Transcriptional landscapes of floral meristems in barley

Organ development in plants predominantly occurs postembryonically through combinatorial activity of meristems; therefore, meristem and organ fate are intimately connected. Inflorescence morphogenesis in grasses (Poaceae) is complex and relies on a specialized floral meristem, called spikelet meristem, that gives rise to all other floral organs and ultimately the grain. The fate of the spikelet determines reproductive success and contributes toward yield-related traits in cereal crops. Here, we examined the transcriptional landscapes of floral meristems in the temperate crop barley (Hordeum vulgare L.) using RNA-seq of laser capture microdissected tissues from immature, developing floral structures. Our unbiased, high-resolution approach revealed fundamental regulatory networks, previously unknown pathways, and key regulators of barley floral fate and will equally be indispensable for comparative transcriptional studies of grass meristems.

Integration of abscisic acid signalling into plant responses

The phytohormone abscisic acid (ABA) plays a major role as an endogenous messenger in the regulation of plant's water status. ABA is generated as a signal during a plant's life cycle to control seed germination and further developmental processes and in response to abiotic stress imposed by salt, cold, drought, and wounding. The action of ABA can target specifically guard cells for induction of stomatal closure but may also signal systemically for adjustment towards severe water shortage. At the molecular level, the responses are primarily mediated by regulation of ion channels and by changes in gene expression. In the last years, the molecular complexity of ABA signal transduction surfaced more and more. Many proteins and a plethora of "secondary" messengers that regulate or modulate ABA-responses have been identified by analysis of mutants including gene knock-out plants and by applying RNA interference technology together with protein interaction analysis. The complexity possibly reflects intensive cross-talk with other signal pathways and the role of ABA to be part of and to integrate several responses. Despite the missing unifying concept, it is becoming clear that ABA action enforces a sophisticated regulation at all levels.

A plant viral "reinitiation" factor interacts with the host translational machinery

The cauliflower mosaic virus transactivator, TAV, controls translation reinitiation of major open reading frames on polycistronic RNA. We show here that TAV function depends on its association with polysomes and eukaryotic initiation factor eIF3 in vitro and in vivo. TAV physically interacts with eIF3 and the 60S ribosomal subunit. Two proteins mediating these interactions were identified: eIF3g and 60S ribosomal protein L24. Transient expression of eIF3g and L24 in plant protoplasts strongly affects TAV-mediated reinitiation activity. We demonstrate that TAV/eIF3/40S and eIF3/TAV/60S ternary complexes form in vitro, and propose that TAV mediates efficient recruitment of eIF3 to polysomes, allowing translation of polycistronic mRNAs by reinitiation, overcoming the normal cell barriers to this process.

ABA signal transduction

Recent advances in the study of abscisic acid signal transduction include the identification of cyclic ADP-ribose as a central mediator of abscisic acid responses. The characterisation of type 2C protein phosphatases, ABI1 and ABI2, implicates negative control and redundant action on the signal pathway of this hormone. In addition, abscisic acid-mediated inhibition of gibberellin-stimulated responses seems to depend on the activation of a phospholipase D during induction of alpha-amylase in barley aleurone cells as well as on a putative acetyltransferase involved in elongation growth.

Signalling of abscisic acid to regulate plant growth

Abscisic acid (ABA) mediated growth control is a fundamental response of plants to adverse environmental cues. The linkage between ABA perception and growth control is currently being unravelled by using different experimental approaches such as mutant analysis and microinjection experiments. So far, two protein phosphatases, ABI1 and ABI2, cADPR, pH, and Ca2+ have been identified as main components of the ABA signalling pathway. Here, the ABA signal transduction pathway is compared to signalling cascades from yeast and mammalian cells. A model for a bifurcated ABA signal transduction pathway exerting a positive and negative control mechanism is proposed.

Interaction between cauliflower mosaic virus inclusion body protein and capsid protein: implications for viral assembly

The cauliflower mosaic virus (CaMV) inclusion body protein (pVI) is able to specifically interact with the viral capsid precursor protein (pIV). By using the yeast two-hybrid system and a blot assay, the pIV region required for the recognition of pVI was mapped to the lysine-rich domain. This region of only 48 amino acids when fused to dihydrofolate reductase (DHFR) mediated pVI and DNA binding in vitro. Competition experiments confirmed that pVI and DNA bind to the same region of pIV. Since pVI is absent from the mature virus, models are discussed in which pVI plays an accessory role in CaMV assembly, in addition to its function in transactivating translation.

Molecular dissection of the cauliflower mosaic virus translation transactivator

The cauliflower mosaic virus (CaMV) transactivator (TAV) is a complex protein that appears to be involved in many aspects of the virus life cycle. One of its roles is to control translation from the polycistronic CaMV 35S RNA. Here we report a molecular dissection of TAV in relation to its ability to enhance dicistronic translation in transient expression experiments. We have identified a protein domain that is responsible and sufficient for that activity. This 'MiniTAV domain' consists of only 140 of the 520 amino acids in the full-length sequence. A further domain located outside the MiniTAV, and therefore dispensable for transactivation, is probably involved in interactions with other molecules. This was identified by its ability to compete with wild-type TAV and some of its deletion mutants. We found, furthermore, that the TAV protein binds RNA. Two regions needed for RNA-binding properties were defined outside the MiniTAV domain and RNA binding seems not to be directly involved in the transactivation mechanism.