Light Quality and Sucrose-Regulated Detached Ripening of Strawberry with Possible Involvement of Abscisic Acid and Auxin Signaling.

The regulation of detached ripening is significant for prolonging fruit shelf life. Although light quality and sucrose affecting strawberry fruit ripening have been widely reported, little information is available about how they co-regulate the strawberry detached ripening process. In this study, different light qualities (red light-RL, blue light-BL, and white light-WL) and 100 mM sucrose were applied to regulate the ripening of initial red fruits detached from the plant. The results showed RL-treated samples (RL + H2O, RL + 100 mM sucrose) had brighter and purer skin color with a higher L*, b*, and C* value, and promoted the ascorbic acid. Almost all light treatments significantly decreased TSS/TA (total soluble solid/titratable acid) and soluble sugar/TA ratio, which is exacerbated by the addition of sucrose. Blue or red light in combination with sucrose notably increased total phenolic content and decreased malondialdehyde (MDA) accumulation. In addition, blue or red light combined with sucrose increased abscisic acid (ABA) content and promoted ABA signaling by inducing ABA-INSENSITIVE 4 (ABI4) expression and inhibiting SUCROSE NONFERMENTING1-RELATED PROTEIN KINASE 2.6 (SnRK2.6) expression. The strawberries exposed to blue and red light significantly improved auxin (IAA) content compared to the control (0 d), whereas the addition of sucrose inhibited IAA accumulation. Moreover, sucrose treatment suppressed the AUXIN/INDOLE-3-ACETIC ACID 11 (AUX/IAA11) and AUXIN RESPONSE FACTOR 6 (ARF6) expression under different light qualities. Overall, these results indicated that RL/BL + 100 mM sucrose might promote the detached ripening of strawberries by regulating abscisic acid and auxin signaling.

Genome-Wide Analysis of the ERF Family and Identification of Potential Genes Involved in Fruit Ripening in Octoploid Strawberry.

Ethylene response factors (ERFs) belonging to the APETALA2/ERF superfamily acted at the end of the ethylene signaling pathway, and they were found to play important roles in plant growth and development. However, the information of ERF genes in strawberry and their involvement in fruit ripening have been limited. Here, a total of 235 ERF members were identified from 426 AP2/ERF genes at octoploid strawberry genome level and classified into six subgroups according to their sequence characteristics and phylogenetic relationship. Conserved motif and gene structure analysis supported the evolutionary conservation of FaERFs. Syntenic analysis showed that four types of duplication events occurred during the expansion of FaERF gene family. Of these, WGD/segmental duplication played a major role. Transcriptomic data of FaERF genes during fruit ripening and in response to abscisic acid screened one activator (FaERF316) and one repressor (FaERF118) that were involved in fruit ripening. Transcriptional regulation analysis showed some transcription factors related to ripening such as ABI4, TCP15, and GLK1 could bind to FaERF316 or FaERF118 promoters, while protein-protein interaction analysis displayed some proteins associated with plant growth and development could interact with FaERF118 or FaERF316. These results suggested that FaERF118 and FaERF316 were potential genes to regulate strawberry ripening. In summary, the present study provides the comprehensive and systematic information on FaERF family evolution and gains insights into FaERF's potential regulatory mechanism in strawberry ripening.

Identification of the Cytosolic Glucose-6-Phosphate Dehydrogenase Gene from Strawberry Involved in Cold Stress Response.

Glucose-6-phosphate dehydrogenase (G6PDH) plays an important role in plant stress responses. Here, five FaG6PDH sequences were obtained in strawberry, designated as FaG6PDH-CY, FaG6PDH-P1, FaG6PDH-P1.1, FaG6PDH-P2 and FaG6PDH-P0, which were divided into cytosolic (CY) and plastidic (P) isoforms based on the bioinformatic analysis. The respective FaG6PDH genes had distinct expression patterns in all tissues and at different stages of fruit development. Notably, FaG6PDH-CY was the most highly expressed gene among five FaG6PDH members, indicating it encoded the major G6PDH isoform throughout the plant. FaG6PDH positively regulated cold tolerance in strawberry. Inhibition of its activity gave rise to greater cold-induced injury in plant. The FaG6PDH-CY transcript had a significant increase under cold stress, similar to the G6PDH enzyme activity, suggesting a principal participant in response to cold stress. Further study showed that the low-temperature responsiveness (LTR) element in FaG6PDH-CY promoter can promote the gene expression when plant encountered cold stimuli. Besides, FaG6PDH-CY was involved in regulating cold-induced activation of antioxidant enzyme genes (FaSOD, FaCAT, FaAPX and FaGR) and RBOH-dependent ROS generation. The elevated FaG6PDH-CY enhanced ROS-scavenging capability of antioxidant enzymes to suppress ROS excessive accumulation and relieved the oxidative damage, eventually improving the strawberry resistance to cold stress.

