EPIDERMAL FEATURES AND SILICA DEPOSITION IN LEMMAS AND AWNS OF ZIZANIA (GRAMINEAE)

In four species of Zizania (Gramineae: Oryzeae) epidermal features of pistillate and staminate lemmas, paleas, and awns were studied by scanning electron microscopy (SEM) and energy dispersive X-ray analysis. Features observed were silica bodies, siliceous papillae, pitted siliceous papillae, stomata, microhairs, and prickle hairs. Staminate lemmas have all of these features. Pistillate lemmas have silica bodies and prickle hairs, lack stomata, and differ among species in occurrences of microhairs and siliceous papillae and pitted siliceous papillae. Awns of pistillate lemmas have silica bodies, prickle hairs, microhairs, and stomata; therefore, they possess a more complete set of features than their attached lemmas. Shapes of silica bodies on pistillate lemmas differ among species. A taxonomic key based on SEM observation of pistillate lemmas separates the four species by the shapes of silica bodies, arrangement of prickle hairs, and occurrences of microhairs and siliceous papillae. The main silica-containing structures are silica bodies, siliceous papillae, pitted siliceous papillae, and to a lesser extent prickle hairs. Pitted siliceous papillae with circular raised rims are formed by collapse or exfoliation of the tops of siliceous papillae; these have not been previously described in grasses. Comparison of epidermal features in the lemmas and leaves of Zizania shows that the former lack three kinds of nonsilicified papillae and epicuticular wax that are present on the latter but the lemmas have siliceous papillae and pits that are absent in leaves.     

ARCHAEOLEERSIA NEBRASKENSIS GEN. ET SP. NOV. (GRAMINEAE-ORYZEAE), A NEW FOSSIL GRASS FROM THE LATE TERTIARY OF NEBRASKA

Archaeoleersia nebraskensis, gen. et sp. nov. was collected from the Late Miocene Ash Hollow Formation in Garden County, Nebraska. The remains consist of anthoecia (fertile lemmas and paleas) preserved as silicifications. Examination of anthoecia by scanning electron microscopy revealed prickle hairs, simple projections, and microhairs. The fossils compare most closely with living Leersia ligularis Trin. of Central and South America. Archaeoleersia nebraskensis resembles more southern living taxa and suggests that climatic deterioration during the Late Tertiary or Quaternary resulted either in a southward migration of ancestral North American forms or in the elimination of North American populations of already widely distributed ancestral forms. Associated flora and fauna indicate a mesic, lacustrine paleoenvironment for Archaeoleersia nebraskensis.     

EPIDERMAL CHARACTERISTICS OF THE CALLUS IN ERIOCHLOA (POACEAE)

A callus or cup at the spikelet base in Eriochloa is a diagnostic character for this genus. In an investigation of Eriochloa and related genera, the callus epidermis of 19 taxa of Eriochloa and the basal portion of Axonopus, Brachiaria, Digitaria, Leptoloma, and Panicum spikelets were examined with a scanning electron microscope. Eriochloa taxa were divided into three types based on callus epidermal characteristics. The callus of E. polystachya more closely resembled the reduced first glume in other panicoid genera than the callus in other taxa of Eriochloa. Bicellular microhairs, silica bodies, and interlocking long cells support the concept that the callus in Eriochloa is in part a remnant of the first glume.     

Effects of salinity and benzyl adenine on development and function of microhairs of Zea mays L

Bicellular microhairs are present on the surfaces of leaves of grasses with the exception of the Pooideae. In some halophytic grasses, these glandular hairs secrete salt, suggesting the intriguing question ‘can the microhairs of grasses that do not normally encounter salinity also secrete salt?’ Microhairs were counted in replicas of the adaxial and abaxial surfaces of leaves of various ages of maize plants growing either in the absence of salt or in the presence of 40, 80 or 120 mM NaCl. The number of microhairs per unit area of adaxial leaf surface of the youngest leaf almost doubled as the salinity increased from zero to 120 mM NaCl; on the abaxial surface, the number of microhairs increased by 50%. Spraying this leaf with benzyl adenine (BA) caused, when averaged across salinities and surfaces, a 32% increase in the number of microhairs. Salinity reduced leaf area but in all the salinity treatments, spraying with BA increased the total number of microhairs per leaf. Washing leaves of plants provided estimates of the loss of salt from those leaves. There were large differences between the Na:K molar ratios in the washing solution and the leaf tissue, indicating a high selectivity for sodium over potassium for loss from the leaf. BA did not influence the efficiency of salt loss, expressed per microhair, at any salinity level, but did increase loss per leaf. Thus, BA increased salt loss from plants due to its influence on the number of microhairs and leaf area, but not due to its effect on the efficiency of the secretion process per se.     

