Differential control of xanthophylls and light-induced stress proteins, as opposed to light-harvesting chlorophyll a/b proteins, during photosynthetic acclimation of barley leaves to light irradiance

Abstract

Barley (Hordeum vulgare L.) plants were grown at different photon flux densities ranging from 100 to 1800 &mgr;mol m-2 s-1 in air and/or in atmospheres with reduced levels of O2 and CO2. Low O2 and CO2 partial pressures allowed plants to grow under high photosystem II (PSII) excitation pressure, estimated in vivo by chlorophyll fluorescence measurements, at moderate photon flux densities. The xanthophyll-cycle pigments, the early light-inducible proteins, and their mRNA accumulated with increasing PSII excitation pressure irrespective of the way high excitation pressure was obtained (high-light irradiance or decreased CO2 and O2 availability). These findings indicate that the reduction state of electron transport chain components could be involved in light sensing for the regulation of nuclear-encoded chloroplast gene expression. In contrast, no correlation was found between the reduction state of PSII and various indicators of the PSII light-harvesting system, such as the chlorophyll a-to-b ratio, the abundance of the major pigment-protein complex of PSII (LHCII), the mRNA level of LHCII, the light-saturation curve of O2 evolution, and the induced chlorophyll-fluorescence rise. We conclude that the chlorophyll antenna size of PSII is not governed by the redox state of PSII in higher plants and, consequently, regulation of early light-inducible protein synthesis is different from that of LHCII.

Figure 1

PSII excitation pressure, as measured by V, in barley leaves exposed to different PFDs in different atmospheres (▴, air; ▵, 70 μg mL⁻¹ CO₂ and 10% O₂; □, 20 μg mL⁻¹ CO₂ and 2% O₂).

Figure 2

Xanthophyll-cycle carotenoids pool (V+A+Z) normalized to the total chlorophyll content or the total carotenoid content ([V+A+Z]/chl, ▴; [V+A+Z]/car, ▪) as a function of the PSII excitation pressure (V) obtained by varying the PFD in air (closed symbols) or by decreasing the CO₂ and O₂ content of the atmosphere (70 μg mL⁻¹ CO₂ and 10% O₂, 750 μmol m⁻² s⁻¹, open symbols). The data were calculated from the data of Table I.

Figure 3

Plots of the photoacoustically monitored Φ in barley leaves grown under different PSII excitation pressures induced by different light and gas environments versus the PFD of the incident light. ▵, Air and 400 μmol photons m⁻² s⁻¹; □, air and 1500 μmol photons m⁻² s⁻¹; ▵, 70 μg mL⁻¹ CO₂ and 10% O₂ at 750 μmol photons m⁻² s⁻¹. Plants were grown for 8 d in the different light and gas environments.

Figure 4

Changes in LHCII abundance after transfer of barley plants from air and low light (100 μmol m⁻² s⁻¹) to air plus strong light (1500 μmol m⁻² s⁻¹) or to low CO₂ and O₂ (50 μg mL⁻¹ CO₂ and 3% O₂) in moderate light (500 μmol m⁻² s⁻¹). The results are expressed relative to the LHCII level before the transfer to the new growth regimes. Lanes on the gels were loaded with equal protein content and were probed with anti-LHCIIb antibodies. The LHCII abundance was measured after 3 and 7 d of growth under the new conditions. Data are mean values of three experiments with sd being lower than 15% for all values.

Figure 5

Maximal level of LHCII mRNA and levels of ELIP mRNA and protein in barley leaves grown for 1 (white bars) or 3 (black bars) d under low or high PSII excitation pressure in air or in 50 μg mL⁻¹ CO₂ plus 3% O₂. Maximum quantum yield for PSII photochemistry (F_m − F_o)/F_m: 0.79 ± 0.01 after 3 d in air at 100 μmol photons m⁻² s⁻¹; 0.81 ± 0.01 after 3 d at 100 μmol photons m⁻² s⁻¹ in low CO₂ and O₂; 0.69 ± 0.02 after 3 d in air at 1500 μmol photons m⁻² s⁻¹; and 0.74 ± 0.02 after 3 d at 500 μmol photons m⁻² s⁻¹ in low CO₂ and O₂. Results are expressed relative to the highest value. Data are mean values of three experiments, with sd being always less than 15%.