The early light-induced proteins (ELIPs) belong to the multigenic family of light-harvesting complexes, which bind chlorophyll and absorb solar energy in green plants. ELIPs accumulate transiently in plants exposed to high light intensities. By using an Arabidopsis thaliana mutant (chaos) affected in the posttranslational targeting of light-harvesting complex-type proteins to the thylakoids, we succeeded in suppressing the rapid accumulation of ELIPs during high-light stress, resulting in leaf bleaching and extensive photooxidative damage. Constitutive expression of ELIP genes in chaos before light stress resulted in ELIP accumulation and restored the phototolerance of the plants to the wild-type level. Free chlorophyll, a generator of singlet oxygen in the light, was detected by chlorophyll fluorescence lifetime measurements in chaos leaves before the symptoms of oxidative stress appeared. Our findings indicate that ELIPs fulfill a photoprotective function that could involve either the binding of chlorophylls released during turnover of pigment-binding proteins or the stabilization of the proper assembly of those proteins during high-light stress.L ight is essential for plants through photosynthetic carbon assimilation. However, when absorbed light exceeds the photosynthetic capacities, reactive O 2 species are generated in the chloroplasts, causing oxidative damage to proteins, lipids, and photosynthetic pigments (1, 2). This effect is amplified by environmental stresses such as low temperature or drought, for example, that inhibit the photosynthetic activity, leading to strong yield reduction in crops. In green plants, solar energy is collected by chlorophyll-and carotenoid-binding lightharvesting complexes (LHCs), which are encoded by a multigene family of LHC genes. The expression of these genes is tightly regulated by light (2-4). High light intensities inhibit transcription of LHC genes and activate synthesis of the early lightinduced proteins (ELIPs), a class of proteins structurally related to the LHCs (5). The ELIPs are predicted to have three transmembrane helices, and they have sequence similarity to the LHCs in the central pair of helices (6, 7). The similarity is not only at the sequence level, however, because both LHCs and ELIPs bind chlorophyll and carotenoids (8). The ELIPs differ from the LHCs by their transient expression under high-light stress (5). Recently, a number of ELIP-type polypeptides, containing LHC motifs and inducible by high light, have been discovered in vascular plants: the one-helix high-light-induced proteins (9) and the two-helix stress-enhanced proteins (10).The physiological role of the ELIPs in vascular plants has not yet been elucidated, although there have been several suggestions (11)(12)(13)(14). The induction of ELIPs by high light intensities suggests a role in the acclimation to light stress rather than a light-harvesting function, but this has not yet been demonstrated. ELIP antisense transgenic tobacco plants did not exhibit any phenotype of sensitivity to high ...

SummaryBiochemical and genetic studies have established that the light-harvesting chlorophyll proteins (LHCPs) of the photosystems use the cpSRP (chloroplast signal recognition particle) pathway for their targeting to thylakoids. Previous analyses of single cpSRP mutants, chaos and ffc, de®cient in cpSRP43 and cpSRP54, respectively, have revealed that half of the LHCPs are still integrated into the thylakoid membranes. Surprisingly, the effects of both mutations are additive in the double mutant ffc/chaos described here. This mutant has pale yellow leaves at all stages of growth and drastically reduced levels of all the LHCPs except Lhcb 4. Although the chloroplasts have a normal shape, the thylakoid structure is affected by the mutation, probably as a consequence of reduction of all the LHCPs. ELIPs (early lightinducible proteins), nuclear-encoded proteins related to the LHCP family and inducible by light stress, were also drastically reduced in the double mutant. However, proteins targeted by other chloroplastic targeting pathways (DpH, Sec and spontaneous pathways) accumulated to similar levels in the wild-type and the double mutant. Therefore, the near total loss of LHCPs and ELIPs in the double mutant suggests that cpSRP is the predominant, if not exclusive, targeting pathway for these proteins. Phenotypic analysis of the double mutant, compared to the single mutants, suggests that the cpSRP subunits cpSRP43 and cpSRP54 contribute to antenna targeting in an independent but additive way.

The chloroplast signal recognition particle (cpSRP) and its receptor (cpFtsY) target proteins both cotranslationally and posttranslationally to the thylakoids. This dual function enables cpSRP to utilize its posttranslational activities for targeting a family of nucleus-encoded light-harvesting chlorophyll binding proteins (LHCPs), the most abundant membrane proteins in plants. Previous in vitro experiments indicated an absolute requirement for all cpSRP pathway soluble components. In agreement, a cpFtsY mutant in Arabidopsis thaliana exhibits a severe chlorotic phenotype resulting from a massive loss of LHCPs. Surprisingly, a double mutant, cpftsy cpsrp54, recovers to a great extent from the chlorotic cpftsy phenotype. This establishes that in plants, a new alternative pathway exists that can bypass cpSRP posttranslational targeting activities. Using a mutant form of cpSRP43 that is unable to assemble with cpSRP54, we complemented the cpSRP43-deficient mutant and found that this subunit is required for the alternative pathway. Along with the ability of cpSRP43 alone to bind the ALBINO3 translocase required for LHCP integration, our results indicate that cpSRP43 has developed features to function independently of cpSRP54/cpFtsY in targeting LHCPs to the thylakoid membranes.