On phytochrome absorption and the phytochrome photoequilibrium in a green leaf: environmental sensitivity and photoequilibrium time

The average, corrected attenuance spectra for both spectral forms of phytochrome in a mature leaf were calculated. Optical masking by chlorophyll together with the detour effect (optical path lengthening effect) due to multiple light scattering led to large changes in both the Qy band shape and wavelength position and the effective intensity of the weak vibrational bands increases. The Pfr/Pr oscillator-strength-ratio between 400-750 nm (0.93 in vitro), becomes 1.63 in a leaf. Thus the dominant absorption form is Pfr. These two values permit calculation of the phytochrome photoequilibrium under conditions of daylight illumination both in vitro and in folia. These values are 0.6 and 0.38 respectively. Previous literature estimates for the situation in vitro, based on the 660/730 nm absorption ratio, yielded values close to 0.6. It is demonstrated that this large decrease in the phytochrome photoequilibrium in a leaf has the effect of translating this parameter to a position on the dose (red/far-red light ratio)-response (Pfr/Ptot) plot towards greater sensitivity to changes in the environmental red/far-red ratio. The increased sensitivity factor is almost five-fold for the daylight environment and is even greater for the various shade-light environments. The approximate time taken to attain photoequilibrium (1/e lifetime) has also been calculated for phytochrome in a leaf in different light environments. For the daylight environment the photoequilibration time is 5 s, which increases into the 20-80 s interval under different degrees of shade light. Thus, despite the strong optical masking by chlorophyll in a mature leaf, the phytochrome photoequilibrium is attained quite rapidly on a physiological time scale

In Silico and Biochemical Analysis of Physcomitrella patens Photosynthetic Antenna: Identification of Subunits which Evolved upon Land Adaptation

Background. In eukaryotes the photosynthetic antenna system is composed of subunits encoded by the light harvesting complex (Lhc) multigene family. These proteins play a key role in photosynthesis and are involved in both light harvesting and photoprotection. The moss Physcomitrella patens is a member of a lineage that diverged from seed plants early after land colonization and therefore by studying this organism, we may gain insight into adaptations to the aerial environment. Principal Findings. In this study, we characterized the antenna protein multigene family in Physcomitrella patens, by sequence analysis as well as biochemical and functional investigations. Sequence identification and analysis showed that some antenna polypeptides, such as Lhcb3 and Lhcb6, are present only in land organisms, suggesting they play a role in adaptation to the sub-aerial environment. Our functional analysis which showed that photo-protective mechanisms in Physcomitrella patens are very similar to those in seed plants fits with this hypothesis. In particular, Physcomitrella patens also activates Non Photochemical Quenching upon illumination, consistent with the detection of an ortholog of the PsbS protein. As a further adaptation to terrestrial conditions, the content of Photosystem I low energy absorbing chlorophylls also increased, as demonstrated by differences in Lhca3 and Lhca4 polypeptide sequences, in vitro reconstitution experiments and low temperature fluorescence spectra. Conclusions. This study highlights the role of Lhc family members in environmental adaptation and allowed proteins associated with mechanisms of stress resistance to be identified within this large family

Photoinhibition in vivo and in vitro involves weakly coupled chlorophyll-protein complexes

In the present study the analysis of the relation between the excited state population in the photosystem 11 (PSII) antenna and photoinactivation has been extended from an in vitro system, isolated thylakoids, to an in vivo system, Chlamydomonas reinhardtii cells. The results indicate that the excited state quenching by an added singlet quencher induces maximal protection against photoinhibition of about 30% of that expected on the basis of the observed light intensity-treatment time reciprocity rule. Similar results, obtained previously with thylakoids, have been interpreted in terms of damaged or incorrectly assembled complexes that play an important role in photoinhibition in the thylakoid membranes (Santabarbara, S., K. Neverov, F. M. Garlaschi, G. Zucchelli and R. C. Jennings [2001] Involvement of uncoupled antenna chlorophylls in photoinhibition in thylakoids. FEBS Lett. 491, 109-113.). In an attempt to better define this aspect, the photoinhibition action spectra were determined for mutant barley thylakoids, lacking the chlorophyll (Chl) a-b complexes of the outer antenna, and for its wild type. The results indicate that in both systems the action spectra are significantly blueshifted (2-4 nm) and are broader than the PSII absorption in the membranes. These data are interpreted in terms of a heterogeneous population of outer and inner antenna pigment-protein complexes that contain significant levels of uncoupled Chl

Light absorption by the chlorophyll a-b complexes of photosystem II in a leaf with special reference to LHCII

To investigate the light-harvesting properties of the Photosystem 11 chlorophyll (chl) a-b complexes (major light-harvesting complex of Photosystem 11 [LHCII], CP24, CP26, CP29) in a mature leaf under natural "daylight" illumination, the absorption spectra of the isolated complexes were converted into the photon absorption spectrum (I - T) within a leaf, using the approach of Rivadossi et al. ([1999] Photosynth. Res. 60, 209-215). In the Q(y) region, significant enhancement of light harvesting by the chl b electronic transitions, with respect to the absorption spectra (optical density [01)]), as well as a large and generalized increase (between two- and four-fold) associated with the vibrational bands of both chi a and b, was observed, which acquires an important light-harvesting role (approximately 30-40% of total). In the Soret region, a small increase in light harvesting by chi b was indicated. To gain more detailed information on these aspects the light harvesting of LHCII in a leaf was investigated. This required describing the pigment absorption (chi a and b, carotenoids) in the LHCII OD spectrum in terms of spectral subbands, which were subsequently used to estimate the relative light harvesting of each pigment type in LHCII of a leaf. When the entire visible spectral interval between 400 and 730 nm is considered, the chi a light harvesting is essentially unchanged with respect to the absorption spectrum (01)) of isolated LHCII, whereas the chi b contribution is 20% higher and the carotenoids are 33% lower. The relative enhancement of the chi b absorption is principally associated with the Q(y) electronic transition region, the light-harvesting contribution of which becomes prominent in the leaf

Revised sequence of the alpha 1B adrenergic receptor promoter region.

GenBank sequence AY53019