Fibrillin (FBN) is a plastid lipid-associated protein found in photosynthetic organisms from cyanobacteria to plants. In this study, 10 CsaFBN genes were identified in genomic DNA sequences of cucumber (Chinese long and Gy14) through database searches using the conserved domain of FBN and the 14 FBN genes of Arabidopsis. Phylogenetic analysis of CsaFBN protein sequences showed that there was no counterpart of Arabidopsis and rice FBN5 in the cucumber genome. FBN5 is essential for growth in

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... dopsis and rice; its absence in cucumber may be because of incomplete genome sequences or that another FBN carries out its functions. Among the 10 CsaFBN genes, CsaFBN1 and CsaFBN9 were the most divergent in terms of nucleotide sequences. Most of the CsaFBN genes were expressed in the leaf, stem and fruit. CsaFBN4 showed the highest mRNA expression levels in various tissues, followed by CsaFBN6, CsaFBN1 and CsaFBN9. High-light stress combined with low temperature decreased photosynthetic efficiency and highly induced transcript levels of CsaFBN1, CsaFBN6 and CsaFBN11, which decreased after 24 h treatment. Transcript levels of the other seven genes were changed only slightly. This result suggests that CsaFBN1, CsaFBN6 and CsaFBN11 may be involved in photoprotection under high-light conditions at low temperature. Plants 2018, 7, 50 2 of 13 anther, chromoplasts in petal and chloroplasts in leaf of oilseed rape (Brassica rapa), which were named PAPs [14] . The fibrillin isolated from pea (Pisum sativum) chloroplasts were named plastoglobulin (PGL) [2] . Recently, it has been suggested that these same proteins, which are called by different names, such as fibrillin, PAP and PGL, should be renamed as fibrillin (FBN) [15] . The FBN protein family has a relatively broad range of molecular weights (21-42 kDa) and isoelectric points (pI; 4-9), presumably because these proteins have a variety of biological functions [16, 17] . In addition, FBNs are present in plastids, including chromoplasts, chloroplasts, elaioplasts, etioplasts and green algal chloroplast eyespots. All 14 of the FBNs in Arabidopsis are predicted to have a plastid transit peptide [16] . It was shown that different FBNs have specific functions in different plastid types and characteristic plastid structures [15, 17] . However, there are few studies on the biological functions of FBN family proteins. Deruere et al. [8] demonstrated fibril reconstitution when FBN was added to a mixture of carotenoids and polar lipids. This finding indicates that FBN structurally functions in fibril assembly. In addition, FBN is either directly or indirectly related to the development of plastoglobules (PG), chromoplast pigment accumulation, hormone responses, protective photosynthetic mechanisms during light inhibition and resistance to biotic/abiotic stresses [1, 4, 12, 13, [18] [19] [20] [21] . There are 14 FBN proteins in Arabidopsis and 7 were PG associated with thylakoids [17]. Vidi et al. [22] reported that large quantities of FBN7a (At3g58010) and FBN1a (At4g04020) were physically associated with PG in fractionation experiments. In addition, GFP fusions of these proteins were used to examine their location in the cell and a punctuate fluorescence pattern was observed in the chloroplast, which is consistent with PG-related function [22] . Through quantitative proteomics, Lundquist et al. [17] showed that FBN protein accounts for 53% of PG proteome mass. Therefore, FBN proteins generally maintain plastid lipid body structure. FBN1a, FBN1b and FBN2 were shown to function in photoprotection, mediated by ABA [20] and modulate jasmonate biosynthesis under cold stress and high light stress [21] . In Arabidopsis and apple, FBN4 is involved in biotic stress response and is related to plastoquinone content [18, 19] . Recently, Arabidopsis and rice FBN5 were shown to be an essential component, along with solenesyl diphosphate synthase (SPS), in the synthesis of the solenesyl diphosphate (SPP) tail of plastoquinone-9 [23, 24] . This list of the representative functions of FBN show that FBNs are involved in photosynthesis and hormone-mediated photoprotection but their specific functions still need to be studied. Cucumber (Cucumis sativus L.) is a major, economically important crop belonging to the Cucurbitaceae family. Cucumber production is 192 million tons, with a land area of 9 million hectares (http://faostat.fao.org). Plants of the Cucurbitaceae family are used as model plants for studies on sex determination and vascular biology [25, 26] . The first genome sequence of Cucumber was completed in 2009 [25] and in 2013, 115 wild-type cucumber genomes were sequenced, allowing comparative studies of domestication and cultivar diversity [27] . The completion of the genome made possible genome-wide analyses of multigene families of transcription factors [28] [29] [30] [31] [32] and powdery mildew MLO genes [33] . Cucumbers are susceptible to chilling damage because of their subtropical origin, which occurs between 0-10 • C. Chilling affects the vegetative parts as well as fruit maturity. Chilling has special effects under high-light conditions. When the temperature is reduced from 25 to 10 degrees under high-light conditions, photosynthetic efficiency drops rapidly [34, 35] . It has been reported that the low-temperature sensitivity of cucumber is related to the degree of saturation of the fatty acids in the chloroplast membrane, as low temperature exposure results in decreased photosynthesis efficiency [36, 37] . However, the genes involved in photoprotection under high light stress in cucumber have not been reported [38] . FBNs were recently proposed to be necessary for optimal photosynthesis [39] . To study whether the FBN family is well conserved in cucumber and their possible function during photoinhibition of photosynthesis under chilling stress, we identified the FBN genes in the cucumber genome databases and investigated their expression in various tissues and under high light conditions.