Interactions between xylem traits linked to hydraulics during xylem development optimize growth performance in conifer seedlings

Climate change has threatened forests globally, challenging tree species ability to track the rapidly changing environment (e..g., drought and temperature rise). Conifer species face strong environmental filters due to climatic seasonality. Investigating how conifers change their hydraulic architecture during xylem development across the season may shed light on possible mechanisms underlying hydraulic adaptation in conifers. Laser microscopy was used to assess the three-dimensional hydraulic architecture of balsam fir (Abies balsamea (Linnaeus) Miller), jack pine (Pinus banksiana Lambert), white spruce (Picea glauca (Moench) Voss), and black spruce (Picea mariana (Miller) Britton, Sterns & Poggenburgh) seedlings. We measured hydraulic-related xylem traits from early to latewood, during four years of plant growth. The xylem development of jack pine seedlings contrasts with the other species for keeping torus overlap (a hydraulic safety-associated xylem trait), relatively constant across the season (from early to latewood) and the years. The tracheids and torus expansion are positively associated with plant growth. Pit aperture-torus covariance is central to the seasonal dynamics of jack pine xylem development, which jointly with a rapid tracheid and pit expansion seems to boost its growth performance. Linking xylem structural changes during xylem development with hydraulics is a major issue for future research to assess conifers vulnerability to climate change.

Exogenous Brassinosteroid Facilitates Xylem Development in Pinus massoniana Seedlings

Brassinosteroids (BRs) are known to be essential regulators for wood formation in herbaceous plants and poplar, but their roles in secondary growth and xylem development are still not well-defined, especially in pines. Here, we treated Pinus massoniana seedlings with different concentrations of exogenous BRs, and assayed the effects on plant growth, xylem development, endogenous phytohormone contents and gene expression within stems. Application of exogenous BR resulted in improving development of xylem more than phloem, and promoting xylem development in a dosage-dependent manner in a certain concentration rage. Endogenous hormone determination showed that BR may interact with other phytohormones in regulating xylem development. RNA-seq analysis revealed that some conventional phenylpropanoid biosynthesis- or lignin synthesis-related genes were downregulated, but the lignin content was elevated, suggesting that new lignin synthesis pathways or other cell wall components should be activated by BR treatment in P. massoniana. The results presented here reveal the foundational role of BRs in regulating plant secondary growth, and provide the basis for understanding molecular mechanisms of xylem development in P. massoniana.

A Small Guanosine Triphosphate Binding Protein PagRabE1b Promotes Xylem Development in Poplar

Rab GTPases are the subfamily of the small guanosine triphosphate (GTP)-binding proteins which participated in the regulation of various biological processes. Recent studies have found that plant Rabs play some specific functions. However, the functions of Rabs in xylem development in trees remain unclear. In this study, functional identification of PagRabE1b in Populus was performed. Quantitative reverse transcription PCR (qRT-PCR) results showed that PagRabE1b was highly accumulated in stems, especially in phloem and xylem tissues. Overexpression of PagRabE1b in poplar enhanced programmed cell death (PCD) and increased the growth rate and the secondary cell wall (SCW) thickness. Quantitative analysis of monosaccharide content showed that various monosaccharides were significantly increased in secondary xylem tissues of the overexpressed lines. Flow cytometry analysis revealed that the number of apoptotic cells in PagRabE1b-OE lines is more than a wild type (WT), which indicated that PagRabE1b may play an important role in PCD. Further studies showed that overexpression of PagRabE1b increased the expression level of genes involved in SCW biosynthesis, PCD, and autophagy. Collectively, the results suggest that PagRabE1b plays a positive role in promoting the xylem development of poplar.

Transcriptomic Analysis of Seasonal Gene Expression and Regulation during Xylem Development in “Shanxin” Hybrid Poplar (Populus davidiana × Populus bolleana)

Xylem development is a key process for wood formation in woody plants. To study the molecular regulatory mechanisms related to xylem development in hybrid poplar P. davidiana × P. bolleana, transcriptome analyses were conducted on developing xylem at six different growth stages within a single growing season. Xylem development and differentially expressed genes in the six time points were selected for a regulatory analysis. Xylem development was observed in stem sections at different growth stages, which showed that xylem development extended from the middle of April to early August and included cell expansion and secondary cell wall biosynthesis. An RNA-seq analysis of six samples with three replicates was performed. After transcriptome assembly and annotation, the differentially expressed genes (DEGs) were identified, and a Gene Ontology (GO) enrichment analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis and expression analysis of the DEGs were performed on each sample. On average, we obtained >20 million clean reads per sample, which were assembled into 84,733 nonredundant transcripts, of which there were 17,603 unigenes with lengths >1 kb. There were 14,890 genes that were differentially expressed among the six stages. The upregulated DEGs were enriched in GO terms related to cell wall biosynthesis between S1 vs. S2 or S3 vs. S4 and, in GO terms, related to phytohormones in the S1 vs. S2 or S4 vs. S5 comparisons. The downregulated DEGs were enriched in GO terms related to cell wall biosynthesis between S4 vs. S5 or S5 vs. S6 and, in GO terms, related to hormones between S1 vs. S2 or S2 vs. S3. The KEGG pathways in the DEGs related to “phenylpropanoid biosynthesis”, “plant hormone signal transduction” and “starch and sucrose metabolism” were significantly enriched among the different stages. The DEGs related to cell expansion, polysaccharide metabolism and synthesis, lignin synthesis, transcription factors and hormones were identified. The identification of genes involved in the regulation of xylem development will increase our understanding of the molecular regulation of wood formation in trees and, also, offers potential targets for genetic manipulation to improve the properties of wood.

