Aerenchyma formation and porosity in root of a mangrove plant,<Emphasis Type="Italic"> Sonneratia alba</Emphasis> (Lythraceae)

Aerenchyma gas spaces are important for plants that grow in flooded and anaerobic sites or habitats, because these gas spaces provide an internal pathway for oxygen transport. The objective of this study is to characterize the development of aerenchyma gas spaces and observe the porosity in roots of Sonneratia alba. Tissue at different developmental stages was collected from four root types, i.e. cable root, pneumatophore, feeding root and anchor root, of S. alba. In S. alba, gas space is schizogenously produced in all root types, and increases in volume from the root meristem to mature root tissues. The aerenchyma formation takes place immediately, or 3–5 mm behind the root apex. At first, cortical cells are relatively round in cross sections (near the root apex); they then become two kinds of cells, rounded and armed, which combine together, forming intercellular spaces behind the root apex. The average dimensions of cortical cells increased more than 1.3 times in the vertical direction and over 3.3 times in the horizontal direction. At maturity, aerenchyma gas spaces are long tuberous structures without diaphragms and with numerous small pores on the lateral walls. Within the aerenchyma, many sclereids grow intrusively. Root porosity in all root types ranged from 0–60%. Pneumatophores and cable roots had the highest aerenchyma area (50–60%).     

Root Cell Ultrastructure in Developing Aerenchyma Tissue of Three Wetland Species

Patterns of root cortex cell development and ultrastructurewere analysed in Sagittaria lancifolia L., Thalia geniculataL. and Pontederia cordata L. using scanning and transmissionelectron microscopy (SEM, TEM). In all three species, cortexcells were arranged in radial columns extending from the endodermisto the hypodermis/epidermis. During gas space formation, thecortex cells elongated parallel to the root radius and shrankin the plane perpendicular to the radius leaving long and thinrows of cortex cells extending from the endodermis to the epidermis.Although the cortex cells appeared collapsed in tissue withwell-developed gas spaces, TEM revealed that the cortical cellsas well as the epidermal cells maintained intact membranes andmany normal organelles. Formation of root cortex tissue withwell-developed gas spaces does not require cell death in thesespecies. Living cortex cells in root tissue with mature gasspaces could provide a symplastic pathway for transport betweenthe root stele and the living epidermal cells. Copyright 2000Annals of Botany Company Sagittaria lancifolia, Thalia geniculata, Pontederia cordata, aerenchyma, root, wetland, development     

MESOCOTYL ROOT FORMATION IN ECHINOCHLOA PHYLLOPOGON (POACEAE) IN RELATION TO ROOT ZONE AERATION

Echinochloa phyllopogon was grown hydroponically under four root zone gassing treatments to determine aeration effects on the growth and development of the plant root system. Although mesocotyl growth and the number of nodal roots were unaffected by the treatments, other aspects of plant growth were altered. Shoot growth was reduced by hypoxic (5 kPa partial pressure O₂ in nitrogen gas) and anoxic conditions (O₂ free nitrogen gas), but not by ethylene (0.1 ppm in air). Seminal root growth was unaffected by hypoxia or ethylene treatments, but was reduced under anoxia. Hypoxic environments stimulated the emergence of roots along the length of the mesocotyl when compared to aerobic controls; anoxic and ethylene treatments had no significant effects. Mesocotyl roots elongated from primordia that were produced de novo in response to the hypoxic treatment. Under hypoxic conditions, aerenchyma was present in the cortex of nodal roots and to a lesser extent in seminal roots, but mesocotyl roots were devoid of aerenchyma under these conditions. The results are compared with the literature concerning flooding and aeration effects on growth and development in other species.     

AERENCHYMA DEVELOPMENT IN WATERLOGGED PLANTS

Aerenchyma development in waterlogged Helianthus annuus, Lycopersicon esculentum, and Salix fragilis was studied. More than half of the root cortical tissue sometimes became an air cavity in willow roots which developed in water. There was no cortical aerenchyma in the terminal portion, but more advanced aerenchyma developed towards the base of the sunflower roots formed in water. Waterlogged sunflower and tomato plants developed lysigenous aerenchyma in the cortex of waterlogged stems within two days.     

