Pitayas (Hylocereus spp.) is an attractive, highly nutritious and commercially valuable tropical fruit. However, low-temperature damage limits crop production. Genome of pitaya has not been sequenced yet. In this study, we sequenced the transcriptome of pitaya as the reference and further investigated the proteome under low temperature. By RNAseq technique, approximately 25.3 million reads were obtained, and further trimmed and assembled into 81,252 unigene sequences. The unigenes were searched against UniProt, NR and COGs at NCBI, Pfam, InterPro and Kyoto Encyclopedia of Genes and Genomes (KEGG) database, and 57,905 unigenes were retrieved annotations. Among them, 44,337 coding sequences were predicted by Trandecoder (v2.0.1), which served as the reference database for label-free proteomic analysis study of pitaya. Here, we identified 116 Differentially Abundant Proteins (DAPs) associated with the cold stress in pitaya, of which 18 proteins were up-regulated and 98 proteins were down-regulated. KEGG analysis and other results showed that these DAPs mainly related to chloroplasts and mitochondria metabolism. In summary, chloroplasts and mitochondria metabolism-related proteins may play an important role in response to cold stress in pitayas.

Pitaya (Hylocereus undatus Britton & Rose) is widely cultivated in subtropical and tropical regions. Pitaya is cold-sensitive; most cultivars are injured during chilling winter periods, especially in subtropical regions. In this study, the effects of shading on the cold tolerance in pitaya plant were investigated. Pitaya plants were grown under full sunlight (control) or a shading net with a light blocking rate of about 60%. Morphological and physiological performance of the overwintering pitaya plants were compared between the control and the shaded treatment. The results showed that shading treatment markedly reduced the chilling induced tissue necrosis. Contents of chlorophyll a, chlorophyll b, and total chlorophylls (chlorophyll a+b), the chlorophyll a/b ratio, Fm and Fv/Fm were all higher in shading treatment than those in the control. Meanwhile, proline content and ascorbic acid peroxidase (APX) activity in the shading treatment were significantly increased, while malondialdehyde (MDA), the superoxide anion (O2.-) and hydrogen peroxide (H2O2) levels were significantly decreased by shading treatment. The results indicated that damage caused by chilling stress on pitaya was, at least partially, light-dependent; and in practical production, shading treatment can be used to reduce chilling injury in pitaya.

In this study, the physiological response of the annual branches of three varieties of pitaya (Xianmi, Fulong, and Zihonglong) in cold stress was investigated using a multivariate statistical method. Physiological change results showed that cold stress could decrease the moisture and chlorophyll contents, on the contrary, increase the relative electric conductivity, the contents of malonadehyde, soluble protein, soluble sugar, and free proline, and enhance the enzyme activities of peroxidase, superoxide dismutase, and catalase. Meanwhile, a comparative proteomic approach was also conducted to clarify the cold resistance-related proteins and pathways in annual pitaya branches. Proteomics results concluded that the cold tolerance of annual pitaya branches could be improved by modulating autophagy. Therefore, we hypothesized that an increased autophagy ability may be an important characteristic of the annual pitaya branches in response to cold stress conditions. Our results provide a good understanding of the physiological responses and molecular mechanisms of the annual pitaya branches in response to cold stress.