Dissection of Functional Modules of AT-HOOK MOTIF NUCLEAR LOCALIZED PROTEIN 4 in the Development of the Root Xylem

Xylem development in the Arabidopsis root apical meristem requires a complex cross talk between plant hormone signaling and transcriptional factors (TFs). The key processes involve fine-tuning between neighboring cells, mediated via the intercellular movement of signaling molecules. As an example, we previously reported that AT-HOOK MOTIF NUCLEAR LOCALIZED PROTEIN (AHL) 4 (AHL4), a member of the 29 AT-hook family TFs in Arabidopsis, moves into xylem precursors from their neighbors to determine xylem differentiation. As part of the effort to understand the molecular functions of AHL4, we performed domain swapping analyses using AHL1 as a counterpart, finding that AHL4 has three functionally distinctive protein modules. The plant and prokaryotes conserved (PPC) domain of AHL4 acts as a mediator of protein–protein interactions with AHL members. The N-terminus of AHL4 is required for the regulation of xylem development likely via its unique DNA-binding activity. The C-terminus of AHL4 confers intercellular mobility. Our characterization of modules in the AHL4 protein will augment our understanding of the complexity of regulation and the evolution of intercellular mobility in AHL4 and its relatives.

Intra-annual dynamics of cambial and xylem phenology in subalpine conifers at Deogyusan National Park in the Republic of Korea

Subalpine conifers are highly sensitive to climatic changes. In these trees, the intra-annual dynamics of cambial activity and phenological process in xylem development are closely associated with climatic conditions. However, these scientific findings have not been verified for subalpine conifers in the Republic of Korea. Therefore, we initiated such a study with four subalpine conifers, viz. Abies koreana, Pinus koraiensis, Taxus cuspidata, and Picea jezoensis, growing between 1573 and 1594 m a.s.l. at Deogyusan National Park. Microcores (Ø 2 mm) of these trees were obtained using a mini borer, called as Trephor, every week between April 7 and September 25 in 2017 to monitor their growing seasons, the intra-annual dynamics of the cambial activity, and the number of cells during phenological phases of cell enlargement and cell-wall thickening. For the study, five trees were selected for each conifer species. Results showed that the cambial activity of A. koreana, P. koraiensis, and P. jezoensis required at least 73.8 heat-sum values, whereas the T. cuspidata needed 109.6 heat-sum. The durations of cambial activity of A. koreana, P. koraiensis, T. cuspidata, and P. jezoensis were 134 (127–144), 113 (92–128), 113 (106–120), and 100 (76–128) days, respectively. The intra-annual variations of the number of cells in the cambium, during the cell enlargement phase and cell-wall thickening phase showed predominantly a bell-shaped curve, with a delay of approximately 2–4 weeks between each other. On the other hand, the number of cumulated mature cells showed an S-shaped curve. Through this study, the first fundamental data on phenological process in xylem development of subalpine conifers in the Republic of Korea have been successfully presented.

Hexose fluxes, mediated by vacuolar SWEET transporters, are important for xylem development in the inflorescence stem of Arabidopsis thaliana

ABSTRACTIn higher plants, the development of the vascular system is controlled by a complex network of transcription factors. However, how nutrient availability in the vascular cells affects their development remains to be addressed. At the cellular level, cytosolic sugar availability is regulated mainly by sugar exchanges at the tonoplast through active and/or facilitated transport. In Arabidopsis thaliana, among the tonoplastic transporters, SWEET16 and SWEET17 have been previously localized in the vascular system. Here, using a reverse genetic approach, we propose that sugar exchanges at the tonoplast, mediated by SWEET16, are important for xylem cell division as revealed in particular by the decreased number of xylem cells in the swt16 mutant and the expression of SWEET16 at the procambium-xylem boundary. In addition, we demonstrate that transport of hexoses mediated by SWEET16 and/or SWEET17 is required to sustain the formation of the xylem secondary cell wall. This result is in line with a defect in the xylem cell wall composition as measured by FTIR in the swt16swt17 double mutant and by upregulation of several genes involved in secondary cell wall synthesis. Our work therefore supports a model in which xylem development is partially dependent on the exchange of hexoses at the tonoplast of xylem-forming cells.

Bipartite influence of abscisic acid on xylem differentiation trajectories is dependent on distinct VND transcription factors in Arabidopsis

SummaryPlants display a remarkable ability to adjust their growth and development to changes in environmental conditions, such as reduction in water availability. This high degree of plasticity is apparent not only as altered root and shoot growth rates, but also as changes to tissue patterning and cell morphology [1,2]. We have previously shown that Arabidopsis thaliana root xylem displays plastic developmental responses to limited water availability, mediated by non-cell autonomous action of abscisic acid, ABA [2]. Here, we show through analyses of ABA response reporters and tissue specific suppression of ABA signalling that xylem cells act as primary signalling centres for mediation of changes to both xylem cell fate and differentiation rate revealing a cell autonomous control of xylem development by ABA. Transcriptomic changes in response to ABA showed that members of the VASCULAR RELATED NAC DOMAIN (VND) transcription factor family are rapidly activated. Molecular and genetic analyses revealed that the two aspects of xylem developmental changes, cell fate and differentiation rate, are dependent on distinct members of this transcription factor family. Thus, this study provides insights into how different aspects of developmental plasticity can be interlinked, yet genetically independent of each other. Moreover, similarities in phenotypic and molecular responses to ABA in diverse species indicate an evolutionary conservation of the ABA-xylem development regulatory network among eudicots. Hence, this study gives molecular insights on how environmental stress promotes anatomical plasticity to key plant traits with potential relevance for water use optimization and adaptation to drought conditions.