Formation of intercellular gas space in the diaphragm during the development of aerenchyma in the leaf petiole of Sagittaria trifolia

The development of aerenchyma in the petiole of Sagittaria trifolia L. was studied by means of light-microscopy, scanning electron microscope, transmission electron microscope and immunofluorescence, focusing on the formation of intercellular spaces in diaphragms and its relationship with the organization of cortical microtubule arrays. A complex and organized honeycomb-like schizogenous aerenchyma formed by cylinders and vascular diaphragms was observed in the petiole of S. trifolia at different developmental stages. Cell division was the primary factor contributing to the increased volume of air spaces at early stages, while cell enlargement became the primary factor at later stages. The cortical microtubules localize at the sites where intercellular spaces and the secondary cell walls will be formed or deposited during the formation of intercellular spaces by the separation of diaphragm cells. Cortical microtubules were observed at the boundary of diaphragm cells and the fringes of intercellular spaces at later developmental stages where cell expansion occurs rapidly. These observations support the hypothesis that reorganization of cortical microtubule arrays might be related to the formation of air spaces in diaphragms and are involved in the deposition of secondary cell walls.     

ORIGIN AND DEVELOPMENT OF LATERAL ROOTS IN TYPHA GLAUCA

Lateral roots of Typha glauca arose from the pericycle of the parent adventitious root. Periclinal divisions of the pericycle gave rise to two layers; the outermost initially produced the ground meristem and protoderm, and the innermost produced the procambium. The immature endodermis of the parent root contributed to the early stages of the root tip as an endodermal covering. Prior to emergence, the ground meristem/protoderm produced cells into the endodermal covering. After emergence, the endodermal covering was replaced by a calyptrogen, which was derived from the ground meristem/protoderm and which, in turn, formed the rootcap. A typical monocotyledonous three-tiered meristem was then produced. An outer ground meristem also arose before emergence to form a hypodermis in many lateral roots; in these, crystalliferous cell production began in midcortex cells before emergence, and a small aerenchyma developed in their cortices. The rootcap columella stored small amounts of starch shortly after emergence. Lateral roots of T. glauca were smaller than their parental adventitious roots; they normally had only two to six poles of xylem and phloem, and the cortex was less than six cells across. During 1–3-cm elongation, the lateral root apical meristem and mature regions narrowed, stored starch disappeared, fewer crystals formed, aerenchyma production ceased, and the roots stopped elongation.     

Aerenchyma tissue development and gas-pathway structure in root of Avicennia marina (Forsk.) Vierh.

The aerenchyma differentiation in cable roots, pneumatophores, anchor roots, and feeding roots of the mangrove plant, Avicennia marina (Verbenaceae) was analyzed using a light microscope and scanning electron microscope. In all types, cortex cells were arranged in longitudinal columns extending from the endodermis to the epidermis. No cells in the cortex had intercellular spaces at the root tip (0–150 m), and aerenchyma started developing at 200 m from the root apex. The aerenchyma formation was due to cell separation (schizogeny) rather than cell lysis. The cell separation occurred between the longitudinal cell columns, forming long intercellular spaces along the root axis. During aerenchyma formation, the cortex cells enlarged longitudinally by 1.8–3.9 times and widened horizontally by 2.2–2.9 times. As a result, the aerenchyma had a pronounced tubular structure that was radially long, elliptical or oval in cross section and that ran parallel to the root axis. The tube had tapering ends, as did vessel elements, although there were no perforated plates. The interconnection between neighboring tubes was made by abundant small pores or canals that were schizogenous intercellular spaces between the wall cells. All aerenchyma tubes in the root were interconnected by these small pores serving as a gas pathway.     

ADVENTITIOUS ROOT DEVELOPMENT IN TYPHA GLAUCA,WITH EMPHASIS ON THE CORTEX

Adventitious root development was investigated in Typha glauca plants grown under experimental conditions with the previous year's dead, sterile stalk either emerged above or submerged below the surface of Hoagland's solution. Adventitious roots emerged from buds in which most primordia had been earlier formed. Most roots elongated to 14–19 cm in 3–4 weeks and produced abundant lateral roots to their tips. Root apical meristem organization was typically monocotyledonous with a single tier of ground meristem/protoderm over the procambium. The ground meristem had zones of periclinal divisions in its innermost and outermost layers; the innermost layer initiated the endodermis and midcortex, and the outermost layers initiated the hypodermis. Crystalliferous cells with raphides were produced in the midcortex, and aerenchyma resulted from the radial expansion of schizogenous air spaces and some lysigeny in the midcortex. The procambium produced a vascular cylinder with 10–13 phloem and xylem poles, 6–9 large metaxylem elements, and central sclerenchyma. As roots stopped elongating, they narrowed, the vascular cylinder diminished in size, typical aerenchyma was lost from the cortex, crystal production ceased, and the rootcap diminished in size with its storage starch used up. Growth was determinate in these adventitious roots. The results suggested that a periclinally derived outer ground meristem was a prerequisite for a hypodermis, which, in turn, was necessary as a structural framework for aerenchyma. Without a hypodermis, typical aerenchyma was not present.     

