Photoinactivation of photosystem II in leaves.

Photoinactivation of Photosystem II (PS II), the light-induced loss of ability to evolve oxygen, inevitably occurs under any light environment in nature, counteracted by repair. Under certain conditions, the extent of photoinactivation of PS II depends on the photon exposure (light dosage, x), rather than the irradiance or duration of illumination per se, thus obeying the law of reciprocity of irradiance and duration of illumination, namely, that equal photon exposure produces an equal effect. If the probability of photoinactivation (p) of PS II is directly proportional to an increment in photon exposure (p = kDeltax, where k is the probability per unit photon exposure), it can be deduced that the number of active PS II complexes decreases exponentially as a function of photon exposure: N = Noexp(-kx). Further, since a photon exposure is usually achieved by varying the illumination time (t) at constant irradiance (I), N = Noexp(-kI t), i.e., N decreases exponentially with time, with a rate coefficient of photoinactivation kI, where the product kI is obviously directly proportional to I. Given that N = Noexp(-kx), the quantum yield of photoinactivation of PS II can be defined as -dN/dx = kN, which varies with the number of active PS II complexes remaining. Typically, the quantum yield of photoinactivation of PS II is ca. 0.1micromol PS II per mol photons at low photon exposure when repair is inhibited. That is, when about 10(7) photons have been received by leaf tissue, one PS II complex is inactivated. Some species such as grapevine have a much lower quantum yield of photoinactivation of PS II, even at a chilling temperature. Examination of the longer-term time course of photoinactivation of PS II in capsicum leaves reveals that the decrease in N deviates from a single-exponential decay when the majority of the PS II complexes are inactivated in the absence of repair. This can be attributed to the formation of strong quenchers in severely-photoinactivated PS II complexes, able to dissipate excitation energy efficiently and to protect the remaining active neighbours against damage by light.

Molecular and functional characterization of CaLEA6, the gene for a hydrophobic LEA protein from Capsicum annuum.

We used differential screening to isolate a full-length dehydration-responsive cDNA clone encoding a hydrophobic late embryogenesis abundant (LEA)-like protein from PEG-treated hot pepper leaves. Named CaLEA6 (for Capsicum annuum LEA), this gene belongs to the atypical hydrophobic LEA Group 6. The full-length CaLEA6 is 709 bp long with an open reading frame encoding 164 amino acids. It is predicted to produce a highly hydrophobic, but cytoplasmic, protein. The putative M(r) of CaLEA6 protein is 18 kDa, with a theoretical pI of 4.63. Based on our Southern blot analysis, CaLEA6 appears to exist as a small gene family. CaLEA6 was not expressed prior to any treatment, but its transcript was rapidly and greatly increased following trials with PEG, ABA, and NaCl. Chilling also induced its rapid induction, but to a much lesser extent. Accumulation of CaLEA6 protein occurred soon after NaCl applications, but considerably delayed after treatment with PEG. Tobacco plants that overexpressed CaLEA6 showed enhanced tolerance to dehydration and NaCl but not to chilling, as defined by their leaf fresh weights, Chl contents, and the general health status of the leaves. Therefore, we suggest that CaLEA6 protein plays a potentially protective role when water deficit is induced by dehydration and high salinity, but not low temperature.

Cold-inducible transcription factor, CaCBF, is associated with a homeodomain leucine zipper protein in hot pepper (Capsicum annuum L.).

C-Repeat/drought responsive element binding factor (CBF1/DREB1b) is a well known transcriptional activator that is induced at low temperature and in turn induces the CBF regulon (CBF-targeted genes). We have cloned and characterized two CBF1-like cDNAs, CaCBF1A and CaCBF1B, from hot pepper. CaCBF1A and CaCBF1B were not produced in response to mechanical wounding or abscisic acid but were induced by low-temperature stress at 4 degrees . When plants were returned to 25 degrees , their transcript levels of the CBF1-like genes decreased markedly within 40 min. Long-term exposure to chilling resulted in continuous expression of these genes. The critical temperature for induction of CaCBF1A was between 10 and 15 degrees . Low temperature led to its transcription in roots, stems, leaves, fruit without seeds, and apical meristems, and a monoclonal antibody against it revealed a significant increase in CaCBF1A protein by 4 h at 4 degrees . Two-hybrid screening led to the isolation of an homeodomain leucine zipper (HD-Zip) protein that interacts with CaCBF1B. Expression of HD-Zip was elevated by low temperature and drought.

Putative effects of pH in intra-chloroplast compartments on photoprotection of functional photosystem II complexes by photoinactivated neighbours and on recovery from photoinactivation in Capsicum annuum leaves.

Leaf segments from Capsicum annuum L. plants grown at 100 (low light) or 500 (high light) µmol photons m-2 s-1 were illuminated in the presence of nigericin, dithiothreitol (DTT), or high [CO2] (1% in air), with or without lincomycin, an inhibitor of chloroplast-encoded protein synthesis. At various times, the remaining fraction (f ) of functional PSII complexes was measured by a dark-adapted chlorophyll fluorescence parameter (1/Fo- 1/Fm; where Fo and Fm are the fluorescence yields corresponding to open and closed PSII traps, respectively), which was calibrated by the oxygen yield per saturating single-turnover flash. The results were interpreted according to a simple kinetic model incorporating the hypothesis that photoinactivated PSII complexes photoprotect functional neighbours (Lee et al. 2001, Planta 105, 377-384), yielding the rate coefficients of photoinactivation and repair, and a parameter, a, which phenomenologically describes the effectiveness of photoprotection by photoinactivated PSII complexes. The presence of the uncoupler nigericin during illumination greatly decreased a by an order of magnitude, suggesting that a sufficiently acidic thylakoid lumen may be required for the photoprotective mechanism to operate. Both nigericin and high [CO2] decreased the rate coefficient of repair several fold, suggesting that the stromal pH was non-optimal for protein synthesis in the presence of nigericin or high [CO2]. The xanthophyll cycle, inhibited by DTT, seemed to have a minimal effect on the rate coefficients of photoinactivation and repair, and on the parameter a. The results underline the importance of optimal pH in both the stroma and lumen for photoprotection, and recovery from photoinactivation of PSII.