Sacha Escamez

Non-Cell-Autonomous Postmortem Lignification of Tracheary Elements in Zinnia elegans

Here, we show that lignification occurs after programmed cell death in xylem tracheary elements (TEs) of Zinnia elegans xylogenic cell cultures. Living, parenchymatic xylem cells surrounding the TEs synthesize and transport lignin monomers and reactive oxygen species to the cell walls of the dead TEs, thereby contributing to TE lignification in a non-cell-autonomous manner. Postmortem lignification of xylem tracheary elements (TEs) has been debated for decades. Here, we provide evidence in Zinnia elegans TE cell cultures, using pharmacological inhibitors and in intact Z. elegans plants using Fourier transform infrared microspectroscopy, that TE lignification occurs postmortem (i.e., after TE programmed cell death). In situ RT-PCR verified expression of the lignin monomer biosynthetic cinnamoyl CoA reductase and cinnamyl alcohol dehydrogenase in not only the lignifying TEs but also in the unlignified non-TE cells of Z. elegans TE cell cultures and in living, parenchymatic xylem cells that surround TEs in stems. These cells were also shown to have the capacity to synthesize and transport lignin monomers and reactive oxygen species to the cell walls of dead TEs. Differential gene expression analysis in Z. elegans TE cell cultures and concomitant functional analysis in Arabidopsis thaliana resulted in identification of several genes that were expressed in the non-TE cells and that affected lignin chemistry on the basis of pyrolysis–gas chromatography/mass spectrometry analysis. These data suggest that living, parenchymatic xylem cells contribute to TE lignification in a non-cell-autonomous manner, thus enabling the postmortem lignification of TEs.

Programmes of cell death and autolysis in tracheary elements: when a suicidal cell arranges its own corpse removal

Tracheary element (TE) differentiation represents a unique system to study plant developmental programmed cell death (PCD). TE PCD occurs after deposition of the secondary cell walls when an unknown signal induces tonoplast rupture and the arrest of cytoplasmic streaming. TE PCD is tightly followed by autolysis of the protoplast and partial hydrolysis of the primary cell walls. This review integrates TE differentiation, programmed cell death (PCD), and autolysis in a biological and evolutionary context. The collective evidence from the evolutionary and molecular studies suggests that TE differentiation consists primarily of a programme for cell death and autolysis under the direct control of the transcriptional master switches VASCULAR NAC DOMAIN 6 (VND6) and VND7. In this scenario, secondary cell walls represent a later innovation to improve the water transport capacity of TEs which necessitates transcriptional regulators downstream of VND6 and VND7. One of the most fascinating features of TEs is that they need to prepare their own corpse removal by expression and accumulation of hydrolases that are released from the vacuole after TE cell death. Therefore, TE differentiation involves, in addition to PCD, a programmed autolysis which is initiated before cell death and executed post-mortem. It has recently become clear that TE PCD and autolysis are separate processes with separate molecular regulation. Therefore, the importance of distinguishing between the cell death programme per se and autolysis in all plant PCD research and of careful description of the morphological, biochemical, and molecular sequences in each of these processes, is advocated.

Genome sequencing and population genomic analyses provide insights into the adaptive landscape of silver birch

Silver birch (Betula pendula) is a pioneer boreal tree that can be induced to flower within 1 year. Its rapid life cycle, small (440-Mb) genome, and advanced germplasm resources make birch an attractive model for forest biotechnology. We assembled and chromosomally anchored the nuclear genome of an inbred B. pendula individual. Gene duplicates from the paleohexaploid event were enriched for transcriptional regulation, whereas tandem duplicates were overrepresented by environmental responses. Population resequencing of 80 individuals showed effective population size crashes at major points of climatic upheaval. Selective sweeps were enriched among polyploid duplicates encoding key developmental and physiological triggering functions, suggesting that local adaptation has tuned the timing of and cross-talk between fundamental plant processes. Variation around the tightly-linked light response genes PHYC and FRS10 correlated with latitude and longitude and temperature, and with precipitation for PHYC. Similar associations characterized the growth-promoting cytokinin response regulator ARR1, and the wood development genes KAK and MED5A.

METACASPASE9 modulates autophagy to confine cell death to the target cells during Arabidopsis vascular xylem differentiation

ABSTRACT We uncovered that the level of autophagy in plant cells undergoing programmed cell death determines the fate of the surrounding cells. Our approach consisted of using Arabidopsis thaliana cell cultures capable of differentiating into two different cell types: vascular tracheary elements (TEs) that undergo programmed cell death (PCD) and protoplast autolysis, and parenchymatic non-TEs that remain alive. The TE cell type displayed higher levels of autophagy when expression of the TE-specific METACASPASE9 (MC9) was reduced using RNAi (MC9-RNAi). Misregulation of autophagy in the MC9-RNAi TEs coincided with ectopic death of the non-TEs, implying the existence of an autophagy-dependent intercellular signalling from within the TEs towards the non-TEs. Viability of the non-TEs was restored when AUTOPHAGY2 (ATG2) was downregulated specifically in MC9-RNAi TEs, demonstrating the importance of autophagy in the spatial confinement of cell death. Our results suggest that other eukaryotic cells undergoing PCD might also need to tightly regulate their level of autophagy to avoid detrimental consequences for the surrounding cells. Summary: In cell cultures that simulate Arabidopsis xylem differentiation, METACASPASE9 modulates the level of autophagy during programmed cell death to prevent ectopic death of the surrounding cells.