A BIOSYSTEMATIC STUDY OF TRIDENS (GRAMINEAE)

Tateoka , Tuguo . (U. Montréal, Montréal, Canada.) A biosystematic study of Tridens (Gramineae) . Amer. Jour. Bot. 48(7): 565–573. Illus. 1961.—The genus Tridens has been studied from cytological, morphological, anatomical and histological points of view. Species of Tridens can be divided into 2 groups according to the characters of caryopses, stigmas, lemmas, paleas and general habit. In accordance with the morphological disparities, the 2 groups have different appearances of the anatomy of leaf margins and midribs and also of the bicellular microhairs on the epidermis of the leaf blade. Further, the 2 groups have different basic chromosome numbers, x=8 and x=10, and some distinction in karyotype. Somatic chromosomes of 8 species have been examined. The evidence given by the combined studies indicates that Tridens should be regarded as a mixture of 2 phylogenetically distant groups. Based on this conclusion, the 5 species representing one of the 2 groups (T. pilosus, T. avenaceus, T. grandiflorus, T. nealleyi and T. pulchellus) are transferred to the genus Erioneuron which is hereby revived. Nomenclatural changes are made for the species of the revived genus.     

Comparative ultrastructure of microhairs in grasses

Ultrastructure of microhairs is described for 17 species of Poaceae, involving four subfamilies and the 'chloridoid', 'panicoid' and 'Enneapogon' morphological types. The plasma membrane invaginations known as partitioning membranes implicated in salt secretion were found in the basal cells of only some of the 'chloridoid type' microhairs. They were absent from 'panicoid type' microhairs, but present in the cap cells of the 'Enneapogon type'. Characteristics common to known secretory tissues, however, were observed in all microhairs studied, implying secretory activity in all the morphological types.     

A DEVELOPMENTAL STUDY OF SILICIFICATION IN THE ABAXIAL EPIDERMAL CELLS OF SUGARCANE LEAF BLADES USING SCANNING ELECTRON MICROSCOPY AND ENERGY DISPERSIVE X-RAY ANALYSIS

A developmental study of the accumulation of silicon and other elements in the abaxial epidermis of sugarcane (Saccharum officinarum L.) leaf blades using scanning electron microscopy and energy dispersive x-ray analysis showed that accumulation of silicon progresses at different rates in each epidermal cell type. In basal cells of two-celled microhairs and in prickles there is accumulation of silicon while the leaf is immature and still enclosed within the spindle cluster of leaves and not involved in transpiration. After transpiration begins, all epidermal cells rapidly accumulate silicon. However, there are differences in the rate of silicon accumulation and in the maximum amount of silicon accumulation among the various cell types. This may relate to differences in their physiology or structure.     

Opaline Silica Deposits in the Leaves of Bidens pilosa L. and their Possible Significance in Cancer

The occurrence of silica deposits in suitably treated leavesof Bidens pilosa L. of African origin was investigated usinglight and scanning electron microscopy and X-ray analyses. Numerousfibres of amorphous silica were located at the edges and surfacesof the leaves. The deposits were always associated with theepidermal microhairs and were confined to the walls. The significance of these accumulations in a dicotyledonousspecies and in relation to possible deposition mechanisms isdiscussed. Their possible role as a cause of cancer is reviewedin the light of recent investigations involving Bidens pilosa. Bidens pilosa L., opaline silica, oesophageal cancer     

Epidermal Structure in the Ligule of Rice (Oryza sativa L.)