The influence of oxygen deficiency on ethylene synthesis, 1-aminocyclopropane-1-carboxylic acid levels and aerenchyma formation in roots of Zea mays

The relationship between ethylene production, 1-aminocyclopropane-l-carboxylic acid (ACC) concentration and aerenchyma formation (ethylene-promoted cavitation of the cortex) was studied using nodal roots of maize (Zea mays L. cv. LG11) subjected to various O₂ treatments. Ethylene evolution was 7–8 fold faster in roots grown at 3 kPa O₂ than in those from aerated solution (21 kPa O₂), and transferring roots from aerated solution to 3 kPa O₂ enhanced ethylene synthesis within less than 2 h. Ethylene production and ACC accumulation were closely correlated in different zones of hypoxic roots, regardless of whether O₂ was furnished to the roots through aerenchyma or external solution. Both ethylene production and ACC concentrations (fresh weight basis) were more than 10-fold greater in the distal 0–10 mm than in the fully expanded zone of roots at 3 kPa O₂. Aerenchyma formation occurred in the apical 20 mm of these roots. Roots transferred from air to anoxia accumulated less than 0. 1 nmol ACC (mg protein)^-1 for the first 1.75 h; no ethylene was produced in this time. The subsequent rise in ACC levels shows that ACC can reach high concentrations even in the absence of O₂, presumably due to a de-repression of ACC synthase. The hypothesis was therefore tested that anoxia in the apical region of the root caused enhanced synthesis of ACC, which was transported to more mature regions (10–20 mm behind the apex), where ethylene could be produced and aerenchyma formation stimulated. Surprisingly, exposure of intact root tips to anoxia inhibited aerenchyma formation in the mature root axis. High osmotic pressures around the growing region or excision of apices had the same effect, demonstrating that a growing apex is required for high rates of aerenchyma formation in the adjacent tissue.     

Cortical Aerenchyma formation in hypocotyl and adventitious roots of Luffa cylindrica subjected to soil flooding

BACKGROUND AND AIMS: Aerenchyma formation is thought to be one of the important morphological adaptations to hypoxic stress. Although sponge gourd is an annual vegetable upland crop, in response to flooding the hypocotyl and newly formed adventitious roots create aerenchyma that is neither schizogenous nor lysigenous, but is produced by radial elongation of cortical cells. The aim of this study is to characterize the morphological changes in flooded tissues and the pattern of cortical aerenchyma formation, and to analyse the relative amount of aerenchyma formed. METHODS: Plants were harvested at 16 d after the flooding treatment was initiated. The root system was observed, and sections of fresh materials (hypocotyl, tap root and adventitious root) were viewed with a light or fluorescence microscope. Distributions of porosity along adventitious roots were estimated by a pycnometer method. KEY RESULTS: Under flooded conditions, a considerable part of the root system consisted of new adventitious roots which soon emerged and grew quickly over the soil surface. The outer cortical cells of these roots and those of the hypocotyl elongated radially and contributed to the development of large intercellular spaces. The elongated cortical cells of adventitious roots were clearly T-shaped, and occurred regularly in mesh-like lacunate structures. In these positions, slits were formed in the epidermis. In the roots, the enlargement of the gas space system began close to the apex in the cortical cell layers immediately beneath the epidermis. The porosity along these roots was 11-45 %. In non-flooded plants, adventitious roots were not formed and no aerenchyma developed in the hypocotyl or tap root. CONCLUSIONS: Sponge gourd aerenchyma is produced by the unique radial elongation of cells that make the expansigeny. These morphological changes seem to enhance flooding tolerance by promoting tissue gas exchange, and sponge gourd might thereby adapt to flooding stress.     