A collection of genetically engineered Populus trees reveals wood biomass traits that predict glucose yield from enzymatic hydrolysis

Wood represents a promising source of sugars to produce bio-based renewables, including biofuels. However, breaking down lignocellulose requires costly pretreatments because lignocellulose is recalcitrant to enzymatic saccharification. Increasing saccharification potential would greatly contribute to make wood a competitive alternative to petroleum, but this requires improving wood properties. To identify wood biomass traits associated with saccharification, we analyzed a total of 65 traits related to wood chemistry, anatomy and structure, biomass production and saccharification in 40 genetically engineered Populus tree lines. These lines exhibited broad variation in quantitative traits, allowing for multivariate analyses and mathematical modeling. Modeling revealed that seven wood biomass traits associated in a predictive manner with saccharification of glucose after pretreatment. Four of these seven traits were also negatively associated with biomass production, suggesting a trade-off between saccharification potential and total biomass, which has previously been observed to offset the overall sugar yield from whole trees. We therefore estimated the “total-wood glucose yield” (TWG) from whole trees and found 22 biomass traits predictive of TWG after pretreatment. Both saccharification and TWG were associated with low abundant, often overlooked matrix polysaccharides such as arabinose and rhamnose which possibly represent new markers for improved Populus feedstocks.

Contribution of cellular autolysis to tissular functions during plant development

Plant development requires specific cells to be eliminated in a predictable and genetically regulated manner referred to as programmed cell death (PCD). However, the target cells do not merely die but they also undergo autolysis to degrade their cellular corpses. Recent progress in understanding developmental cell elimination suggests that distinct proteins execute PCD sensu stricto and autolysis. In addition, cell death alone and cell dismantlement can fulfill different functions. Hence, it appears biologically meaningful to distinguish between the modules of PCD and autolysis during plant development.

Quick Histochemical Staining Methods to Detect Cell Death in Xylem Elements of Plant Tissues

Histochemical assays of xylem cell death cannot take advantage of the conventional methods for detection of cell death, such as staining with propidium iodide or trypan/Evans blue or the TUNEL staining. This chapter presents two alternative histochemical methods that can be used to detect xylem cell death quickly and reliably using light microscopy. The first method is a viability stain that can be used to detect cell death of different types of xylem elements in basically any plant species. The second method reveals cell death in xylem vessel elements based on their functionality in transport of water and small water-soluble stains.

Life Beyond Death: The Formation of Xylem Sap Conduits

Xylem is the vascular tissue conducting water and minerals in plants. The conduction of the hydro-mineral sap in this tissue is enabled by specific conduit cells named tracheary elements (TEs). These vascular cells undergo a distinct differentiation programme which requires programmed cell death (PCD) to functionalise the cell for sap conduction: PCD empties the cell lumen leaving a hollow corpse delimited only by its cell wall to form the future vascular cylinder. In contrast to many other cell types, PCD initiates the “physiological life” of TEs to enable the cell to conduct the hydro-mineral sap. This central role of PCD appeared as the first distinct differentiation event of TE ancestor cells during plant evolution. Breakthrough studies combining real-time live-cell imaging and TE differentiation in cell suspension cultures enabled to define the temporal succession of the pre-mortem TE differentiation events—cellulose and hemicellulose depositions in the secondary cell wall—and the post-mortem events including cell wall lignification and the clearing of the residual protoplast. The coordination between these different events and the exact timing of PCD is controlled by specific signalling molecules.

Cell death in cells overlying lateral root primordia contributes to organ growth in Arabidopsis

Unlike animal development, plant organ growth is widely accepted to be determined by cell division without any contribution of cell elimination. We investigated this paradigm during Arabidopsis lateral root formation when growth of the new primordia (LRP) from pericycle-derived stem cells deep inside the root is reportedly facilitated by remodeling of the walls of overlying cells without apparent cell death. However, we observed the induction of marker genes for cell types undergoing developmental cell death in several cells overlying the growing LRP. Transmission electron microscopy, time-lapse confocal and light sheet microscopy techniques were used to establish that cell death occurred at least in a subset of endodermal LRP-overlying cells during organ emergence. Significantly, organ emergence was retarded in mutants lacking a positive cell death regulator, and restored by inducing cell death in cells overlying LRP. Hence, we conclude that in the case of LRP, cell elimination contributes to organ growth.