The structure of the abaxial epidermis of the ligule of rice(Oryza sativa L.) as seen in the light and scanning electronmicroscope is described. Long cells, silica cells, cork cells,prickle hairs, microhairs and stomata-like structures were found.Reasons for considering these latter structures to be true stomataare given. Results of an X-ray analysis for silicon are presentedand the distribution of silica discussed in relation to itsfunctional role in the ligule. Oryza sativa L, rice, Gramineae, ligule, epidermis, scanning electron microscopy, X-ray analysis, silica, stomata     

Relationships between gas-exchange characteristics and stomatal structural modifications in some desert grasses under high salinity

Two populations, one from lesser saline Derawar Fort (DF) and the other from highly saline Ladam Sir (LS) in the Cholistan desert, for each of the five grass species, Aeluropus lagopoides, Cymbopogon jwarancusa, Lasiurus scindicus, Ochthochloa compressa, and Sporobolus ioclados were examined to investigate the influence of salinity on structural and functional characteristics of stomata. Salinity tolerance in A. lagopoides mainly depended on controlled transpiration rate (E) and high water-use efficiency (WUE), which was found to be regulated by fewer and smaller stomata on both leaf surfaces as well as stomatal encryption by epidermal invaginations. C. jwarancusa had sunken stomata on the abaxial surface only, which largely reflected a reduced E, but less affected stomatal conductance (g _s) or WUE. L. scindicus had fewer but larger stomata along with hairs/trichomes which may function to avoid water loss through transpiration, and hence, to attain a high WUE. In O. compressa stomata were found only on the abaxial surface and these were completely encrypted by epidermal invaginations as well as a dense covering of microhairs, which was associated with a low E and high WUE under salinity stress. In S. ioclados, the traits of increased stomatal density and decreased stomatal area may be critical for stomatal regulation under salt-prone environments. High stomatal regulation depended largely on stomatal density, area, and degree of encryption under salinity, which is of great ecophysiological significance for plants growing under osmotic stresses.     

DRO1 influences root system architecture in Arabidopsis and Prunus species

Roots provide essential uptake of water and nutrients from the soil, as well as anchorage and stability for the whole plant. Root orientation, or angle, is an important component of the overall architecture and depth of the root system; however, little is known about the genetic control of this trait. Recent reports in Oryza sativa (rice) identified a role for DEEPER ROOTING 1 (DRO1) in influencing the orientation of the root system, leading to positive changes in grain yields under water‐limited conditions. Here we found that DRO1 and DRO1‐related genes are present across diverse plant phyla, and fall within the IGT gene family. The IGT family also includes TAC1 and LAZY1, which are known to affect the orientation of lateral shoots. Consistent with a potential role in root development, DRO1 homologs in Arabidopsis and peach showed root‐specific expression. Promoter–reporter constructs revealed that AtDRO1 is predominantly expressed in both the root vasculature and root tips, in a distinct developmental pattern. Mutation of AtDRO1 led to more horizontal lateral root angles. Overexpression of AtDRO1 under a constitutive promoter resulted in steeper lateral root angles, as well as shoot phenotypes including upward leaf curling, shortened siliques and narrow lateral branch angles. A conserved C‐terminal EAR‐like motif found in IGT genes was required for these ectopic phenotypes. Overexpression of PpeDRO1 in Prunus domestica (plum) led to deeper‐rooting phenotypes. Collectively, these data indicate a potential application for DRO1‐related genes to alter root architecture for drought avoidance and improved resource use.     

THE COMPARATIVE AND DEVELOPMENTAL MORPHOLOGY OF THE ROOT SYSTEM OF SELAGINELLA SELAGINOIDES (L.) LINK

All of the roots of Selaginella selaginoides are attached laterally to the base of the shoot, which has monopolar growth as is characteristic of Selaginella. The first three roots are produced by meristematic activity in the cortex of the hypocotyl as in several other species of Selaginella. The fourth root is produced in the same way as the first three, except that not all of the cortical cells which become meristematic mature into root tissue. Some of the meristematic tissue remains undifferentiated and continues to produce additional roots. Potentially an unlimited number of roots could be produced, but no plant was found to have more than eight. There is some secondary growth in the cortex of the basal swelling on the hypocotyl, but no secondary vascular tissue is produced and no cambium of any sort is ever organized. On the basis of comparisons with other living species of Selaginella. the centralized root system of S. selaginoides is interpreted as having been modified from a noncentralized type of root system by the persistence of the juvenile mode of root production.     