Water in aerenchyma spaces in roots. A fast diffusion path for solutes

During a study of the diffusivity of sulphorhodamine G in the cortical apoplast of maize roots widely discrepant rates were found between different samples. In roots which had developed large aerenchyma spaces, the diffusion in some regions was very fast, indistinguishable from the rate in water. In other regions the rate was as much as 100 times slower. Examination of frozen intact roots with the cryo-scanning electron microscope showed the presence of liquid filling some of the aerenchyma spaces, while other spaces of the same root contained air. X-ray microanalysis of the liquid (for oxygen) showed that the liquid was water with few detectable ions. Similar liquid was present in small intercellular spaces within the spoke-like radial files of cells between the large spaces, or between remnants of collapsed cell walls at the edges of the large spaces. It is proposed that regions of roots with high diffusivity are those in which some of the aerenchyma spaces are filled with water. In seeking the origin of this liquid, the progress of aerenchyma formation could be followed in the frozen tissues. The first change observed in a group of contiguous cells was a loss of vacuolar solutes and of cell turgor. Next the walls broke apart and collapsed back onto the surrounding turgid cells leaving a volume of ion-poor liquid. The liquid was probably not that found in some aerenchyma spaces of the mature roots, because the final stage of space formation was a loss of the liquid, leaving an air filled cavity surrounded by a composite lining formed from the collapsed walls of the broken cells. It is likely that the liquid in the spaces of mature aerenchyma is exuded from the remaining living cortical cells at times when the root turgor is high. This would be consistent with several recent studies which have shown periodic exudation of water from the surface of turgid roots. The spasmodic occurrence of root cortex tissue with enhanced diffusivity would have important implications for the transport of nutrient ions across the root.Abbreviations CSEM cryo-scanning electron microscope - EDX energy dispersive X-ray microanalysis - SR-G sulphorhodamine G     

Aerenchyma Formed Under Phosphorus Deficiency Contributes to the Reduced Root Hydraulic Conductivity in Maize Roots

Root hydraulic conductivity has been shown to decrease under phosphorus （P） deficiency. This study Investigated how the formation of aerenchyma is related to this change. Root anatomy, as well as root hydraulic conductivity was studied In maize （Zea mays L.） roots under different phosphorus nutrition conditions. Plant roots under P stress showed enhanced degradation of cortical cells and the aerenchyma formation was associated with their reduced root hydraulic conductivity, supporting our hypothesis that air spaces that form in the cortex of phosphorusstressed roots Impede the radial transport of water in a root cylinder. Further evidence came from the variation In aerenchyma formation due to genotypic differences. Five maize inbred lines with different porosity in their root cortex showed a significant negative correlation with their root hydraulic conductivity. Shoot relative water content was also found lower In P-deficient maize plants than that in P-sufficient ones when such treatment was prolonged enough, suggesting a limitation of water transport due to lowered root hydraulic conductivity of P-deficient plants.     

Development and Structure of the Root Cortex in Caltha palustris L. and Nymphaea odorata Ait.

Structural features of the mature root cortex and its apoplasticpermeability to dyes have been determined for two dicotyledonouswetland plants of differing habitats: Nymphaea odorata, growingrooted in water and mud, and Caltha palustris, growing in temporalwetlands among cattails. In mature roots, movement of the apoplasticdyes, berberine and safranin, into the roots was blocked atthe hypodermis, indicating the presence of an exodermis. A hypodermiswith an exodermis, i.e. Casparian bands in the outermost uniseriatelayer plus suberin lamellae, is present in both species. InN. odorata, hypodermal walls are further modified with cellulosicsecondary walls. Roots of N. odorata and C. palustris have anendodermis with Casparian bands only. A honeycomb aerenchymais produced by differential expansion in N. odorata and includesastrosclereids and diaphragms, while roots of C. palustris haveno aerenchyma, but some irregular lacunae are found in old roots.These aspects of cortex structure are related to an open meristemorganization, with unusual patterns of cell divisions in certainground meristem cells (called semi-regular hexagon cells) ofN. odorata. The correlation between aerenchyma pattern and hypodermalstructure appears to be related to habitat differences.Copyright2000 Annals of Botany Company Caltha palustris, Nymphaea odorata, root development, cortex, endodermis, aerenchyma, exodermis, hypodermis, permeability, wetland plants     

Roots of willow (Salix viminalis L.) show marked tolerance to oxygen shortage in flooded soils and in solution culture

