Vegetative propagation through somatic embryogenesis is critical in conifer biotechnology towards multivarietal forestry that uses elite varieties to cope with environmental and socio-economic issues. An important and still sub-optimal process during in vitro maturation of somatic embryos (SE) is the biosynthesis and deposition of storage proteins, which are rich in amino acids with high nitrogen (N) content, such as arginine. Mobilization of these N-rich proteins is essential for the germination and production of vigorous somatic seedlings. Somatic embryos accumulate lower levels of N reserves than zygotic embryos (ZE) at a similar stage of development. To understand the molecular basis for this difference, the arginine metabolic pathway has been characterized in maritime pine (Pinus pinaster Ait.). The genes involved in arginine metabolism have been identified and GFP-fusion constructs were used to locate the enzymes in different cellular compartments and clarify their metabolic roles during embryogenesis and germination. Analysis of gene expression during somatic embryo maturation revealed high levels of transcripts for genes involved in the biosynthesis and metabolic utilization of arginine. By contrast, enhanced expression levels were only observed during the last stages of maturation and germination of ZE, consistent with the adequate accumulation and mobilization of protein reserves. These results suggest that arginine metabolism is unbalanced in SE (simultaneous biosynthesis and degradation of arginine) and could explain the lower accumulation of storage proteins observed during the late stages of somatic embryogenesis.

The biotic resistance hypothesis sensu stricto is also known as the diversity-invasibility hypothesis. It proposes that ecosystems with high biodiversity are more resistant against non-native species than ecosystems with lower biodiversity. It is a classic hypothesis of the field and our systematic literature search identified 155 empirical studies that examined it. Most of these studies question the hypothesis. The frequency of supportive observational field studies is only about 15%. Although the frequency of supportive experimental studies, which are typically done at smaller spatial scales, is significantly higher, it is still below 50%. The island susceptibility hypothesis is topically similar and posits that continents are more resistant against non-native species than islands. In more specific terms, the island susceptibility hypothesis states that non-native species are more likely to become established and have major ecological impacts on islands than on continents. Our literature search only identified 17 empirical tests of this hypothesis with five of them being supportive. Thus, the biotic resistance and island susceptibility hypotheses are not frequently supported by existing empirical evidence. Most studies addressing them examined the number of non-native species or their establishment success, whereas relatively few studies measured impacts of non-native species. Studies that measured abundance, biomass or cover of non-native species - which are related to impact - more frequently supported the resistance hypothesis than other studies. A promising way forward might thus be to narrow the definition and scope of both hypotheses (and possibly rename them), so that 'resistance' and 'susceptibility' are related to impact of nonnative species. The next steps will then be to critically test these revised hypotheses and further refine the relevant ecological contexts that mediate the importance or magnitude of resistance.

Biogenesis of the secondary cell wall in trees involves the massive biosynthesis of the phenylalanine-derived polymer lignin. Arogenate dehydratase (ADT) catalyzes the last, and rate-limiting, step of the main pathway for phenylalanine biosynthesis. In this study, we found that transcript levels for several members of the large ADT gene family, including ADT-A and ADT-D, were enhanced in compression wood of maritime pine, a xylem tissue enriched in lignin. Transcriptomic analysis of maritime pine silenced for PpMYB8 revealed that this gene plays a critical role in coordinating the deposition of lignin with the biosynthesis of phenylalanine. Specifically, it was found that ADT-A and ADT-D were strongly down-regulated in PpMYB8-silenced plants and that they were transcriptionally regulated through direct interaction of this transcription factor with regulatory elements present in their promoters. Another transcription factor, PpHY5, exhibited an expression profile opposite to that of PpMYB8 and also interacted with specific regulatory elements of ADT-A and ADT-D genes, suggesting that it is involved in transcriptional regulation of phenylalanine biosynthesis. Taken together, our results reveal that PpMYB8 and PpHY5 are involved in the control of phenylalanine formation and its metabolic channeling for lignin biosynthesis and deposition during wood formation in maritime pine.