The mystery of underground death: cell death in roots during ontogeny and in response to environmental factors

The relationship between autophagy and respiratory viruses

Respiratory viruses have caused severe global health problems and posed essential challenges to the medical community. In recent years, the role of autophagy as a critical process in cells in viral respiratory diseases has been noticed. One of the vital catabolic biological processes in the body is autophagy. Autophagy contributes to energy recovery by targeting and selectively directing foreign microorganisms, organelles, and senescent intracellular proteins to the lysosome for degradation and phagocytosis. Activation or suppression of autophagy is often initiated when foreign pathogenic organisms such as viruses infect cells. Because of its antiviral properties, several viruses may escape or resist this process by encoding viral proteins. Viruses can also use autophagy to enhance their replication or prolong the persistence of latent infections. Here, we provide an overview of autophagy and respiratory viruses such as coronavirus, rhinovirus, parainfluenza, influenza, adenovirus, and respiratory syncytial virus, and examine the interactions between them and the role of autophagy in the virus-host interaction process and the resulting virus replication strategy.

The wild plant Gnaphalium lavandulifolium as a sentinel for biomonitoring the effects of environmental heavy metals in the metropolitan area of México Valley

Biomonitoring is a valuable tool for assessing the presence and effects of air pollutants such as heavy metals (HM); due to their toxicity and stability, these compounds can affect human health and the balance of ecosystems. To assess its potential as a sentinel organism of HM pollution, the wild plant Gnaphalium lavandulifolium was exposed to four sites in the metropolitan area of México Valley (MAMV): Altzomoni (ALT) Coyoacán (COY), Ecatepec (ECA), and Tlalnepantla (TLA) during 2, 4, and 8 weeks, between October and November 2019. Control plants remained under controlled conditions. The chemical analysis determined twelve HM (Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V, and Zn) in the leaves. Macroscopic damage to the leaves, later determined in semi-thin sections under light microscopy, lead to a finer analysis. Transmission electron microscope (TEM) showed major structural changes: chromatin condensation, protoplast shrinkage, cytoplasm vacuolization, cell wall thinning, decreased number and size of starch grains, and plastoglobules in chloroplasts. All these characteristics of stress-induced programed cell death (sPCD) were related to the significant increase of toxic HM in the leaves of the exposed plants compared to the control (p < 0.05). Immunohistochemistry revealed a significant amount of proteases with caspase 3-like activity in ECA and TLA samples during long exposure times. Ultrastructural changes and sPCD features detected confirmed the usefulness of G. lavandulifolium as a good biomonitor of HM contamination. They supported the possibility of considering subcellular changes as markers of abiotic stress conditions in plants.

The hidden half comes into the spotlight: Peeking inside the black box of root developmental phases

Efficient use of natural resources (e.g., light, water, and nutrients) can be improved with a tailored developmental program that maximizes the lifetime and fitness of plants. In plant shoots, a developmental phase represents a time window in which the meristem triggers the development of unique morphological and physiological traits, leading to the emergence of leaves, flowers, and fruits. Whereas developmental phases in plant shoots have been shown to enhance food production in crops, this phenomenon has remained poorly investigated in roots. In light of recent advances, we suggest that root development occurs in three main phases: root apical meristem appearance, foraging, and senescence. We provide compelling evidence suggesting that these phases are regulated by at least four developmental pathways: autonomous, non-autonomous, hormonal, and periodic. Root developmental pathways differentially coordinate organ plasticity, promoting morphological alterations, tissue regeneration, and cell death regulation. Furthermore, we suggest how nutritional checkpoints may allow progression through the developmental phases, thus completing the root life cycle. These insights highlight novel and exciting advances in root biology that may help maximize the productivity of crops through more sustainable agriculture and the reduced use of chemical fertilizers.

Autophagy-an underestimated coordinator of construction and destruction during plant root ontogeny

MAIN CONCLUSION

Autophagy is a key but undervalued process in root ontogeny, ensuring both the proper development of root tissues as well as the senescence of the entire organ. Autophagy is a process which occurs during plant adaptation to changing environmental conditions as well as during plant ontogeny. Autophagy is also engaged in plant root development, however, the limitations of belowground studies make it challenging to understand the entirety of the developmental processes. We summarize and discuss the current data pertaining to autophagy in the roots of higher plants during their formation and degradation, from the beginning of root tissue differentiation and maturation; all the way to the aging of the entire organ. During root growth, autophagy participates in the processes of central vacuole formation in cortical tissue development, as well as vascular tissue differentiation and root senescence. At present, several key issues are still not entirely understood and remain to be addressed in future studies. The major challenge lies in the portrayal of the mechanisms of autophagy on subcellular events in belowground plant organs during the programmed control of cellular degradation pathways in roots. Given the wide range of technical areas of inquiry where root-related research can be applied, including cutting-edge cell biological methods to track, sort and screen cells from different root tissues and zones of growth, the identification of several lines of evidence pertaining to autophagy during root developmental processes is the most urgent challenge. Consequently, a substantial effort must be made to ensure whether the analyzed process is autophagy-dependent or not.

