Molecular cloning and functional characterization of genes associated with flowering in citrus using an early-flowering trifoliate orange (Poncirus trifoliata L. Raf.) mutant

Genome-wide screening and characterization of long non-coding RNAs involved in flowering development of trifoliate orange (Poncirus trifoliata L. Raf.)

Long non-coding RNAs (lncRNAs) have been demonstrated to play critical regulatory roles in post-transcriptional and transcriptional regulation in Arabidopsis. However, lncRNAs and their functional roles remain poorly characterized in woody plants, including citrus. To identify lncRNAs and investigate their role in citrus flowering, paired-end strand-specific RNA sequencing was performed for precocious trifoliate orange and its wild-type counterpart. A total of 6,584 potential lncRNAs were identified, 51.6% of which were from intergenic regions. Additionally, 555 lncRNAs were significantly up-regulated and 276 lncRNAs were down-regulated in precocious trifoliate orange, indicating that lncRNAs could be involved in the regulation of trifoliate orange flowering. Comparisons between lncRNAs and coding genes indicated that lncRNAs tend to have shorter transcripts and lower expression levels and that they display significant expression specificity. More importantly, 59 and 7 lncRNAs were identified as putative targets and target mimics of citrus miRNAs, respectively. In addition, the targets of Pt-miR156 and Pt-miR396 were confirmed using the regional amplification reverse-transcription polymerase chain reaction method. Furthermore, overexpression of Pt-miR156a1 and Pt-miR156a1 in Arabidopsis resulted in an extended juvenile phase, short siliques, and smaller leaves in transgenic plants compared with control plants. These findings provide important insight regarding citrus lncRNAs, thus enabling in-depth functional analyses.

Comparative proteomic analysis of the shoot apical meristem in maize between a ZmCCT-associated near-isogenic line and its recurrent parent

The ZmCCT, one of the most important genes affecting photoperiod response, delays flowering under long-day conditions in maize (Zea mays). In this study we used the isobaric tags for relative and absolute quantification (iTRAQ) technique-based proteomics approach to identify differentially expressed proteins between a near-isogenic line (NIL) and its recurrent parent, contrasting in alleles of ZmCCT. A total of 5,259 distinct proteins were identified. Among them, 386 proteins were differentially expressed between NIL-cml line (ZmCCT-positive) and H4 line (ZmCCT-negative). Functional categorization showed that the differentially proteins were mainly involved in energy production, photosynthesis, signal transduction, and cell organization and biogenesis. Our results showed that during shoot apical meristem (SAM) development cell division proteins, carbohydrate metabolism–related proteins, and flower inhibition-related proteins were more abundant in the ZmCCT-positive line than the ZmCCT-negative line. These results, taken together with morphological observations, showed that the effect of ZmCCT on flowering might be caused by its effect on one or all of these biological processes. Although the exact roles of these putative related proteins remain to be examined, our results obtained using the proteomics approach lead to a better understanding of the photoperiodicity mechanism in maize plants.

Genome-wide classification and evolutionary and expression analyses of citrus MYB transcription factor families in sweet orange

MYB family genes are widely distributed in plants and comprise one of the largest transcription factors involved in various developmental processes and defense responses of plants. To date, few MYB genes and little expression profiling have been reported for citrus. Here, we describe and classify 177 members of the sweet orange MYB gene (CsMYB) family in terms of their genomic gene structures and similarity to their putative Arabidopsis orthologs. According to these analyses, these CsMYBs were categorized into four groups (4R-MYB, 3R-MYB, 2R-MYB and 1R-MYB). Gene structure analysis revealed that 1R-MYB genes possess relatively more introns as compared with 2R-MYB genes. Investigation of their chromosomal localizations revealed that these CsMYBs are distributed across nine chromosomes. Sweet orange includes a relatively small number of MYB genes compared with the 198 members in Arabidopsis, presumably due to a paralog reduction related to repetitive sequence insertion into promoter and non-coding transcribed region of the genes. Comparative studies of CsMYBs and Arabidopsis showed that CsMYBs had fewer gene duplication events. Expression analysis revealed that the MYB gene family has a wide expression profile in sweet orange development and plays important roles in development and stress responses. In addition, 337 new putative microsatellites with flanking sequences sufficient for primer design were also identified from the 177 CsMYBs. These results provide a useful reference for the selection of candidate MYB genes for cloning and further functional analysis forcitrus.

