Abstract
There are a number of highly conserved photosystem II light-harvesting antenna proteins in moss whose functions are unclear. Here, we investigated the involvement of chlorophyll-binding proteins, Lhcb6 and Lhcb5, in light-harvesting and photosynthesis regulation in Physcomitrella patens. Lhcb6 or Lhcb5 knock-out resulted in a disordered thylakoid arrangement, a decrease in the number of grana membranes, and an increase in the number of starch granule. The absence of Lhcb6 or Lhcb5 did not noticeably alter the electron transport rates. However, the non-photochemical quenching activity in the lhcb5 mutant was dramatically reduced when compared to wild-type or lhcb6 plants under abiotic stress. Lhcb5 plants were more sensitive to photo-inhibition, while lhcb6 plants showed little difference compared to the wild-type plants under high-light stress. Moreover, both mutants showed a growth malformation phenotype with reduced chlorophyll content in the gametophyte. These results suggested that Lhcb6 or Lhcb5 played a unique role in plant development, thylakoid organization, and photoprotection of PSII in Physcomitrella, especially when exposed to high light or osmotic environments.
Highlights
To harvest solar energy efficiently, photosynthetic organisms use groups of light-harvesting antenna proteins that bind carotenoids and chlorophylls
The photosystem (PS) II outer antenna consists of different heterotrimers of Lhcb1, Lhcb2, and Lhcb3, which is encoded by Lhcb1, Lhcb2, and Lhcb3 genes, while minor antenna complexes, such as chlorophyll protein 29 (CP29), CP26, and CP24 are encoded by Lhcb4, Lhcb5, and Lhcb6, respectively [1]
There are two mechanisms that regulate the absorption of light energy from Lhc: (1) the light-harvesting antennas quench excess harvested light, which is known as non-photochemical quenching (NPQ) [5,6]; (2) state transition, which is a process associated with PSII
Summary
To harvest solar energy efficiently, photosynthetic organisms use groups of light-harvesting antenna proteins that bind carotenoids and chlorophylls In higher plants, these proteins are composed of an inner and outer antenna. Other minor antennas, such as Lhcb and Lhcb, participate in the xanthophyll cycle, which can alleviate light damage [12,13,14,15]. The PSI protein super-complex component Lhcb was reported to change the absorption properties of PSII by harboring red-shifted chlorophyll [18,19] Physcomitrella contains both LhcSR (light-harvesting complex stress-related) and PsbS (PSII subunit S) [20]. Our results indicated that minor antenna protein Lhcb and Lhcb of Physcomitrella played significant roles in the function and structure of PSII, especially under abiotic stress
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