Effects of Temperature and Photoperiod on the Flower Potential in Everbearing Strawberry as Evaluated by Meristem Dissection

The growing interest in using everbearing (EB) strawberry cultivars to extend the cultivation period has faced some challenges. These include poor runner production due to its perpetual flowering nature; irregular flowering behavior and extended periods of high temperature have caused floral inhibition and reduced yield. As flowering is an interplay between temperature and photoperiod, it is important to investigate the effects of this interaction on the cultivation. Therefore, this study used meristem dissection as a tool to study the effect of temperature and photoperiod on meristem development. Tray plants of two EB strawberry cultivars ‘Florentina’ and ‘Favori’ were grown at 20 °C, 25 °C, and 30 °C under short day (SD) conditions, and subsequently at 20 °C under long day (LD) conditions. The meristem development was analysed every 6 weeks for a 15-week period in SD and for 14 weeks in LD conditions using meristem dissection. The plants showed similar flowering patterns to previously studied everbearing cultivars, which was qualitative LD plants at high temperatures and quantitative LD plants at lower temperatures. Our results show that meristem dissection can be used to determine the temperature and photoperiodic effect on meristem development, and for the occurrence of cropping peaks, and can therefore be used to decide the environmental input and to evaluate yield potential.

PERPETUAL FLOWERING2 coordinates the vernalization response and perennial flowering in Arabis alpina

ABSTRACTThe floral repressor APETALA2 (AP2) in Arabidopsis regulates flowering through the age pathway. The AP2 orthologue in the alpine perennial Arabis alpina, PERPETUAL FLOWERING 2 (PEP2), was previously reported to regulate flowering through the vernalization pathway by enhancing the expression of another floral repressor PERPETUAL FLOWERING 1 (PEP1), the orthologue of Arabidopsis FLOWERING LOCUS C (FLC). However, PEP2 also regulates flowering independently of PEP1. To characterize the function of PEP2 we analyzed the transcriptomes of pep2 and pep1 mutants. The majority of differentially expressed genes were detected between pep2 and the wild type or between pep2 and pep1, highlighting the importance of the PEP2 role that is independent of PEP1. Here we demonstrate that PEP2 prevents the upregulation of the A. alpina floral meristem identity genes FRUITFUL (AaFUL), LEAFY (AaLFY) and APETALA1 (AaAP1) which ensure floral commitment during vernalization. Young pep2 seedlings respond to vernalization, suggesting that PEP2 regulates the age-dependent response to vernalization independently of PEP1. The major role of PEP2 through the PEP1-dependent pathway takes place after vernalization, when it facilitates PEP1 activation both in the main shoot apex and in the axillary branches. These multiple roles of PEP2 in vernalization response contribute to the A. alpina life-cycle.HIGHLIGHTThe Arabis alpina APETALA2 orthologue, PERPETUAL FLOWERING2, regulates the age-dependent response to vernalization and it is required to facilitate the activation of the A. alpina FLOWERING LOCUS C after vernalization.

Identification of WRKY Gene Family from Dimocarpus longan and Its Expression Analysis during Flower Induction and Abiotic Stress Responses

Longan is an important fruit tree in the subtropical region of Southeast Asia and Australia. However, its blooming and its yield are susceptible to stresses such as droughts, high salinity, and high and low temperature. To date, the molecular mechanisms of abiotic stress tolerance and flower induction in longan have not been elucidated. WRKY transcription factors (TFs), which have been studied in various plant species, play important regulatory roles in plant growth, development, and responses to stresses. However, there is no report about WRKYs in longan. In this study, we identified 55 WRKY genes with the conserved WRKY domain and zinc finger motif in the longan genome. Based on the structural features of WRKY proteins and topology of the phylogenetic tree, the longan WRKY (DlWRKY) family was classified into three major groups (I–III) and five subgroups (IIa–IIe) in group II. Tissue expression analysis showed that 25 DlWRKYs were highly expressed in almost all organs, suggesting that these genes may be important for plant growth and organ development in longan. Comparative RNA-seq and qRT-PCR-based gene expression analysis revealed that 18 DlWRKY genes showed a specific expression during three stages of flower induction in “Sijimi” (“SJ”), which exhibited the “perpetual flowering” (PF) habit, indicating that these 18 DlWRKY genes may be involved in the flower induction and the genetic control of the perpetual flowering trait in longan. Furthermore, the RT-qPCR analysis illustrated the significant variation of 27, 18, 15, 17, 27, and 23 DlWRKY genes under SA (Salicylic acid), MeJA (Methyl Jasmonate), heat, cold, drought, or high salinity treatment, respectively, implicating that they might be stress- or hormone-responsive genes. In summary, we systematically and comprehensively analyzed the structure, evolution, and expression pattern of the DlWRKY genes. The results presented here increase our understanding of the WRKY family in fruit trees and provide a basis for the further elucidation of the biological function of DlWRKY genes in longan.

Perpetual Flowering in Strawberry Species

The genetic control of flowering habit in many species of Fragaria has not been well studied. Identification of flowering traits and patterns for these taxa could be used in the quest for perpetual flowering (PF) genes and for the octoploids, broaden the genepool of available PF parents for breeding programs. As such, clones from the Fragaria germplasm collection housed at the USDA-ARS National Clonal Germplasm Repository in Corvallis, OR, were evaluated to describe flowering habits in various taxa and identify PF clones. Flower presence was recorded monthly for 962 clones of 36 taxa from the first of May through October in 2015 and 2016 to determine flowering habit and pairwise comparisons between taxa were examined using Pearson’s Chi-squared test. Taxa with the largest percent of PF accessions were F. vesca subsp. vesca f. semperflorens, F. vesca subsp. vesca f. alba, F. vesca subsp. americana, and F. virginiana subsp. glauca. These taxa had similar flowering habits to each other but were significantly different (α = 0.05) from most other taxa in which the seasonal flowering (SF) trait was predominant. Fifteen clones that demonstrated the PF phenotype in both 2015 and 2016 were identified. Differing genetic controls have been observed for flowering habit in F. ×ananassa and F. vesca. Additional studies are needed to determine genetic control of flowering in other Fragaria taxa.