Clones coding for the two small early light‐inducible proteins (ELIP)‐gene families of 13.5 and 17 kDa have been used as markers to study the effect of high‐light intensities on gene expression in cultivars of barley which were bred for growth in Southern and Northern Europe. The mRNA levels of the light‐harvesting chlorophyll a/b protein (LHC‐II) and the small subunit of ribulose‐1,5‐bisphosphate carboxylase (SSU) were determined in addition. These data were correlated to the decay of PSII activity during high‐light stress and its recovery. In all cultivars, the induction of ELIP mRNAs by high light was accompanied by a correspondent reduction of the LHC‐II mRNA level. Furthermore, the LHC‐II mRNA levels observed under low‐light conditions used for the growth of the plants, were in all cases found to be inversely related to the amount of the ELIP which could be induced by a high‐light treatment. In contrast, the amount of the SSU mRNA was reduced only at the highest investigated light intensity of 2 000 μmol m−2 s−1. During recovery from light‐stress, the activity of PSII was quickly restored in all European cultivars. Of these cultivars, however, the cv. Otis which expressed the highest ELIP levels recovered considerably faster than the cultivar p4266N which accumulated the lowest amounts of ELIP under high light. Thus, it appears likely that ELIPs contribute to the restoration of PSII activity during and after photoinhibition.
The appearance of the light harvesting II (LHC II) protein in etiolated bean leaves, as monitored by immunodetection in LDS-solubilized leaf protein extracts, is under phytochrome control. A single red light pulse induces accumulation of the protein, in leaves kept in the dark thereafter, which follows circadian oscillations similar to those earlier found for Lhcb mRNA (Tavladoraki et al. (1989) Plant Physiol 90: 665-672). These oscillations are closely followed by oscillations in the capacity of the leaf to form Chlorophyll (Chl) in the light, suggesting that the synthesis of the LHC II protein and its chromophore are in close coordination. Experiments with levulinic acid showed that PChl(ide) resynthesis does not affect the LHC II level nor its oscillations, but new Chl a synthesis affects LHC II stabilization in thylakoids, implicating a proteolytic mechanism. A proteolytic activity against exogenously added LHC II was detected in thylakoids of etiolated bean leaves, which was enhanced by the light pulse. The activity, also under phytochrome control, was found to follow circadian oscillations in verse to those in the stabilization of LHC II protein in thylakoids. Such a proteolytic mechanism therefore, may account for the circadian changes observed in LHC II protein level, being implicated in pigment-protein complex assembly/stabilization during thylakoid biogenesis.
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