Plant meristems and organogenesis: the new era of plant developmental research

Sustainable Cannabis Nutrition: Elevated root-zone phosphorus significantly increases leachate P and does not improve yield or quality

Phosphorus (P) is an essential but often over-applied nutrient in agricultural systems. Because of its detrimental environmental effects, P fertilization is well studied in crop production. Controlled environment agriculture allows for precise control of root-zone P and has the potential to improve sustainability over field agriculture. Medical Cannabis is uniquely cultivated for the unfertilized female inflorescence and mineral nutrition can affect the yield and chemical composition of these flowers. P typically accumulates in seeds, but its partitioning in unfertilized Cannabis flowers is not well studied. Here we report the effect of increasing P (25, 50, and 75 mg P per L) in continuous liquid fertilizer on flower yield, cannabinoid concentration, leachate P, nutrient partitioning, and phosphorus use efficiency (PUE) of a high-CBD Cannabis variety. There was no significant effect of P concentration on flower yield or cannabinoid concentration, but there were significant differences in leachate P, nutrient partitioning, and PUE. Leachate P increased 12-fold in response to the 3-fold increase in P input. The P concentration in the unfertilized flowers increased to more than 1%, but this did not increase yield or quality. The fraction of P in the flowers increased from 25 to 65% and PUE increased from 31 to 80% as the as the P input decreased from 75 to 25 mg per L. Avoiding excessive P fertilization can decrease the environmental impact of Cannabis cultivation.

Fine-tuning of auxin homeostasis governs the transition from floral stem cell maintenance to gynoecium formation

To ensure successful plant reproduction and crop production, the spatial and temporal control of the termination of the floral meristem must be coordinated. In Arabidopsis, the timing of this termination is determined by AGAMOUS (AG). Following its termination, the floral meristem underdoes gynoecium formation. A direct target of AG, CRABS CLAW (CRC), is involved in both floral meristem determinacy and gynoecium development. However, how floral meristem termination is coordinated with gynoecium formation is not understood. Here, we identify a mechanistic link between floral meristem termination and gynoecium development through fine-tuning of auxin homeostasis by CRC. CRC controls auxin homeostasis in the medial region of the developing gynoecium to generate proper auxin maxima. This regulation partially occurs via direct transcriptional repression of TORNADO2 (TRN2) by CRC. Plasma membrane-localized TRN2 modulates auxin homeostasis. We propose a model describing how regulation of auxin homeostasis mediates the transition from floral meristem termination to gynoecium development.

In Arabidopsis, the timing of floral meristem termination is determined by AGAMOUS. Here, the authors show that the CRC transcription factor, itself a direct target of AGAMOUS, coordinates meristem termination with subsequent gynoecium formation partly by repressing TRN2 expression and regulating auxin homeostasis.

Analysis of global gene expression profiles to identify differentially expressed genes critical for embryo development in Brassica rapa

Embryo development represents a crucial developmental period in the life cycle of flowering plants. To gain insights into the genetic programs that control embryo development in Brassica rapa L., RNA sequencing technology was used to perform transcriptome profiling analysis of B. rapa developing embryos. The results generated 42,906,229 sequence reads aligned with 32,941 genes. In total, 27,760, 28,871, 28,384, and 25,653 genes were identified from embryos at globular, heart, early cotyledon, and mature developmental stages, respectively, and analysis between stages revealed a subset of stage-specific genes. We next investigated 9,884 differentially expressed genes with more than fivefold changes in expression and false discovery rate ≤ 0.001 from three adjacent-stage comparisons; 1,514, 3,831, and 6,633 genes were detected between globular and heart stage embryo libraries, heart stage and early cotyledon stage, and early cotyledon and mature stage, respectively. Large numbers of genes related to cellular process, metabolism process, response to stimulus, and biological process were expressed during the early and middle stages of embryo development. Fatty acid biosynthesis, biosynthesis of secondary metabolites, and photosynthesis-related genes were expressed predominantly in embryos at the middle stage. Genes for lipid metabolism and storage proteins were highly expressed in the middle and late stages of embryo development. We also identified 911 transcription factor genes that show differential expression across embryo developmental stages. These results increase our understanding of the complex molecular and cellular events during embryo development in B. rapa and provide a foundation for future studies on other oilseed crops.

Stem cells and regeneration in plants

BACKGROUND

Plants are characterized by indeterminate post-embryonic development that is evident, for example, in the continuous branching of shoots and roots. High competence to regenerate tissues is another consequence of such intrinsic developmental plasticity in plants. It has been suggested that specialized groups of cells within plant meristems should be compared to stem cells in animals, but the utility of this label in the context of post-embryonic plant development and regeneration is often debated.

SUMMARY

This paper is organized into 3 short sections, where (a) key observations and experimental results on tissue regeneration in plants - mainly in the model system Arabidopsis thaliana, (b) stem cell activity and (c) their role in regeneration are described. The main focus is maintained on the critical aspects of defining stem cell-ness in plants, particularly in the context of tissue regeneration. A number of recent excellent reviews are cited throughout the text to give the reader the appropriate tools to dig deeper into the various stimulating topics introduced here.

KEY MESSAGES

Despite the remarkable somatic developmental plasticity characterizing post-embryonic development in plants, use of the classic concept of stem cells has been imported from the animal literature with the goal of facilitating our understanding and description of plant developmental processes. It is not clear if this is the case, especially in light of the recent experimental results on root regeneration in Arabidopsis mutants.