Nodulin 26-like intrinsic proteins (NIPs) are a family of channel-forming transmembrane proteins that function in the transport of water and other small molecules.  Some NIPs can mediate silicon transport across plasma membranes and lead to silicon accumulation in plants, which is beneficial for the growth and development of plants.  Cucumber is one of the most widely consumed vegetables; however, the functions of NIPs in this crop are still largely unknown.  Here, we report the functional characteristics of CsNIP2;2.  It was found that CsNIP2;2 is a tandem repeat of CsNIP2;1, which had been demonstrated to be a silicon influx transporter gene.  CsNIP2;2 has a selectivity filter composed of cysteine, serine, glycine and arginine (CSGR), which is different from all previously characterized silicon influx transporters in higher plants at the second helix position.  Xenopus laevis oocytes injected with CsNIP2;2 cRNA demonstrated a higher uptake of silicon than the control, and the uptake remained unchanged under low temperature.  CsNIP2;2 was found to be expressed in the root, stem, lamina and petiole, and exogenous silicon treatment decreased its expression in the stem but not in other tissues.  Transient expression of CsNIP2;2-eGFP fusion sequence in onion epidermal cells showed that CsNIP2;2 was localized to the cell nucleus, plasma membrane and an unknown structure inside the cell.  The results suggest that CsNIP2;2 is a silicon influx transporter in cucumber, and its subcellular localization and the selectivity filter are different from those of the previously characterized silicon influx transporters in other plants.  These findings may be helpful for understanding the functions of NIPs in cucumber plants.

Tomato (Solanum lycopersicum) is a model plant for research on fruit development and stress response, in which gene expression analysis is frequently conducted.  Quantitative PCR (qPCR) is a widely used technique for gene expression analysis, and the selection of reference genes may affect the accuracy of results and even conclusions.  Although there have been some frequently used reference genes in tomato, it has been shown that the expressions of some of these genes are not constant in different tissues and environmental conditions.  Moreover, little information on genomic identification of reference genes is available in tomato.  Here, we mined the publicly available transcriptional sequencing data and screened out fifteen candidate reference genes, and the expression stability of these candidate genes and seven traditionally used ones were evaluated under stress and hormone treatment.  The results showed that over half of the selected candidate references were housekeeping genes in tomato cells.  Among the candidate reference genes and the traditionally used ones, the most stably expressed genes varied under different treatments, and most of these genes were recommended as preferred reference genes at least once except Solyc04g009030 and Solyc07g066610, two traditionally used reference genes.  This study provides some novel reference genes in tomato, and the preferred reference genes under different environmental stimuli, which may be useful for future research.  Our study suggests that excavating stably expressed genes from transcriptome sequencing data is a reliable approach to screening reference genes for qPCR analysis.  

Silicon can improve drought tolerance of plants, but the mechanism still remains unclear.  Previous studies have mainly concentrated on silicon-accumulating plants, whereas less work has been conducted in silicon-excluding plants, such as tomato (Solanum lycopersicum L.).  In this study, we investigated the effects of exogenous silicon (2.5 mmol L^–1) on the chlorophyll fluorescence and expression of photosynthesis-related genes in tomato seedlings (Zhongza 9) under water stress induced by 10% (w/v) polyethylene glycol (PEG-6000).  The results showed that under water stress, the growth of shoot and root was inhibited, and the chlorophyll and carotenoid concentrations were decreased, while silicon addition improved the plant growth and increased the concentrations of chlorophyll and carotenoid.  Under water sterss, chlorophyll fluorescence parameters such as PSII maximum photochemical efficiency (F_v/F_m), effective quantum efficiency, actual photochemical quantum efficiency (Ф_PSII), photosynthetic electron transport rate (ETR), and photochemical quenching coefficient (q_P) were decreased; while these changes were reversed in the presence of added silicon.  The expressions of some photosynthesis-related genes including PetE, PetF, PsbP, PsbQ, PsbW, and Psb28 were down-regulated under water stress, and exogenous Si could partially up-regulate their expressions.  These results suggest that silicon plays a role in the alleviation of water stress by modulating some photosynthesis-related genes and regulating the photochemical process, and thus promoting photosynthesis.