Genome-Wide Characterization of Snf1-Related Protein Kinases (SnRKs) and Expression Analysis of SnRK1.1 in Strawberry.

The plant sucrose nonfermenting 1 (SNF1)-related protein kinases (SnRKs) are key regulators in the interconnection of various signaling pathways. However, little is known about the SnRK family in strawberries. In this study, a total of 26 FvSnRKs including one FvSnRK1, nine FvSnRK2s and 16 FvSnRK3s were identified from the strawberry genome database. They were respectively designated as FvSnRK1.1, FvSnRK2.1 to FvSnRK2.9 and FvSnRK3.1 to FvSnRK3.16, according to the conserved domain of each subfamily and multiple sequence alignment with Arabidopsis. FvSnRK family members were unevenly distributed in seven chromosomes. The number of exons or introns varied among FvSnRK1s, FvSnRK2s and FvSnRK3s, but highly conserved in the same subfamily. The FvSnRK1.1 had 10 exons. Most of FvSnRK2s had nine exons or eight introns, except FvSnRK2.4, FvSnRK2.8 and FvSnRK2.9. FvSnRK3 genes were divided into intron-free and intron-harboring members, and the number of introns in intron-harboring group ranged from 11 to 15. Moreover, the phylogenetic analysis showed SnRK1, SnRK2 and SnRK3 subfamilies respectively clustered together in spite of the different species of strawberry and Arabidopsis, indicating the genes were established prior to the divergence of the corresponding taxonomic lineages. Meanwhile, conserved motif analysis showed that FvSnRK sequences that belonged to the same subgroup contained their own specific motifs. Cis-element in promoter and expression pattern analyses of FvSnRK1.1 suggested that FvSnRK1.1 was involved in cold responsiveness, light responsiveness and fruit ripening. Taken together, this comprehensive analysis will facilitate further studies of the FvSnRK family and provide a basis for the understanding of their function in strawberry.

Variability in CitXET expression and XET activity in Citrus cultivar Huangguogan seedlings with differed degrees of etiolation.

Considering the known effects of xyloglucan endotransglycosylase (XET) on plant growth and development, we aimed to determine whether XETs help to regulate the growth and elongation of Huangguogan shoots and roots. We confirmed a possible role for XET during seedling etiolation. Our results revealed that the roots of etiolated seedlings (H-E) were longer than those of green seedlings (H-G). However, shoot length exhibited the opposite pattern. We also observed positive and negative effects on the xyloglucan-degrading activity of XET in the root sub-apical region and shoots of etiolated Huangguogan seedling, respectively. There was a significant down-regulation in CitXET expression in the etiolated shoots at 15 days after seed germination. On the contrary, it was significantly increased in the root sub-apical region of etiolated and multicolored seedlings at 15 days after seed germination. The XET coding sequence (i.e., CitXET) was cloned from Huangguogan seedlings using gene-specific primers. The encoded amino acid sequence was predicted by using bioinformatics-based methods. The 990-bp CitXET gene was highly homologous to other XET genes. The CitXET protein was predicted to contain 319 amino acids, with a molecular mass of 37.45 kDa and an isoelectric point of 9.05. The predicted molecular formula was C1724H2548N448O466S14, and the resulting protein included only one transmembrane structure. The CitXET secondary structure consisted of four main structures (i.e., 21% α-helix, 30.72% extended strand, 9.09% ß-turn, and 39.18% random coil). Analyses involving the NCBI Conserved Domains Database (NCBI-CDD), InterPro, and ScanProsite revealed that CitXET was a member of the glycosyl hydrolase family 16 (GH16), and included the DEIDFEFLG motif. Our results indicate that the differed degrees of etiolation influenced the CitXET expression pattern and XET activity in Huangguogan seedlings. The differential changes in XET activity and CitXET expression levels in Huangguogan seedlings may influence the regulation of root and shoot development, and may be important for seedling etiolation.