Molecular analysis of SCARECROW function reveals a radial patterning mechanism common to root and shoot

Mutation of the SCARECROW (SCR) gene results in a radial pattern defect, loss of a ground tissue layer, in the root. Analysis of the shoot phenotype of scr mutants revealed that both hypocotyl and shoot inflorescence also have a radial pattern defect, loss of a normal starch sheath layer, and consequently are unable to sense gravity in the shoot. Analogous to its expression in the endodermis of the root, SCR is expressed in the starch sheath of the hypocotyl and inflorescence stem. The SCR expression pattern in leaf bundle sheath cells and root quiescent center cells led to the identification of additional phenotypic defects in these tissues. SCR expression in a pin-formed mutant background suggested the possible origins of the starch sheath in the shoot inflorescence. Analysis of SCR expression and the mutant phenotype from the earliest stages of embryogenesis revealed a tight correlation between defective cell divisions and SCR expression in cells that contribute to ground tissue radial patterning in both embryonic root and shoot. Our data provides evidence that the same molecular mechanism regulates the radial patterning of ground tissue in both root and shoot during embryogenesis as well as postembryonically.     

Anatomy of seedling tap roots of loblolly pine (Pinus taeda L.)

The development of tap root anatomical features was investigated in seedlings of loblolly pine (Pinus taeda L.) under both pot and pouch growth regimes. The roots possessed the three anatomical zones previously observed in jack pine (Pinus banksiana Lamb) and Eucalyptus pilularis Sm. - white, condensed tannin (CT), and cork - suggesting that this developmental sequence is preserved over species and growth conditions. Xylem development was centripetal and similar to that found earlier in P. sylvestris. Tracheids with lignified, secondary walls were detected distal to the point of endodermal Casparian band deposition. However, tests for ability to conduct fluid indicated that the protoxylem was capable of transport only proximal to the Casparian bands. Detailed examination of suberin lamella deposition in the endodermis demonstrated that passage cells were present through the white and CT zones. Progressive, centripetal cortical death in the CT zone did not include the endodermis, which remained alive until the cork layer formed, at which point the endodermis was crushed. Therefore, passage cells remain as functional portals for nutrient and water uptake in the CT zone even though the central cortex is dead. Tracer tests indicated that the endodermis provides an apoplastic barrier to tracer diffusion into the stele and that this function was taken over by the young cork layers. Results of this study point to a strong role for the endodermis in the regulation of nutrient and water uptake until the maturation of the first cork layer.     

Cell-type action specificity of auxin on Arabidopsis root growth

The plant hormone auxin plays a critical role in root growth and development; however, the contributions or specific roles of cell-type auxin signals in root growth and development are not well understood. Here, we mapped tissue and cell types that are important for auxin-mediated root growth and development by manipulating the local response and synthesis of auxin. Repressing auxin signaling in the epidermis, cortex, endodermis, pericycle or stele strongly inhibited root growth, with the largest effect observed in the endodermis. Enhancing auxin signaling in the epidermis, cortex, endodermis, pericycle or stele also caused reduced root growth, albeit to a lesser extent. Moreover, we established that root growth was inhibited by enhancement of auxin synthesis in specific cell types of the epidermis, cortex and endodermis, whereas increased auxin synthesis in the pericycle and stele had only minor effects on root growth. Our study thus establishes an association between cellular identity and cell type-specific auxin signaling that guides root growth and development.     

Root growth in Arabidopsis requires gibberellin/DELLA signalling in the endodermis

Gibberellins (GAs) are key regulators of plant growth and development. They promote growth by targeting the degradation of DELLA repressor proteins; however, their site of action at the cellular, tissue or organ levels remains unknown. To map the site of GA action in regulating root growth, we expressed gai, a non-degradable, mutant DELLA protein, in selected root tissues. Root growth was retarded specifically when gai was expressed in endodermal cells. Our results demonstrate that the endodermis represents the primary GA-responsive tissue regulating organ growth and that endodermal cell expansion is rate-limiting for elongation of other tissues and therefore of the root as a whole.     