Comprehensive analysis of CCCH-type zinc finger gene family in citrus (Clementine mandarin) by genome-wide characterization

The CCCH-type zinc finger proteins comprise a large gene family of regulatory proteins and are widely distributed in eukaryotic organisms. The CCCH proteins have been implicated in multiple biological processes and environmental responses in plants. Little information is available, however, about CCCH genes in plants, especially in woody plants such as citrus. The release of the whole-genome sequence of citrus allowed us to perform a genome-wide analysis of CCCH genes and to compare the identified proteins with their orthologs in model plants. In this study, 62 CCCH genes and a total of 132 CCCH motifs were identified, and a comprehensive analysis including the chromosomal locations, phylogenetic relationships, functional annotations, gene structures and conserved motifs was performed. Distribution mapping revealed that 54 of the 62 CCCH genes are unevenly dispersed on the nine citrus chromosomes. Based on phylogenetic analysis and gene structural features, we constructed 5 subfamilies of 62 CCCH members and integrative subfamilies from citrus, Arabidopsis, and rice, respectively. Importantly, large numbers of SNPs and InDels in 26 CCCH genes were identified from Poncirus trifoliata and Fortunella japonica using whole-genome deep re-sequencing. Furthermore, citrus CCCH genes showed distinct temporal and spatial expression patterns in different developmental processes and in response to various stress conditions. Our comprehensive analysis of CleC3Hs is a valuable resource that further elucidates the roles of CCCH family members in plant growth and development. In addition, variants and comparative genomics analyses deepen our understanding of the evolution of the CCCH gene family and will contribute to further genetics and genomics studies of citrus and other plant species.

The Vaccinium corymbosum FLOWERING LOCUS T-like gene (VcFT): a flowering activator reverses photoperiodic and chilling requirements in blueberry

KEY MESSAGE

The blueberry FLOWERING LOCUS T ( FT )-like gene ( VcFT ) cloned from the cDNA of a tetraploid, northern highbush blueberry ( Vaccinium corymbosum L.) is able to reverse the photoperiodic and chilling requirements and drive early and continuous flowering.

ABSTRACT

Blueberry is a woody perennial bush with a longer juvenile period than annual crops, requiring vernalization to flower normally. Few studies have been reported on the molecular mechanism of flowering in blueberry or other woody plants. Because FLOWERING LOCUS T (FT) from Arabidopsis thaliana plays a multifaceted role in generating mobile molecular signals to regulate plant flowering time, isolation and functional analysis of the blueberry (Vaccinium corymbosum L.) FT-like gene (VcFT) will facilitate the elucidation of molecular mechanisms of flowering in woody plants. Based on EST sequences, a 525-bpVcFT was identified and cloned from the cDNA of a tetraploid, northern highbush blueberry cultivar, Bluecrop. Ectopic expression of 35S:VcFT in tobacco induced flowering an average of 28 days earlier than wild-type plants. Expression of the 35S:VcFT in the blueberry cultivar Aurora resulted in an extremely early flowering phenotype, which flowered not only during in vitro culture, a growth stage when nontransgenic shoots had not yet flowered, but also in 6-10-week old, soil-grown transgenic plants, in contrast to the fact that at least 1 year and 800 chilling hours are required for the appearance of the first flower of both nontransgenic 'Aurora' and transgenic controls with the gusA. These results demonstrate that the VcFT is a functional floral activator and overexpression of the VcFT is able to reverse the photoperiodic and chilling requirements and drive early and continuous flowering.

Identification of MV-generated ROS responsive EST clones in floral buds of Litchi chinensis Sonn

A suppression subtractive hybridization library was constructed using inflorescence primordia of 'Nuomici' litchi to identify EST clones responsive to MV-generated ROS. 93 ESTs could be aligned as unique gene sequences in the inflorescence primordia of litchi. Litchi is an evergreen woody tree widely cultivated in subtropical and tropical regions. However, defective flowering is a pending problem of litchi production. Our previous study indicated that reactive oxygen species (ROS) induced by methyl viologen dichloride hydrate (MV) promotes flowering in litchi. In the present study, a suppression subtractive hybridization (SSH) library was constructed using inflorescence primordia of 'Nuomici' with the aim to find out ROS responsive clones during floral differentiation. 1856 Expressed sequence tag (EST) clones were randomly selected. Clones carrying single exogenous fragments were screened by reverse northern analysis to identify those responsive to MV-generated ROS. A total of 783 differentially expressed EST clones were identified as MV responsive cDNA and were subjected to sequencing. Among them, 26 clones were represented more than three times. 783 clones were aligned to 93 unique gene sequences. The unique genes were classified into 9 categories. 16 % of them were involved in transport facilitation, 11 % in transcription regulation, 4 % in stress response, 9 % in carbohydrate metabolism, 1 % in secondary metabolism, 14 % in intracellular signaling, and 25 % in other metabolism, while 9 % were genes with unknown functions and 11 % were genes with no match in the database.