Review: Emerging roles of brassinosteroid in nutrient foraging

Brassinosteroids (BRs) are well-characterized growth hormones that are critical for plant growth, development, and productivity. Genetic and molecular studies have revealed the key components of BR biosynthesis and signaling pathways. The membrane-localized BR signaling receptor, BRASSINOSTEROID INSENSITIVE1 (BRI1) binds directly to its ligand and initiates series of signaling events that led to the activation of BR transcriptional regulators, BRASSINAZOLE RESISTANT1 (BZR1) and BRI1-ETHYL METHANESULFONATE-SUPPRESSOR1 (BES1/BZR2) to regulate the cellular processes. Insights from Arabidopsis research revealed tissue and cell type-specific roles of BR in controlling cell elongation and maintenance of stem cell niche in roots. More recently, BRs have gained much attention in regulating the root growth during nutrient deficiency such as nitrogen, phosphorus, and boron. Differential distribution of nutrients in the rhizosphere alters BR hormone levels and signaling to reprogram spatial distribution of root system architecture (RSA) such as a change in primary root growth, lateral root numbers, length, and angle, root hair formation and elongation. These morpho-physiological changes in RSA are also known as an adaptive root trait or foraging response of the plant. In this review, we highlight the role of BRs in regulating RSA to increase root foraging response during fluctuating nutrient availability.

Occurrence of autophagy during pioneer root and stem development in Populus trichocarpa

Autophagy is involved in developmentally programmed cell death and is identified during the early development of phloem, as well as xylem with a dual role, as both an inducer and executioner of cell death. The regulation of primary and secondary development of roots and stems is important for the establishment of root systems and for the overall survival of trees. The molecular and cellular basis of the autophagic processes, which are used at distinct moments during the growth of both organs, is crucial to understand the regulation of their development. To address this, we use Populus trichocarpa seedlings grown in a rhizotron system to examine the autophagy processes involved in root and stem development. To monitor the visual aspects of autophagy, transmission electron microscopy (TEM) and immunolocalization of AuTophaGy-related protein (ATG8) enabled observations of the phenomenon at a structural level. To gain further insight into the autophagy process at the protein and molecular level, we evaluated the expression of ATG gene transcripts and ATG protein levels. Alternations in the expression level of specific ATG genes and localization of ATG8 proteins were observed during the course of root or stem primary and secondary development. Specifically, ATG8 was present in the cells exhibiting autophagy, during the differentiation and early development of xylem and phloem tissues, including both xylary and extraxylary fibers. Ultrastructural observations revealed tonoplast invagination with the formation of autophagic-like bodies. Additionally, the accumulation of autophagosomes was identifiable during the differentiation of xylem in both organs, long before the commencement of cell death. Taken together, these results provide evidence in support of the dual role of autophagy in developmental PCD. A specific role of the controller of cell death, which is a committed step with the release of hydrolytic enzymes from the vacuole and final digestion of protoplast, from which there is no return once initiated, is only attributed to mega-autophagy.

Phosphoethanolamine N-methyltransferase 1 contributes to maintenance of root apical meristem by affecting ROS and auxin-regulated cell differentiation in Arabidopsis

The continuous growth of roots requires the balance between cell division and differentiation. Reactive oxygen species (ROS) and auxin are important regulators of root development by affecting cell division and differentiation. The mechanism controlling the coordination of cell division and differentiation is not well understood. Using a forward genetic screen, we isolated a mutant, defective primary root 2 (dpr2), defective in root apical meristem (RAM) maintenance. The DPR2 gene encodes phosphoethanolamine N-methyltransferase 1 (PEAMT1) that catalyzes phosphocholine biosynthesis in Arabidopsis. We characterized the primary root phenotypes of dpr2 using various marker lines, using histochemical and pharmacological analysis to probe early root development. Loss-of-function of DPR2/PEAMT1 resulted in RAM consumption by affecting root stem cell niche, division zone, elongation and differentiation zone (EDZ). PIN-FORMED (PIN) protein abundance, PIN2 polar distribution and general endocytosis were impaired in the root tip of dpr2. Excess hydrogen peroxide and auxin accumulate in the EDZ of dpr2, leading to RAM consumption by accelerating cell differentiation. Suppression of ROS over-accumulation or inhibition of auxin signalling partially prevent RAM differentiation in dpr2 after choline starvation. Taken together, we conclude that the EDZ of the root tip is most sensitive to choline shortage, leading to RAM consumption through an ROS-auxin regulation module.

