Senescence Meets Dedifferentiation

Know when and how to die: gaining insights into the molecular regulation of leaf senescence

Senescence is the ultimate phase in the life cycle of leaves which is crucial for recycling of nutrients to maintain plant fitness and reproductive success. The earliest visible manifestation of leaf senescence is their yellowing, which usually commences with the breakdown of chlorophyll. The degradation process involves a gradual and highly coordinated disassembly of macromolecules resulting in the accumulation of nutrients, which are subsequently mobilized from the senescing leaves to the developing organs. Leaf senescence progresses under overly tight genetic and molecular control involving a well-orchestrated and intricate network of regulators that coordinate spatio-temporally with the influence of both internal and external cues. Owing to the advancements in omics technologies, the availability of mutant resources, scalability of molecular analyses methodologies and the advanced capacity to integrate multidimensional data, our understanding of the genetic and molecular basis of leaf ageing has greatly expanded. The review provides a compilation of the multitier regulation of senescence process and the interrelation between the environment and the terminal phase of leaf development. The knowledge gained would benefit in devising the strategies for manipulation of leaf senescence process to improve crop quality and productivity.

Identification of Shoot Differentiation-Related Genes in Populus euphratica Oliv

De novo shoot regeneration is one of the important manifestations of cell totipotency in organogenesis, which reflects a survival strategy organism evolved when facing natural selection. Compared with tissue regeneration, and somatic embryogenesis, de novo shoot regeneration denotes a shoot regeneration process directly from detatched or injured tissues of plant. Studies on plant shoot regeneration had identified key genes mediating shoot regeneration. However, knowledge was derived from Arabidopsis; the regeneration capacity is hugely distinct among species. To achieve a comprehensive understanding of the shoot regeneration mechanism from tree species, we select four genetic lines of Populus euphratica from a natural population to be sequenced at transcriptome level. On the basis of the large difference of differentiation capacity, between the highly differentiated (HD) and low differentiated (LD) groups, the analysis of differential expression identified 4920 differentially expressed genes (DEGs), which were revealed in five groups of expression patterns by clustering analysis. Enrichment showed crucial pathways involved in regulation of regeneration difference, including “plant hormone signal transduction”, “cell differentiation”, "cellular response to auxin stimulus", and “auxin-activated signaling pathway”. The expression of nine genes reported to be associated with shoot regeneration was validated using quantitative real-time PCR (qRT-PCR). For the specificity of regeneration mechanism with P. euphratica, large amount of DEGs involved in "plant-pathogen interaction", ubiquitin-26S proteosome mediated proteolysis pathway, stress-responsive DEGs, and senescence-associated DEGs were summarized to possibly account for the differentiation difference with distinct genotypes of P. euphratica. The result in this study helps screening of key regulators in mediating the shoot differentiation. The transcriptomic characteristic in P. euphratica further enhances our understanding of key processes affecting the regeneration capacity of de novo shoots among distinct species.

Somatic Embryogenesis Induction in Woody Species: The Future After OMICs Data Assessment

Very early somatic embryogenesis has been recognized as a powerful method to propagate plants in vitro. For some woody species and in particular for some coniferous trees, somatic embryogenesis induction has become a routine procedure. For the majority, the application of this technology presents yet many limitations especially due to the genotype, the induction conditions, the number of embryos produced, maturation, and conversion, among other factors that compromise the systematic use of somatic embryogenesis for commercial purposes especially of woody species and forest trees in particular. The advancements obtained on somatic embryogenesis in Arabidopsis and the development of OMIC technologies allowed the characterization of genes and the corresponding proteins that are conserved in woody species. This knowledge will help in understanding the molecular mechanisms underlying the complex regulatory networks that control somatic embryogenesis in woody plants. In this revision, we report on developments of OMICs (genomics, transcriptomics, metabolomics, and proteomics) applied to somatic embryogenesis induction and its contribution for understanding the change of fate giving rise to the expression of somatic embryogenesis competence.

Integration of multi-omics techniques and physiological phenotyping within a holistic phenomics approach to study senescence in model and crop plants

The study of senescence in plants is complicated by diverse levels of temporal and spatial dynamics as well as the impact of external biotic and abiotic factors and crop plant management. Whereas the molecular mechanisms involved in developmentally regulated leaf senescence are very well understood, in particular in the annual model plant species Arabidopsis, senescence of other organs such as the flower, fruit, and root is much less studied as well as senescence in perennials such as trees. This review addresses the need for the integration of multi-omics techniques and physiological phenotyping into holistic phenomics approaches to dissect the complex phenomenon of senescence. That became feasible through major advances in the establishment of various, complementary 'omics' technologies. Such an interdisciplinary approach will also need to consider knowledge from the animal field, in particular in relation to novel regulators such as small, non-coding RNAs, epigenetic control and telomere length. Such a characterization of phenotypes via the acquisition of high-dimensional datasets within a systems biology approach will allow us to systematically characterize the various programmes governing senescence beyond leaf senescence in Arabidopsis and to elucidate the underlying molecular processes. Such a multi-omics approach is expected to also spur the application of results from model plants to agriculture and their verification for sustainable and environmentally friendly improvement of crop plant stress resilience and productivity and contribute to improvements based on postharvest physiology for the food industry and the benefit of its customers.

Senescence and nitrogen use efficiency in perennial grasses for forage and biofuel production

Organ senescence is an important developmental process in plants that enables recycling of nutrients, such as nitrogen, to maximize reproductive success. Nitrogen is the mineral nutrient required in greatest amount by plants, although soil-N limits plant productivity in many natural and agricultural systems, especially systems that receive little or no fertilizer-N. Use of industrial N-fertilizers in agriculture increased crop yields several fold over the past century, although at substantial cost to fossil energy reserves and the environment. Therefore, it is important to optimize nitrogen use efficiency (NUE) in agricultural systems. Organ senescence contributes to NUE in plants and manipulation of senescence in plant breeding programs is a promising approach to improve NUE in agriculture. Much of what we know about plant senescence comes from research on annual plants, which provide most of the food for humans. Relatively little work has been done on senescence in perennial plants, especially perennial grasses, which provide much of the forage for grazing animals and promise to supply much of the biomass required by the future biofuel industry. Here, we review briefly what is known about senescence from studies of annual plants, before presenting current knowledge about senescence in perennial grasses and its relationship to yield, quality, and NUE. While higher yield is a common target, desired N-content diverges between forage and biofuel crops. We discuss how senescence programs might be altered to produce high-yielding, stress-tolerant perennial grasses with high-N (protein) for forage or low-N for biofuels in systems optimized for NUE.