Responses to soil flooding and oxygen shortage were studied in field, glasshouse and controlled environment conditions. Established stools ofSalix viminalis L., were compared at five field sites in close proximity but with contrasting water table levels and flooding intensities during the preceding winter. There was no marked effect of site on shoot extension rate, time to half maximum length or final length attained. When rooted cuttings were waterlogged for 4 weeks in a glasshouse, soil redox potentials quickly decreased to below zero. Shoot extension was slowed after a delay of 20 d, while, in the upper 100 mm of soil, formation and outgrowth of unbranched adventitious roots with enhanced aerenchyma development was promoted after 7 d. At depths of 100–200 mm and 200–300 mm, extension by existing root axes was halted by soil flooding, while adventitious roots from above failed to penetrate these deeper zones. After 4 weeks waterlogging, all arrested root tips recommenced elongation when the soil was drained; their extension rates exceeding those of roots that were well-drained throughout. Growth in fresh mass was also stimulated. The additional aerenchyma found in adventitious roots in the upper 100 mm of soil may have been ethylene regulated since gas space development was inhibited by silver nitrate, an ethylene action inhibitor. The effectiveness of aerenchyma was tested by blocking the entry of atmospheric oxygen into plants with lanolin applied to lenticels of woody shoots of plants grown in solution culture. Root extension was halved, while shoot growth remained unaffected. H Lambers Section editor     

Ethylene-Induced Activation of Xylanase in Adventitious Roots of Maize as a Response to the Stress Effect of Root Submersion

Submersion of roots of ten-day-old maize (Zea maysL.) seedlings was accompanied by a decrease in pO₂and an increase in pCO₂of the medium adjacent to the roots. These changes stimulated ethylene evolution in intact plants. Enhanced biosynthesis of ethylene was accompanied by xylanase activation in adventitious roots. As a result, an enhanced formation of aerenchyma was observed in the cortex of adventitious roots. Therefore, these processes resulted in the development of a ventilation system by which O₂can reach the root system exposed to hypoxia. The volume of aerenchyma was assessed by the volume of gas cavities (porosity). In contrast to the main root, the growth of adventitious roots was not inhibited under these conditions. Enlargement of the stem base and increase in the number of aerenchymatous adventitious roots facilitated the oxygen supply to the submerged organs of the plants.     

Ontogeny and distribution of cell‐wall glycans during aerenchyma formation in roots of Pistia stratiotes (Araceae)

Aerenchyma is widely known to be lysigenous, schizogenous or, more recently, expansigenous. The interpretation and understanding of its function is questionable, given the lack of extensive knowledge on the development and cellular changes of this tissue. The aerenchyma of Pistia stratiotes roots reportedly originates from packet lysigeny. However, our observations suggest schizogenous development. Our objective was to analyse ontogeny of aerenchyma in P. stratiotes roots and evaluate the morphological and chemical changes in the cell wall during the formation of aerenchyma. The aerenchymatous inner cortex of schizogenous origin was observed under light and electron microscopy. Lacunae are formed by the separation, division and stretching of cells, which remain alive until maturity. Analyses using monoclonal anti‐glycan antibodies show that formation of that type of aerenchyma apparently proceeds through the same mechanisms as the genesis of intercellular spaces. However, the greatest changes occur when cells undergo stretching, including the loss of methyl‐esterification and detection of arabinans, which are not directly involved in cell separation. Thus, other factors may account for the formation of schizogenous aerenchyma.     

Process of aerenchyma formation and reactive oxygen species induced by waterlogging in wheat seminal roots

The development and regulation of aerenchyma in waterlogged conditions were studied in the seminal roots of wheat. Evans blue staining and the first cell death position indicated that the cortical cell death began at the root mid-cortex cells in flooding conditions. Continuous waterlogging treatment caused the spread of cell death from the mid-cortex to the neighboring cells and well-developed aerenchyma was formed after 72 h. Meanwhile, the formation of radial oxygen loss barrier was observed in the exodermis owing to the induction of Casparian bands and lignin deposition. Analysis of aerenchyma along the wheat root revealed that aerenchyma formed at 10 mm from the root tip, significantly increased toward the center of the roots, and decreased toward the basal region of the root. In situ detection of radial oxygen species (ROS) showed that ROS accumulation started in the mid-cortex cells, where cell death began indicating that cell death was probably accompanied by ROS production. Further waterlogging treatments resulted in the accumulation of ROS in the cortical cells, which were the zone for aerenchyma development. Accumulation and distribution of H₂O₂ at the subcellular level were revealed by ultracytochemical localization, which further verified the involvement of ROS in the cortical cell death process (i.e., aerenchyma formation). Furthermore, gene expression analysis indicated that ROS production might be the result of up-regulation of genes encoding for ROS-producing enzymes and the down-regulation of genes encoding for ROS-detoxifying enzymes. These results suggest that aerenchyma development in wheat roots starts in the mid-cortex cells and its formation is regulated by ROS.     