Structural Evidence of Programmed Cell Death Induction by Tungsten in Root Tip Cells of Pisum sativum

Previous studies have shown that excess tungsten (W), a rare heavy metal, is toxic to plant cells and may induce a kind of programmed cell death (PCD). In the present study we used transmission electron microscopy (TEM) and confocal laser scanning microscopy (CLSM) to investigate the subcellular malformations caused by W, supplied as 200 mg/L sodium tungstate (Na₂WO₄) for 12 or 24 h, in root tip cells of Pisum sativum (pea), The objective was to provide additional evidence in support of the notion of PCD induction and the presumed involvement of reactive oxygen species (ROS). It is shown ultrastructurally that W inhibited seedling growth, deranged root tip morphology, induced the collapse and deformation of vacuoles, degraded Golgi bodies, increased the incidence of multivesicular and multilamellar bodies, and caused the detachment of the plasma membrane from the cell walls. Plastids and mitochondria were also affected. By TEM, the endoplasmic reticulum appeared in aggregations of straight, curved or concentric cisternae, frequently enclosing cytoplasmic organelles, while by CLSM it appeared in bright ring-like aggregations and was severely disrupted in mitotic cells. However, no evidence of ROS increase was obtained. Overall, these findings support the view of a W-induced vacuolar destructive PCD without ROS enhancement.

Plant organ senescence - regulation by manifold pathways

Senescence is the final stage of plant ontogeny before death. Senescence may occur naturally because of age or may be induced by various endogenous and exogenous factors. Despite its destructive character, senescence is a precisely controlled process that follows a well-defined order. It is often inseparable from programmed cell death (PCD), and a correlation between these processes has been confirmed during the senescence of leaves and petals. Despite suggestions that senescence and PCD are two separate processes, with PCD occurring after senescence, cell death responsible for senescence is accompanied by numerous changes at the cytological, physiological and molecular levels, similar to other types of PCD. Independent of the plant organ analysed, these changes are focused on initiating the processes of cellular structural degradation via fluctuations in phytohormone levels and the activation of specific genes. Cellular structural degradation is genetically programmed and dependent on autophagy. Phytohormones/plant regulators are heavily involved in regulating the senescence of plant organs and can either promote [ethylene, abscisic acid (ABA), jasmonic acid (JA), and polyamines (PAs)] or inhibit [cytokinins (CKs)] this process. Auxins and carbohydrates have been assigned a dual role in the regulation of senescence, and can both inhibit and stimulate the senescence process. In this review, we introduce the basic pathways that regulate senescence in plants and identify mechanisms involved in controlling senescence in ephemeral plant organs. Moreover, we demonstrate a universal nature of this process in different plant organs; despite this process occurring in organs that have completely different functions, it is very similar. Progress in this area is providing opportunities to revisit how, when and which way senescence is coordinated or decoupled by plant regulators in different organs and will provide a powerful tool for plant physiology research.

Proteasome Activity Profiling Uncovers Alteration of Catalytic β2 and β5 Subunits of the Stress-Induced Proteasome during Salinity Stress in Tomato Roots

The stress proteasome in the animal kingdom facilitates faster conversion of oxidized proteins during stress conditions by incorporating different catalytic β subunits. Plants deal with similar kind of stresses and also carry multiple paralogous genes encoding for each of the three catalytic β subunits. Here, we investigated the existence of stress proteasomes upon abiotic stress (salt stress) in tomato roots. In contrast to Arabidopsis thaliana, tomato has a simplified proteasome gene set with single genes encoding each β subunit except for two genes encoding β2. Using proteasome activity profiling on tomato roots during salt stress, we discovered a transient modification of the catalytic subunits of the proteasome coinciding with a loss of cell viability. This stress-induced active proteasome disappears at later time points and coincides with the need to degrade oxidized proteins during salt stress. Subunit-selective proteasome probes and MS analysis of fluorescent 2D gels demonstrated that the detected stress-induced proteasome is not caused by an altered composition of subunits in active proteasomes, but involves an increased molecular weight of both labeled β2 and β5 subunits, and an additional acidic pI shift for labeled β5, whilst labeled β1 remains mostly unchanged. Treatment with phosphatase or glycosidases did not affect the migration pattern. This stress-induced proteasome may play an important role in PCD during abiotic stress.