Aquaporins (AQPs) are members of a superfamily of integral membrane proteins and play a significant role in the transportation of small molecules across membranes. However, currently little is known about the AQP genes in Chinese cabbage (Brassica rapa ssp. pekinensis). In this study, a genome-wide analysis was carried out to identify the AQP genes in Chinese cabbage. In total, 53 non-redundant AQP genes were identified that were located on all of the 10 chromosomes. The number of AQP genes in Chinese cabbage was greater than in Arabidopsis. They were classified into four subfamilies, including PIP, TIP, NIP, and SIP. Thirty-three groups of AQP orthologous genes were identified between Chinese cabbage and Arabidopsis, but orthologs corresponding to AtNIP1;1 and AtPIP2;8 were not detected. Seventeen groups of paralogous genes were identified in Chinese cabbage. Three-dimensional models of the AQPs of Chinese cabbage were constructed using Phyre2, and ar/R selectivity filters were analyzed comparatively between Chinese cabbage and Arabidopsis. Generally, gene structure was conserved within each subfamily, especially in the SIP subfamily. Intron loss events have occurred during the evolution of the PIP, TIP, and NIP subfamilies. The expression of AQP genes in Chinese cabbage was analyzed in different organs. Most AQP genes were downregulated in response to salt stress. This work shows that the AQP genes of Chinese cabbage have undergone triplication and subsequent biased gene loss.

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Circular RNA (circRNA) is a newly discovered non-coding RNA, which play significant roles in the function and transcriptional regulation of microRNA. To date, in Chinese cabbage, the functional characteristic of circRNAs in response to calcium deficiency-induced tip-burn have not been reported. In this study, 730 circRNAs were isolated from Chinese cabbage leaves, of which 23 and 22 were differentially expressed in different calcium deficiency stages compared with the control. Forty-six host genes of the differentially expressed circRNAs were identified, and one circRNA was found to act as miRNAs sponges. Based on the functional analysis of host genes and target mRNAs of the corresponding miRNAs, the identified circRNAs might participated in response to stimulus, electron carrier activity, ATPase activity, cell wall metabolism, transcription factors and plant hormone signal transduction. ABF2, a positive regulator of the abiotic stress response in the abscisic acid (ABA) pathway, may play a role in calcium deficiency tolerance through a circRNA regulatory pathway. Correspondingly, the concentration of ABA is also increased during the Ca2+ deficiency stress. Our results suggest that circRNAs participate in a broad range of biological processes and physiological functions in the response to calcium deficiency-induced tip-burn and provide a basis for further studies of the biological roles that circRNAs play in the plant stress response.

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