Changes in cell structure during the formation of root aerenchyma inSAGITTARIA LANCIFOLIA (Alismataceae)

In many wetland species, root aerenchyma is produced by the predictable collapse of root cortex cells, indicating a programmed cell death (PCD). The objective of this study was to characterize the cellular changes that accompany this PCD in the marsh species Sagittaria lancifolia. Structural changes in membranes and organelles were examined during development of root cortex cells to compare with previous examples of PCD. The organization of cortical microtubule (CMT) arrays in root cells from S. lancifolia was also evaluated as a possible predictor of cell lysis. Nuclear fragmentation and condensation were the earliest changes observed in cells undergoing lysis. Breakdown of the tonoplast and other organelles and disruption of the plasma membrane followed. After loss of cytoplasm, cells collapsed to form gas spaces. These results were compared to collapse of root cortical cells of Zea mays and Oryza sativa during aerenchyma development. Changes in the appearance of the cytoplasm of all three species were similar at later stages of aerenchyma development. The relative timing of disintegration of the tonoplast and middle lamella appeared to differ among the three species. Changes in the organization of CMT arrays did not appear to be a predictor of PCD in S. lancifolia. Aerenchyma production in plants involves a type of PCD that is morphologically distinct from PCD described from many animals.     

Enhanced Sensitivity to Ethylene in Nitrogen- or Phosphate-Starved Roots of Zea mays L. during Aerenchyma Formation

Adventitious roots of maize (Zea mays L. cv TX 5855), grown in a well-oxygenated nutrient solution, were induced to form cortical gas spaces (aerenchyma) by temporarily omitting nitrate and ammonium (-N), or phosphate (-P), from the solution. Previously this response was shown (MC Drew, CJ He, PW Morgan [1989] Plant Physiology 91: 266-271) to be associated with a slower rate of ethylene biosynthesis, contrasting with the induction of aerenchyma by hypoxia during which ethylene production is strongly stimulated. In the present paper, we show that aerenchyma formation induced by nutrient starvation was blocked, under noninjurious conditions, by addition of low concentrations of Ag⁺, an inhibitor of ethylene action, or of aminoethoxyvinyl glycine, an inhibitor of ethylene biosynthesis. When extending roots were exposed to low concentrations of ethylene in air sparged through the nutrient solution, N or P starvation enhanced the sensitivity to exogenous ethylene at concentrations as low as 0.05 microliters ethylene per liter air, promoting a more rapid and extensive formation of aerenchyma than in unstarved roots. We conclude that temporary deprivation of N or P enhances the sensitivity of ethylene-responsive cells of the root cortex, leading to cell lysis and aerenchyma.     

Transduction of an Ethylene Signal Is Required for Cell Death and Lysis in the Root Cortex of Maize during Aerenchyma Formation Induced by Hypoxia

Ethylene has been implicated in signaling cell death in the lysigenous formation of gas spaces (aerenchyma) in the cortex of adventitious roots of maize (Zea mays) subjected to hypoxia. Various antagonists that are known to modify particular steps in signal transduction in other plant systems were applied at low concentrations to normoxic and hypoxic roots of maize, and the effect on cell death (aerenchyma formation) and the increase in cellulase activity that precedes the appearance of cell degeneration were measured. Both cellulase activity and cell death were inhibited in hypoxic roots in the presence of antagonists of inositol phospholipids, Ca2+- calmodulin, and protein kinases. By contrast, there was a parallel promotion of cellulase activity and cell death in hypoxic and normoxic roots by contact with reagents that activate G-proteins, increase cytosolic Ca2+, or inhibit protein phosphatases. Most of these reagents had no effect on ethylene biosynthesis and did not arrest root extension. These results indicate that the transduction of an ethylene signal leading to an increase in intracellular Ca2+ is necessary for cell death and the resulting aerenchyma development in roots of maize subjected to hypoxia.