This recent research article in Science has received a decent amount of attention for good reason given the possible impact it could have on crop productivity through increasing photosynthetic efficiency. However, the approach to increasing efficiency in this paper varies considerably from the efforts to transport C4 photosynthesis into C3 crops more regularly seen.
We have a good understanding of the working of photosynthesis and its use of photons and excited electrons to fix carbon. But built into this system is a protection mechanism that kicks in when the intensity of light is too great for the CO2 fixation capacity of the photosystems, a damaging state for the plant to be in. When the excitation energy is too great, the energy is dissipated as heat, a process called nonphotochemical quenching of chlorophyll fluorescence (NPQ).
At high light intensity, NPQ is a useful process. However, NPQ at light intensities lower than that which could cause damage to the delicate photosynthetic components results in a reduction in CO2 fixation. When a leaf goes from high light intensity to low light intensity, NPQ reduces accordingly. However, the transition of NPQ lags behind the transition of the leaf from high to low light intensity, resulting in a temporary reduction in CO2 fixation and, therefore, plant growth. Earlier research indicated that these losses were in the range of 7.5 to 30% of fixation rates.
These researchers therefore tested the possibility of increasing the speed with which the transition from photoprotection to full resumption of carbon fixation could occur.
NPQ results from a conformational change to the photosystem II antennae from an unquenched to quenched state, which results in the excess excitation energy being dissipated. NPQ levels correlate with the amount of photosystem II subunit S (PsbS) and the flux of xanthophyll cycle. While over-expression of PsbS in plants can enhance NPQ and photoprotection as well as increase the rate of change between quenched and unquenched states, overexpression can also reduce CO2 fixation rates when light intensity is at a level lower than that which could cause damage. Further, the lag between transition from quenched to unquenched states due to the effects of PsbS on NPQ is minimal (10 to 90 seconds) compared to the to the same transition when NPQ is triggered by zeaxanthin (10 to 15 minutes), a product of the xanthophyll cycle.
Figure 1 from article showing the factors affecting NPQ under different light intensity.
Therefore, the researchers looked at whether adjusting the xanthophyll cycle could assist the transition rate of NPQ. Specifically, they hypothesised whether accelerating the cycle and simultaneously increasing PsbS would result in a faster reduction of NPQ when leaves transition from high to low light intensity.
The researchers transformed tobacco plants with Arabidopsis-derived sequences for violaxanthin de-epoxidase, zeaxanthin epoxidase and PsbS and promoters for their expression in leaves. Transcript and protein levels of the three sequences were shown to be increased in the transformed plants compared to the wild type controls. Leaves of transformed and wild type plants were then subjected to fluctuating intensities of light. The NPQ relaxation rate due to the altered xanthophyll cycle increased significantly compared to the wild type, having an average relaxation time of 753 seconds compared to 2684 seconds in the wild type while relaxation due to the additional PsbS expression decreased from 21 to 15 seconds on average.
The recovery of CO2 assimilation was also analysed under the same fluctuating light. After transitioning from high to low light the CO2 assimilation decreased and was at a minimum 30 seconds after the transition before increasing again as the photoprotection relaxed. The rate of CO2 fixation increased faster in the transgenic lines with 9% higher fixation rates compared to the wild type tobacco plants.
Further testing the effects of the overexpressed genes on CO2 fixation, the researchers looked at the variation in CO2 fixation rates in response to variations in light. Two tests were performed: vary light intensity leaving enough time at each intensity to allow the fluorescence and gas exchange to reach a steady state, and varying light intensity every 4 minutes.
During the steady state experiment, the maximum CO2 fixation didn’t vary between the transgenic and wild type plants (both averaging 0.092 CO2 per absorbed photon), indicating that the overexpression didn’t effect the photosynthetic capacity generally as was seen in previous experiments.
Figure 4 from article showing CO2 fixation per photon, quantum yield per photon and NPQ levels in the wild type and the three transgenic lines.
Under the alternating light experiment the CO2 fixed per absorbed photon was decreased compared to the steady state experiment but was greater in the transgenic lines (0.066CO2/photon) than the wild type (0.058CO2/photon), an 11.3% increase. Similar findings were observed in relation to quantum yield of whole-chain electron transport.
Plants grown under field conditions showed the same differences in fixation rates. Further, a randomised block design consisting of 12 blocks was used to test the agronomic performance of experimental and control lines, with 14 to 20% greater dry weight observed in the transgenic lines compared to the wild type with noted increases in leaf, stem and root weights and leaf area.
Finally, whether the transgenic lines may suffer from altered photoprotection under high intensity was tested in seedlings. After 2 hours being exposed to excessive light the photoprotection appeared to be similar or higher in transgenic lines.
In the discussion section the researchers point out that under field conditions an individual chloroplast can be subjected to instantaneous and repetitive changes from high to low light conditions due to shading from other parts of the same plant or from nearby plants. The ability to reduce the response time of the photoprotective system will significantly assist crop productivity.
Further, stomatal conductance increases under high light conditions and remains so for minutes after transferring back to shade, resulting in excess water loss compared to when the leaves receive less than a harmful level of light. Speeding the relaxation of NPQ, the researchers point out, should also result in better water use efficiency.
This results of this research are really novel and demonstrate just how complex and evolved the photosynthesis machinery is. The xanthophyll cycle and PsbS are found in vascular plants, leaving open the possibility of transferring these faster transition rates to important crops.