The document discusses approaches to improving photosynthesis efficiency through genetic engineering. It describes how a USDA research team discovered an enzyme that governs the rate of carbon dioxide absorption in leaves and used genetic engineering to create an altered version of this enzyme that has higher activity. It also discusses research at Cornell University where they genetically engineered a tobacco plant to replace its natural carbon-fixing enzyme, Rubisco, with a faster version from cyanobacteria. Potential genetic engineering targets discussed to further improve photosynthesis include transferring genes between species to optimize carbon fixation pathways, engineering improved versions of Rubisco, or replacing the entire carbon fixation cycle.

1. Assignment -5

2. Improving Photosynthesis by Genetic
Engineering
William L. Ogren, research leader, USDA/ARS, and professor of agronomy
 In photosynthesis, a plant uses sunlight to remove carbon dioxide from the atmosphere
and converts it into sugar and starch. The plant uses these for growth of seeds, fruits,
tubers, or other plant parts of economic value, such as leaves, roots, and stems. Because
crop productivity increases when photosynthesis occurs more rapidly, much research is
directed toward improving this process.
 One approach being taken by a USDA research team at the University of Illinois
Department of Agronomy is to speed up the first step of photosynthesis. Five years ago,
this team discovered an enzyme that governs the rate at which leaves absorb carbon
dioxide. The gene that determines the structure of this enzyme, and thus the enzyme's
activity, was then obtained. Using genetic engineering, several different changes were
made in the gene to alter the properties of its enzyme product.
 It cannot be predicted how changes in enzymes will affect their activity. Because enzymes
have highly organized structures, changes usually result in less activity. In this case,
however, one of the altered enzymes had more activity than normal. Attempts are now
being made to incorporate the modified genes into plants, using standard methods of
genetic transformation. Following transformation, photosynthesis measurements of the
transgenic plants will be made. Such tests will determine the possibility of improved
plant productivity by modifying this component of photosynthesis.

3. A genetically engineered tobacco plant,
developed with two genes from blue-green algae
(cyanobacteria) at CORNELL UNIVERSITY
.
 Plants photosynthesize – convert carbon dioxide, water and light into oxygen and
sucrose, a sugar used for energy and for building new plant tissue – but cyanobacteria
can perform photosynthesis significantly more quickly than many crops can.
 “This is the first time that a plant has been created through genetic engineering to fix all
of its carbon by a cyanobacterial enzyme,” said Maureen Hanson, a co-author of the
study and Liberty Hyde Bailey Professor of Plant Molecular Biology at Cornell.
 “It is an important first step in creating plants with more efficient photosynthesis,”
Hanson said.
 The study is published Sept. 17 in the journal Nature. Myat Lin, a postdoctoral
fellow in Hanson’s lab, and Alessandro Occhialini, a scientist at the U.K.’s
Rothamsted Research, are co-lead authors of the study.
 Crops with cyanobacteria’s faster carbon fixation would produce more, according to a
computer modeling study by Justin McGrath and Stephen Long at the University of
Illinois. Producing more crops on finite arable land is a necessity as the world’s
population is projected to pass nine billion by 2050.
 Though others have tried and failed, the Cornell and Rothamsted researchers have
successfully replaced the gene for a carbon-fixing enzyme called Rubisco in a tobacco
plant with two genes for a cyanobacterial version of Rubisco, which works faster than the
plant’s original enzyme.

4.  All plants require Rubisco to fix carbon during photosynthesis. Rubisco reacts
with both carbon dioxide and oxygen in the air, but when it reacts with oxygen,
a plant’s rate of photosynthesis slows down, leading to lower yields.
 In many crop plants, including tobacco, Rubisco is less reactive with oxygen,
but a trade-off leads to slower carbon fixing and photosynthesis, and thus,
smaller yields. The Rubisco in cyanobacteria fixes carbon faster, but it is more
reactive with oxygen. As a result, in cyanobacteria, Rubisco is protected in
special micro-compartments (called carboxysomes) that keep oxygen out and
concentrate carbon dioxide for efficient photosynthesis.
 In previous research, Lin, Hanson and colleagues inserted blue-green algae
genes in tobacco to create carboxysomes in the plant cells. In future work, the
researchers will need to combine genes for cyanobacterial Rubisco with genes
for carboxysomes in the tobacco’s chloroplasts, the site in the cell where
photosynthesis takes place.
 Co-authors include Martin Parry, a professor of plant biology, and researcher
John Andralojc, both at Rothamsted Research. The study was funded by the
National Science Foundation, the Biotechnology and Biological Sciences
Research Council, the National Institutes of Health and the 20:20 Wheat
Institute Strategic Program.

5. Approaches in improving photosynthesis
efficiency

6. Potential Targets for Genetic Engineering
 One obvious route to the improvement of carbon fixation is the transfer of genes from
one species to another.
 Attempts to introduce C4 photosynthesis or other carbon-concentrating systems into C3
plants, engineer improved versions of RuBisCO, or even replace the entire Calvin-Benson
cycle, are ongoing (reviewed in: Blankenship et al., 2011; Langdale, 2011).
 But how can the light reactions of photosynthesis in plants benefit from replacing
components by their counterparts from other species? The most pronounced differences
between the light reactions in photoautotrophic oxygen-evolving organisms reside in
their light-harvesting antenna systems.
 cyanobacteria, glaucophytes, and red algae contain the aforementioned
phycobilisomes, whereas land plants use LHCs. To enhance light-harvesting
indirectly, research efforts are underway in crop plants to enable more light to
penetrate to lower levels of the canopy. These involve either modifying plant
architecture or decreasing chlorophyll content (reviewed in: Blankenship et al.,
2011).
 The introduction of prokaryotic pigments that absorb further into the near-infrared
(Blankenship et al., 2011), or even entire prokaryotic light-harvesting systems (to
supplement or replace LHCs), into plants might make it possible to expand the
absorption spectrum of photosynthesis and thus increase photosynthetic efficiency at
low-light levels.
 Moreover, the potential of genetic engineering is not restricted to the
modification or replacement of LHCs. The removal or overexpression of single
components of the photosynthetic light reactions can improve the efficiency of
photosynthesis, at least under certain conditions.

7.  Overexpression of either plastocyanin, the soluble electron transporter that reduces
photosystem I (PSI), or its algal substitute cytochrome c6, increases biomass in A. thaliana ).
Similarly, increased phosphorylation of thylakoid proteins, achieved by inactivating the
thylakoid phosphatase TAP38, improves photosynthetic electron flow under certain light
conditions (Pribil et al., 2010). This indicates that simple single-gene genetic engineering
of photosynthetic light reaction might have practical applications.
 Moreover, these findings, together with the observation that manipulation of photosynthetic
carbon fixation inArabidopsis (by introducing a prokaryotic glycolate catabolic pathway to
bypass photorespiration) also increases biomass accumulation (Kebeish et al., 2007), argue
strongly against the idea that plants are limited in their sink, but not in their source, capacity
(reviewed in: Kant et al., 2012). Nevertheless, it is clear that neutralization of the feedback
mechanisms that down-regulate photosynthesis when sink capacity becomes limiting, and
increasing sink capacity per se, are both necessary to get the most out of improved
photosynthetic light reactions.
 Hence, genetic engineering of the light reactions of photosynthesis should focus primarily on
modifying light-harvesting and regulators of photosynthetic electron flow, as well as on
increasing the sink capacity of plants to cope with an enhanced photosynthetic rate. The
resulting plants should exhibit more efficient photosynthesis under controlled conditions, e.g.,
in greenhouses, or in regions that cannot otherwise be extensively used for agriculture because
of their short growing seasons.