中国蜘蛛抱蛋属植物营养器官的解剖学研究

首次对中国蜘蛛抱蛋属 9种植物的营养器官进行了解剖学研究。结果表明 :该属 9种植物的解剖结构基本相同。根和根状茎都由表皮、皮层和维管柱组成。根表皮下有单层薄壁细胞 ;内、外皮层均为一层五面加厚的厚壁细胞 ,内皮层外切向壁薄 ,呈马蹄形 ,外皮层内切向壁薄而外切向壁特别加厚 ,并栓质化 ,在横切面上形成一明显的厚壁组织环带 ;根为多原形。根状茎有明显的内皮层 ,周木维管束散生于维管柱的基本组织中 ;具有次生结构 ,次生保护组织为周皮。叶为等面叶 ,具有C4植物结构特征 ;气孔在上、下表皮均有分布 ,保卫细胞肾形 ,属四轮列型。该属植物的解剖结构与其生态环境相适应 ,体现了结构与功能的统一。     

The development of the endoder mis andPhi layer of apple roots

Summary Suberin lamellae and a tertiary cellulose wall in endodermal cells are deposited much closer to the tip of apple roots than of annual roots. Casparian strips and lignified thickenings differentiate in the anticlinal walls of all endodermal andphi layer cells respectively, 4–5 mm from the root tip. 16 mm from the root tip and only in the endodermis opposite the phloem poles, suberin lamellae are laid down on the inner surface of the cell walls, followed 35 mm from the root tip by an additional cellulosic layer. Coincidentally with this last development, the suberin and cellulose layers detach from the outer tangential walls and the cytoplasm fragments. 85 mm from the root tip the xylem pole endodermis (50% of the endodermis) develops similarly, but does not collapse. 100–150 mm from the root tip, the surface colour of the root changes from white to brown, a phellogen develops from the pericycle and sloughing of the cortex begins. A few secondary xylem elements are visible at this stage.Plasmodesmata traverse the suberin and cellulose layers of the endodermis, but their greater frequency in the outer tangential and radial walls of thephi layer when compared with the endodermis suggests that this layer may regulate the inflow of water and nutrients to the stele.     

Genetic evidence that the endodermis is essential for shoot gravitropism in Arabidopsis thaliana

Shoots of higher plants exhibit negative gravitropism. However, little is known about the mechanism or site of gravity perception in shoots. We have identified two loci that are essential for normal shoot gravitropism in Arabidopsis thaliana . Genetic analysis demonstrated that the shoot gravitropism mutants sgr1 and sgr7 are allelic to the radial pattern mutants, scr and shr , respectively. Characterization of the aerial phenotype of these mutants revealed that the primary defect is the absence of a normal endodermis in hypocotyls and inflorescence stems. This indicates that the endodermis is essential for shoot gravitropism and strongly suggests that this cell layer functions as the gravity-sensing cell layer in dicotyledonous plant shoots. These results also demonstrate that, in addition to their previously characterized role in root radial patterning, SCR and SHR regulate the radial organization of the shoot axial organs in Arabidopsis .      

Focus Issue on Roots: Beyond the Barrier: Communication in the Root through the Endodermis

The root endodermis is characterized by the Casparian strip and by the suberin lamellae, two hydrophobic barriers that restrict the free diffusion of molecules between the inner cell layers of the root and the outer environment. The presence of these barriers and the position of the endodermis between the inner and outer parts of the root require that communication between these two domains acts through the endodermis. Recent work on hormone signaling, propagation of calcium waves, and plant-fungal symbiosis has provided evidence in support of the hypothesis that the endodermis acts as a signaling center. The endodermis is also a unique mechanical barrier to organogenesis, which must be overcome through chemical and mechanical cross talk between cell layers to allow for development of new lateral organs while maintaining its barrier functions. In this review, we discuss recent findings regarding these two important aspects of the endodermis.Soil contains water and dissolved nutrients needed for plant growth, but also holds pathogens and toxic compounds that can be detrimental to the plant. The root system, which is directly in contact with soil particles, can integrate environmental cues to adjust its development in order to optimize nutrient (Péret et al., 2011; Lynch, 2013) and water uptake (Cassab et al., 2013; Lynch, 2013; Bao et al., 2014) or avoid regions of high salinity (Galvan-Ampudia et al., 2013). Once anchored in the soil, roots must deal with the constraints of their local environment and develop specific barriers to balance uptake of nutrients, water, and interactions with symbionts with protection against detrimental biotic and abiotic factors.In young roots, these barriers are mainly formed by the deposition of hydrophobic polymers such as lignin and suberin within the primary cell wall of the endodermis, which separates the pericycle from the cortex (Fig. 1), and of the exodermis, which lies between the cortex and the epidermis (Nawrath et al., 2013). Although formation of an exodermis is species dependent, the endodermis is a distinguishing figure of extant vascular plants (Raven and Edwards, 2001). Within this layer, two barriers (i.e. the Casparian strip and the suberin lamellae) are sequentially deposited and regulate water and nutrient movements between the inner and outer parts of the root. In this review, we discuss how the presence of these two major endodermal barriers affects communication between the different cell layers of the root. We focus on recent articles highlighting the importance of the endodermis in this communication during various biological and developmental processes.Open in a separate windowFigure 1.Endodermal barriers affect radial movement of water and solutes through the root. A, At the root tip, to move from the soil to the outer tissues of the root and then into the stele, water and solute molecules can use either the apoplastic (black lines), symplastic (dotted lines), or transcellular (dashed lines) pathways. B, The deposition of the Casparian strip in the endodermis prevents the free apoplastic diffusion of molecules between the outer part and the inner part of the root forcing molecules to pass through the symplast of endodermal cells. C, The deposition of suberin lamellae prevents the uptake of molecules from the apoplast directly into the endodermis forcing molecules to enter the symplast from more outer tissue layers. Suberin deposition is also likely to prevent the backflow of water and ions out of the stele. Passage cells are unsuberized and may facilitate the uptake of water and nutrients in older parts of the root. Cor, Cortex; End, endodermis; Epi, epidermis; Peri, pericycle; Vasc, vasculature. Figure redrawn and modified from Geldner et al. (2013).     

