic model of Rubisco turnover in cereal leaves” LJ Irving & D Robinson (2006) New Phytologist

Plant responses to stress

Lucerne (Medicago sativa) is a widely used legume for animal fodder, due to its high nutritive and rapid growth rate. Lucerne is notably drought tolerant due to its deep tap root; however, lucerne is waterlogging intolerant. Waterlogging is a common problem in many areas where lucerne is cultivated, for example in the Yellow river basin in China.

Past work has looked at the reductions in photosynthetic rates in waterlogged lucerne plants. This concluded that during the autumn, nitrogen deficiency may cause increased senescence and be the primary factor limiting growth. However, in a second experiment conducted in the spring, the lucerne plants exhibited much higher rates of senescence and death than the first experiment. We hypothesize that the plants suffered higher levels of photo-oxidative damage in the second experiment, and we are currently conducting further trials to test this hypothesis.

In a second theme, we recently investigated the role of anthocyanins in providing protection against photo-oxidative stress in petunia plants. Pentunia control (MP) and anthocyanin overexpressing (Lc) plants were grown under either low (250 mmol photons m-2 s-1) or high (700 mmol photons m-2 s-1) light conditions for one week.
Picture from Albert et al. (2009).

At the start of the experiment the photosynthetic characteristics of all the plants were identical; however, after a week the anthocyanin overexpressing plants had turned deep purple at high light, while relatively little colouration was noted in the low light plants. High light plants had maximum photosynthetic rates approximately twice those of low light plants. At high light, the anthocyanin overexpressing plants required significantly higher irradiances to reach this maximum, however, suggesting that anthocyanins act as a light shield under photo-oxidative stress conditions.

Relevant papers;

“Physiological effects of waterlogging on two lucerne varieties grown under glasshouse conditions” LJ Irving, Y Sheng, D Wooley & C Matthew (2007) Journal of Agronomy and Crop Science.

“Light induced vegetative anthocyanin pigmentation in Petunia” NW Albert, DH Lewis, H Zhang, LJ Irving, PE Jamieson, & KM Davies (2009). Journal of Experimental Botany.

Transport processes in vascular plants.

Due to their sessile nature plants need to transport nutrients and sugars between organs. Sugars are produced in mature leaves, which can then be used as an energy source either for growing new leaves, new stems or for root maintenance and development. Sugars are transported primarily in the phloem, while nitrogen and water taken up by the roots is transported via the xylem. Main stems, young primary tillers and old primary tillers of guinea grass were 14C labeled to study the transfer of carbon between organs. Surprisingly, young tillers supplied a large proportion of the root carbon during the day, although appeared to represent a carbon sink at night. Older tillers supplied much less carbon to the other plant parts, suggesting reduced vascular connectivity in these organs.This may have implications for nitrogen remobilization and yield, as well as growth dynamics in other grasses. Significant differences were noted between morphologically distinct varieties.
From Carvalho et al. (2006). The bold triangle represents the labeled tiller. Triangle size is indicative of tiller size, and the numbers represent the percentage of radiocarbon localized to that tiller class with errors.

Reference;

“Distribution of current photosynthate in two Guinea grass (Panicum maximum Jacq.) cultivars.” DD Carvalho, LJ Irving, RA Carnevalli, J Hodgson & C Matthew (2006). Journal of Experimental Botany.

Plant parasitism.

There are approximately 3000 species of parasitic plant worldwide. Parasitic plants derive carbon, nitrogen, water and other resources from their host plant typically reducing the growth and fecundity of the host. Parasites can be broadly split into two groups, phloem feeders and xylem feeders.

Xylem feeders typically derive water and nitrogen from their hosts, along with other things like potassium, phosphate, and hormones. Xylem feeders often gain about 20% of their carbon from the host plant, although this is typically as amino acids.

Phloem feeders gain essentially all their resources, including carbon from their host plants, and are often incapable of photosynthesis.

From a plant physiological perspective, xylem feeding plants often act as a “dead branch”, drawing water and N from the host, while phloem feeders can impose a significant carbon deficit on the host plant, with both of these having profound effects on the host plant physiology and biology, and making them an interesting system for study. Recently, we showed that host choice can have a significant effect on both parasitic plant and host plant photosynthetic rates.

References;

“Suppression of host photosynthesis by the parasitic plant Rhinanthus minor”. DD Cameron, JM Geniez, WE Seel & LJ Irving (2008). Annals of Botany.

“You are what you eat; Interactions between root parasitic plants and their hosts.” LJ Irving & DD Cameron (2009). Advances in Botanical Research.

Mathematical modeling of plant systems – seed germination

During my time at Massey University a Chinese PhD student, HongXiang Zhang, visited Massey for 5 months. HongXiang was interested in the effects of salt on seed germination rates. She had a small dataset she had collected in China, which suggested that the germination of salt-treated seeds could not be well modeled u sing the current hydrothermal time models. We conducted a large experiment to diffuse the effects of temperature, salt and osmotic potential using barley. As well as being a halophytic cereal, able to tolerate relatively salty environment, barley has large seeds, allowing us to measure the salt contents of individual seeds.

We were able to show that seeds in salt solution were able to germinate faster and at lower water potentials than seeds exposed to osmotically equivalent PEG solutions.Furthermore, analysis of the seeds showed that the salt exposed seeds contained higher levels of salt than the PEG controls, and that the amount of salt in each seed correlated with the solution in which the seeds were incubated. The seeds were taking the salt up and using it as an osmoticum in order to germinate in environments where they would be otherwise unable to.

Germination times and rates were likewise affected by temperature. Increased temperature led to increased germination rates; however, the final germination percentages decreased with increasing temperatures.

Since entering Tohoku University I have initiated a collaboration with Professor Izumi Takagi and Associate Professor Yuu Hariya at Tohoku University Maths Department, working on a new mathematical model of seed water uptake and germination. Experiments are ongoing, and we have been further able to expand our descriptions, for example demonstrating that small seeds within a population take water up and germinate faster than large seeds, especially under osmotic stress. This, coupled with the distribution of seed size within a population gives rise to the classic S-shaped (sigmoidal) germination curves through time. We anticipate several publications from this line of work in the next few months.

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Due to their sessile nature plants need to transport nutrients and sugars between organs. Sugars are produced in mature leaves, which can then be used as an energy source either for growing new leaves, new stems or for root maintenance and development. Sugars are transported primarily in the phloem, while nitrogen and water taken up by the roots is transported via the xylem. Main stems, young primary tillers and old primary tillers of guinea grass were 14C labeled to study the transfer of carbon between organs. Surprisingly, young tillers supplied a large proportion of the root carbon during the day, although appeared to represent a carbon sink at night. Older tillers supplied much less carbon to the other plant parts, suggesting reduced vascular connectivity in these organs.This may have implications for nitrogen remobilization and yield, as well as growth dynamics in other grasses. Significant differences were noted between morphologically distinct varieties. From Carvalho et al. (2006). The bold triangle represents the labeled tiller. Triangle size is indicative of tiller size, and the numbers represent the percentage of radiocarbon localized to that tiller class with errors.