The spider mite Tetranychus urticae is a cosmopolitan agricultural pest with an extensive host plant range and an extreme record of pesticide resistance. Here we present the completely sequenced and annotated spider mite genome, representing the first complete chelicerate genome. At 90 megabases T. urticae has the smallest sequenced arthropod genome. Compared with other arthropods, the spider mite genome shows unique changes in the hormonal environment and organization of the Hox complex, and also reveals evolutionary innovation of silk production. We find strong signatures of polyphagy and detoxification in gene families associated with feeding on different hosts and in new gene families acquired by lateral gene transfer. Deep transcriptome analysis of mites feeding on different plants shows how this pest responds to a changing host environment. The T. urticae genome thus offers new insights into arthropod evolution and plant–herbivore interactions, and provides unique opportunities for developing novel plant protection strategies.

The Arabidopsis thaliana transcription factor APETALA2 (AP2) has numerous functions, including roles in seed development, stem cell maintenance, and specification of floral organ identity. To understand the relationship between these different roles, we mapped direct targets of AP2 on a genome-wide scale in two tissue types. We find that AP2 binds to thousands of loci in the developing flower, many of which exhibit AP2-dependent transcription. Opposing, logical effects are evident in AP2 binding to two microRNA genes that influence AP2 expression, with AP2 positively regulating miR156 and negatively regulating miR172, forming a complex direct feedback loop, which also included all but one of the AP2-like miR172 target clade members. We compare the genome-wide direct target repertoire of AP2 with that of SCHLAFMÜ TZE, a closely related transcription factor that also represses the transition to flowering. We detect clear similarities and important differences in the direct target repertoires that are also tissue specific. Finally, using an inducible expression system, we demonstrate that AP2 has dual molecular roles. It functions as both a transcriptional activator and repressor, directly inducing the expression of the floral repressor AGAMOUS-LIKE15 and directly repressing the transcription of floral activators like SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1.

The appropriate timing of flowering is crucial for plant reproductive success. It is therefore not surprising that intricate genetic networks have evolved to perceive and integrate both endogenous and environmental signals, such as carbohydrate and hormonal status, photoperiod and temperature. In contrast to our detailed understanding of the vernalization pathway, little is known about how flowering time is controlled in response to changes in the ambient growth temperature. In Arabidopsis thaliana, the MADS-box transcription factor genes FLOWERING LOCUS M (FLM) and SHORT VEGETATIVE PHASE (SVP) have key roles in this process. FLM is subject to temperature-dependent alternative splicing. Here we report that the two main FLM protein splice variants, FLM-β and FLM-δ, compete for interaction with the floral repressor SVP. The SVP-FLM-β complex is predominately formed at low temperatures and prevents precocious flowering. By contrast, the competing SVP-FLM-δ complex is impaired in DNA binding and acts as a dominant-negative activator of flowering at higher temperatures. Our results show a new mechanism that controls the timing of the floral transition in response to changes in ambient temperature. A better understanding of how temperature controls the molecular mechanisms of flowering will be important to cope with current changes in global climate.

Several endogenous and environmental factors need to be integrated to time the onset of flowering. Genetic and molecular analyses, primarily in Arabidopsis thaliana and rice, have shown that CONSTANS (CO) and FLOWERING LOCUS T (FT) play central roles in photoperiod-dependent flowering. The overall picture is that CO acts in the phloem companion cells of leaves and that its main effect is to induce FT mRNA in these cells. Surprisingly, FT, a small globular protein of 20 kDa, interacts at the shoot apex with the bZIP transcription factor FLOWERING LOCUS D (FD) to induce downstream targets. Given that green fluorescent protein (GFP), which as a monomer is 27 kDa, can be easily exported to sink tissue including flowers when expressed in phloem companion cells, the latter finding strongly implied that FT protein is the mobile floral-inductive signal. In agreement with this hypothesis, an FT-GFP fusion, just like GFP, can be exported from the phloem of both rice and Arabidopsis. It has been unknown, however, whether mobile FT protein is sufficient for transmitting the flowering signal. Here we show that FT mRNA is required in phloem companion cells where it acts partially redundant with its paralog TWIN SISTER OF FT (TSF) to induce flowering. Furthermore, we have devised a method that uncouples FT mRNA and protein effects in vivo. We demonstrate that export of FT protein from phloem companion cells is sufficient to induce flowering.