Assembly of LHCA5 into PSI blue shifts the far-red fluorescence emission in higher plants.

In higher plants, the PSI core complex is associated with light-harvesting complex I (LHCI), forming the PSI-LHCI super-complex. In vascular plants, four major antenna proteins (LHCA1-4) are assembled in the order of LHCA1, LHCA4, LHCA2, and LHCA3 into a crescent-shaped LHCI, while LHCA5 and LHCA6 are minor antenna proteins. By contrast, in moss and green algae, LHCA5 or LHCA5-like protein functions as one of the major antenna proteins by residing at the second site of LHCI. In order to learn the effect of binding different LHCA proteins, i.e. LHCA4 or LHCA5, within the PSI-LHCI super-complex on photosynthetic properties of plants, we constructed LHCA5 overexpression plants with a wild type (WT) background and an lhca4 mutant background in Arabidopsis thaliana. The results showed that: (i) there are little difference in phenotype, pigment composition and chlorophyll fluorescence parameters between the transgenic Arabidopsis and their corresponding background materials; (ii) in spite of a small amount of LHCA5, the LHCA5-included PSI-LHCI super-complex can be obtained by extracting samples incubated with anti-FLAG M2 Affinity Gel, in which LHCA5 is found to substitute for LHCA4 as analyzed by immunoblotting analysis; (iii) the replacement of LHCA4 with LHCA5 within PSI-LHCI super-complex leads to a blue shift in low temperature fluorescence emission, suggesting a decrease in far-red absorbance. These results provide new clues for understanding the position and function of LHCA4 and LHCA5 during the evolution of green plants from aquatic to terrestrial lifestyles.

LHCA4 residues surrounding red chlorophylls allow for fine-tuning of the spectral region for photosynthesis in Arabidopsis thaliana.

Improving far-red light utilization could be an approach to increasing crop production under suboptimal conditions. In land plants, only a small part of far-red light can be used for photosynthesis, which is captured by the antenna proteins LHCAs of photosystem I (PSI) through the chlorophyll (Chl) pair a603 and a609. However, it is unknown how the energy level of Chls a603-a609 is fine-tuned by the local protein environment in vivo. In this study, we investigated how changing the amino acid ligand for Chl a603 in LHCA4, the most red-shifted LHCA in Arabidopsis thaliana, or one amino acid near Chl a609, affected the energy level of the resulting PSI-LHCI complexes in situ and in vitro. Substitutions of the Chl a603 ligand N99 caused a blue shift in fluorescence emission, whereas the E146Q substitution near Chl a609 expanded the emission range to the red. Purified PSI-LHCI complexes with N99 substitutions exhibited the same fluorescence emission maxima as their respective transgenic lines, while the extent of red shift in purified PSI-LHCI with the E146Q substitution was weaker than in the corresponding transgenic lines. We propose that substituting amino acids surrounding red Chls can tune their energy level higher or lower in vivo, while shifting the absorption spectrum more to the red could prove more difficult than shifting to the blue end of the spectrum. Here, we report the first in vivo exploration of changing the local protein environment on the energy level of the red Chls, providing new clues for engineering red/blue-shifted crops.