类短命植物阿尔泰独尾草的解剖学研究

通过常规石蜡切片法,对类短命植物阿尔泰独尾草的根、茎、叶、花等器官进行了解剖结构的研究。结果表明：阿尔泰独尾草的根系有明显的二型性,即有储藏根和吸收根的结构与功能分化,是其对生长发育快速、年休眠期长的类短命植物生活习性高度适应的结果;其营养器官表现出明显的旱生植物特征;储藏根与吸收根木质部的二型性及其内皮层的带状凯氏带增厚结构则说明独尾草属植物可能具有较为特殊的系统演化地位。     

The Arabidopsis cytochrome P450 CYP86A1 encodes a fatty acid omega-hydroxylase involved in suberin monomer biosynthesis

The lipophilic biopolyester suberin forms important boundaries to protect the plant from its surrounding environment or to separate different tissues within the plant. In roots, suberin can be found in the cell walls of the endodermis and the hypodermis or periderm. Apoplastic barriers composed of suberin accomplish the challenge to restrict water and nutrient loss and prevent the invasion of pathogens. Despite the physiological importance of suberin and the knowledge of the suberin composition of many plants, very little is known about its biosynthesis and the genes involved. Here, a detailed analysis of the Arabidopsis aliphatic suberin in roots at different developmental stages is presented. This study demonstrates some variability in suberin amount and composition along the root axis and indicates the importance of omega-hydroxylation for suberin biosynthesis. Using reverse genetics, the cytochrome P450 fatty acid omega-hydroxylase CYP86A1 (At5g58860) has been identified as a key enzyme for aliphatic root suberin biosynthesis in Arabidopsis. The corresponding horst mutants show a substantial reduction in omega-hydroxyacids with a chain length 相似文献   

SCHIZORIZA controls an asymmetric cell division and restricts epidermal identity in the Arabidopsis root

The primary root of Arabidopsis has a simple cellular organisation. The fixed radial cell pattern results from stereotypical cell divisions that occur in the meristem. Here we describe the characterisation of schizoriza (scz), a mutant with defective radial patterning. In scz mutants, the subepidermal layer (ground tissue) develops root hairs. Root hairs normally only form on epidermal cells of wild-type plants. Moreover, extra periclinal divisions (new wall parallel to surface of the root) occur in the scz root resulting in the formation of supernumerary layers in the ground tissue. Both scarecrow (scr) and short root (shr) suppress the extra periclinal divisions characteristic of scz mutant roots. This results in the formation of a single layered ground tissue in the double mutants. Cells of this layer develop root hairs, indicating that mis-specification of the ground tissue in scz mutants is uncoupled to the cell division defect. This suggests that during the development of the ground tissue SCZ has two distinct roles: (1) it acts as a suppressor of epidermal fate in the ground tissue, and (2) it is required to repress periclinal divisions in the meristem. It may act in the same pathway as SCR and SHR.