Analysis of ambient temperature-responsive transcriptome in shoot apical meristem of heat-tolerant and heat-sensitive broccoli inbred lines during floral head formation

BACKGROUND

Head formation of broccoli (Brassica oleracea var. italica) is greatly reduced under high temperature (22 °C and 27 °C). Broccoli inbred lines that are capable of producing heads at high temperatures in summer are varieties that are unique to Taiwan. However, knowledge of the early-activated pathways of broccoli head formation under high temperature is limited.

RESULTS

We compared heat-tolerant (HT) and heat-sensitive (HS) transcriptome of broccoli under different temperatures. Weighted gene correlation network analysis (WGCNA) revealed that genes involved in calcium signaling pathways, mitogen-activated protein kinase (MAPK) cascades, leucine-rich repeat receptor-like kinases (LRR-RLKs), and genes coding for heat-shock proteins and reactive oxygen species homeostasis shared a similar expression pattern to BoFLC1, which was highly expressed at high temperature (27 °C). Of note, these genes were less expressed in HT than HS broccoli at 22 °C. Co-expression analysis identified a model for LRR-RLKs in survival-reproduction tradeoffs by modulating MAPK- versus phytohormones-signaling during head formation. The difference in head-forming ability in response to heat stress between HT and HS broccoli may result from their differential transcriptome profiles of LRR-RLK genes. High temperature induced JA- as well as suppressed auxin- and cytokinin-related pathways may facilitate a balancing act to ensure fitness at 27 °C. BoFLC1 was less expressed in HT than HS at 22 °C, whereas other FLC homologues were not. Promoter analysis of BoFLC1 showed fewer AT dinucleotide repeats in HT broccoli. These results provide insight into the early activation of stress- or development-related pathways during head formation in broccoli. The identification of the BoFLC1 DNA biomarker may facilitate breeding of HT broccoli.

CONCLUSIONS

In this study, HT and HS broccoli genotypes were used to determine the effect of temperature on head formation by transcriptome profiling. On the basis of the expression pattern of high temperature-associated signaling genes, the HS transcriptome may be involved in stress defense instead of transition to the reproductive phase in response to heat stress. Transcriptome profiling of HT and HS broccoli helps in understanding the molecular mechanisms underlying head-forming capacity and in promoting functional marker-assisted breeding.

Expression and function of the zinc finger transcription factor Sp6-9 in the spider Parasteatoda tepidariorum

Zinc finger transcription factors of the Sp6-9 group are evolutionarily conserved in all metazoans and have important functions in, e.g., limb formation and heart development. The function of Sp6-9-related genes has been studied in a number of vertebrates and invertebrates, but data from chelicerates (spiders and allies) was lacking so far. We have isolated the ortholog of Sp6-9 from the common house spider Parasteatoda tepidariorum and the cellar spider Pholcus phalangioides. We show that the Sp6-9 gene in these spider species is expressed in the developing appendages thus suggesting a conserved role in limb formation. Indeed, RNAi with Sp6-9 in P. tepidariorum leads not only to strong limb defects, but also to the loss of body segments and head defects in more strongly affected animals. Together with a new expression domain in the early embryo, these data suggest that Sp6-9 has a dual role P. tepidariorum. The early role in head and body segment formation is not known from other arthropods, but the role in limb formation is evolutionarily highly conserved.

Formation of the Embryonic Head in the Mouse: Attributes of a Gene Regulatory Network

The embryonic head is the first major body part to be constructed during embryogenesis. The allocation and the assembly of the progenitor tissues, which start at gastrulation, are accompanied by the spatiotemporal activity of transcription factors and signaling pathways that drives lineage specification, germ layer formation, and cell/tissue movement. The morphogenesis, regionalization, and patterning of the brain and craniofacial structures rely on the function of LIM-domain, homeodomain, and basic helix-loop-helix transcription factors. These factors constitute the central nodes of a gene regulatory network (GRN) which encompasses and intersects with signaling pathways involved with head formation. It is predicted that the functional output of this "head GRN" impacts on cellular function and cell-cell interactions that are essential for lineage differentiation and tissue modeling, which are key processes underpinning the formation of the head.

G protein-coupled receptors Flop1 and Flop2 inhibit Wnt/β-catenin signaling and are essential for head formation in Xenopus

Patterning of the vertebrate anterior-posterior axis is regulated by the coordinated action of growth factors whose effects can be further modulated by upstream and downstream mediators and the cross-talk of different intracellular pathways. In particular, the inhibition of the Wnt/β-catenin signaling pathway by various factors is critically required for anterior specification. Here, we report that Flop1 and Flop2 (Flop1/2), G protein-coupled receptors related to Gpr4, contribute to the regulation of head formation by inhibiting Wnt/β-catenin signaling in Xenopus embryos. Using whole-mount in situ hybridization, we showed that flop1 and flop2 mRNAs were expressed in the neural ectoderm during early gastrulation. Both the overexpression and knockdown of Flop1/2 resulted in altered embryonic head phenotypes, while the overexpression of either Flop1/2 or the small GTPase RhoA in the absence of bone morphogenetic protein (BMP) signaling resulted in ectopic head induction. Examination of the Flops' function in Xenopus embryo animal cap cells showed that they inhibited Wnt/β-catenin signaling by promoting β-catenin degradation through both RhoA-dependent and -independent pathways in a cell-autonomous manner. These results suggest that Flop1 and Flop2 are essential regulators of Xenopus head formation that act as novel inhibitory components of the Wnt/β-catenin signaling pathway.