Root cap specific expression of an endo-β-1,4--glucanase (cellulase): a new marker to study root development in Arabidopsis

The sloughing of root cap cells from the root tip is important because it assists the growing root in penetrating the soil. Using a promoter–reporter (GUS) and RT-PCR analysis, we identified an endo--1,4-glucanase (AtCel5) of Arabidopsis thaliana that is expressed exclusively in root cap cells of both primary and secondary roots. Expression is inhibited by high concentrations of IAA, both exogenous and internal, as well as by ABA. AtCel5 expression begins once the mature tissue pattern is established and continues for 3weeks. GUS staining is observed in both root cap cells that are still attached and cells that have already been shed. Using AtCel5-GUS as a marker, we observed that the root cap cells begin to separate at the sides of the tip while the cells of the central region of the tip separate last. Separation involves sequential tiers of intact cells that separate from the periphery of the root tip. A homozygous T-DNA insertion mutant that does not express AtCel5 forms the root cap and sheds root cap cells but sloughing is less efficient compared to wild type. The reduction in sloughing in the mutant does not affect the overall growth performance of the plant in loose media. The modest effect of abolishing AtCel5 expression suggests that there are multiple redundant genes regulating the process of sloughing of the root cap, including AtCel3/At1g71380, the paralog of the AtCel5 gene that is also expressed in the root cap cells. Thus, these two endo-1,4--d-glucanases may have a role in the sloughing of border cells from the root tip. We propose that AtCel5, provides a new molecular marker to further analyze the process of root cap cell separation and a root cap specific promoter for targeting to the environment genes with beneficial properties for plant growth.     

The wavy growth 3 E3 ligase family controls the gravitropic response in Arabidopsis roots

Regulation of the root growth pattern is an important control mechanism during plant growth and propagation. To better understand alterations in root growth direction in response to environmental stimuli, we have characterized an Arabidopsis thaliana mutant, wavy growth 3 (wav3), whose roots show a short‐pitch pattern of wavy growth on inclined agar medium. The wav3 mutant shows a greater curvature of root bending in response to gravity, but a smaller curvature in response to light, suggesting that it is a root gravitropism‐enhancing mutation. This wav3 phenotype also suggests that enhancement of the gravitropic response in roots strengthens root tip impedance after contact with the agar surface and/or causes an increase in subsequent root bending in response to obstacle‐touching stimulus in these mutants. WAV3 encodes a protein with a RING finger domain, and is mainly expressed in root tips. RING‐containing proteins often function as an E3 ubiquitin ligase, and the WAV3 protein shows such activity in vitro. There are three genes homologous to WAV3 in the Arabidopsis genome [EMBRYO SAC DEVELOPMENT ARREST 40 (EDA40), WAVH1 and WAVH2 ], and wav3 wavh1 wavh2 triple mutants show marked root gravitropism abnormalities. This genetic study indicates that WAV3 functions positively rather than negatively in root gravitropism, and that enhancement of the gravitropic response in wav3 roots is dependent upon the function of WAVH2 in the absence of WAV3. Hence, our results demonstrate that the WAV3 family of proteins are E3 ligases that are required for root gravitropism in Arabidopsis.     

基于高通量测序分析西藏沙棘根瘤内生菌的多样性

根瘤是微生物侵染植物根部并与之形成的共生结构,这些微生物都可被称为植物内生菌。豆科植物根瘤中的内生菌常常又被称为根瘤菌,而侵染非豆科植物形成根瘤的主要是放线菌弗兰克氏菌,这些非豆科植物又被称为放线菌结瘤植物。西藏沙棘是一种典型的放线菌结瘤植物,由于其分布生境的特殊性,对其根瘤内生菌的研究具有重要的生态意义。对于西藏沙棘根瘤内生菌的研究,培养方法因难以模拟自然条件而不易获得纯培养,高通量测序技术对其多样性的研究提供了便利。因此,本研究以生长在甘肃省天祝县金强河河滩地的西藏沙棘根瘤为材料,采用16S rRNA基因扩增子高通量测序方法,结合OTU分析,对西藏沙棘根瘤内生菌的多样性进行探讨。实验结果表明,西藏沙棘根瘤内生菌具有丰富的多样性,根瘤内的优势属为共生固氮的弗兰克氏菌属（Frankia）,其相对丰度为47.63%,共检测到7个弗兰克氏菌属的OTUs;根瘤内除弗兰克氏菌外,还存在大量的非弗兰克氏菌,共检测到1523个OTUs,隶属于22个门、33个纲、69个目、113个科和202个属,相对丰度排名前9的属中有25个非弗兰克氏菌属的OTUs。该研究也表明,西藏沙棘根瘤内生菌具有丰富的多样性,西藏沙棘根瘤中不仅存在着可共生固氮的弗兰克氏菌,并且还分布着非弗兰克氏菌;在同一根瘤样品中,弗兰克氏菌属还具有不同的物种。本研究不仅拓展了西藏沙棘根瘤内生菌多样性的研究方法,还为同一寄主植物中弗兰克氏菌多样性的研究提供了分析思路。     

Multiple cyclic nucleotide‐gated channels coordinate calcium oscillations and polar growth of root hairs

Calcium gradients underlie polarization in eukaryotic cells. In plants, a tip‐focused Ca²⁺‐gradient is fundamental for rapid and unidirectional cell expansion during epidermal root hair development. Here we report that three members of the cyclic nucleotide‐gated channel family are required to maintain cytosolic Ca²⁺ oscillations and the normal growth of root hairs. CNGC6, CNGC9 and CNGC14 were expressed in root hairs, with CNGC9 displaying the highest root hair specificity. In individual channel mutants, morphological defects including root hair swelling and branching, as well as bursting, were observed. The developmental phenotypes were amplified in the three cngc double mutant combinations. Finally, cngc6/9/14 triple mutants only developed bulging trichoblasts and could not form normal root hair protrusions because they burst after the transition to the rapid growth phase. Prior to developmental defects, single and double mutants showed increasingly disturbed patterns of Ca²⁺ oscillations. We conclude that CNGC6, CNGC9 and CNGC14 fulfill partially but not fully redundant functions in generating and maintaining tip‐focused Ca²⁺ oscillations, which are fundamental for proper root hair growth and polarity. Furthermore, the results suggest that these calmodulin‐binding and Ca²⁺‐permeable channels organize a robust tip‐focused oscillatory calcium gradient, which is not essential for root hair initiation but is required to control the integrity of the root hair after the transition to the rapid growth phase. Our findings also show that root hairs possess a large ability to compensate calcium‐signaling defects, and add new players to the regulatory network, which coordinates cell wall properties and cell expansion during polar root hair growth.     

Salt-induced plasticity of root hair development is caused by ion disequilibrium in Arabidopsis thaliana

Root hair development is controlled by environmental signals. Studies on root hair plasticity in Arabidopsis thaliana have mainly focused on phosphate and iron deficiency. Root hair growth and development and their physiological role in response to salt stress are largely unknown. Here, we show that root epidermal cell types and root hair development are highly regulated by salt stress. Root hair length and density decreased significantly in a dose-dependent manner on both primary roots and junction sites between roots and shoots. The root hair growth and development were sensitive to inhibition by ions but not to osmotic stress. High salinity also alters anatomical structure of roots, leading to a decrease in cell number in N positions and enlargement of the cells. Moreover, analysis of the salt overly sensitive mutants indicated that salt-induced root hair response is caused by ion disequilibrium and appears to be an adaptive mechanism that reduces excessive ion uptake. Finally, we show that genes WER, GL3, EGL3, CPC, and GL2 might be involved in cell specification of root epidermis in stressed plants. Taken together, data suggests that salt-induced root hair plasticity represents a coordinated strategy for early stress avoidance and tolerance as well as a morphological sign of stress adaptation.     

Nitrate induction of root hair density is mediated by TGA1/TGA4 and CPC transcription factors in Arabidopsis thaliana

Root hairs are specialized cells that are important for nutrient uptake. It is well established that nutrients such as phosphate have a great influence on root hair development in many plant species. Here we investigated the role of nitrate on root hair development at a physiological and molecular level. We showed that nitrate increases root hair density in Arabidopsis thaliana. We found that two different root hair defective mutants have significantly less nitrate than wild‐type plants, suggesting that in A. thaliana root hairs have an important role in the capacity to acquire nitrate. Nitrate reductase‐null mutants exhibited nitrate‐dependent root hair phenotypes comparable with wild‐type plants, indicating that nitrate is the signal that leads to increased formation of root hairs. We examined the role of two key regulators of root hair cell fate, CPC and WER, in response to nitrate treatments. Phenotypic analyses of these mutants showed that CPC is essential for nitrate‐induced responses of root hair development. Moreover, we showed that NRT1.1 and TGA1/TGA4 are required for pathways that induce root hair development by suppression of longitudinal elongation of trichoblast cells in response to nitrate treatments. Our results prompted a model where nitrate signaling via TGA1/TGA4 directly regulates the CPC root hair cell fate specification gene to increase formation of root hairs in A. thaliana.