Meristematic endodermis and secretory structures in adventitious roots of Richterago Kuntze (Mutisieae-Asteraceae)

Endoderme meristemática e estruturas secretoras em raízes adventícias de Richterago Kuntze (Mutisieae-Asteraceae)

Meristematic endodermis and secretory structures in adventitious roots of Richterago Kuntze (Mutisieae-Asteraceae)

Endoderme meristemática e estruturas secretoras em raízes adventícias de Richterago Kuntze (Mutisieae-Asteraceae)

Gladys F.A. Melo-de-PinnaI,¹; Nanuza L. Menezes^II

^IUniversidade Federal de Pernambuco, Centro de Ciências Biológicas, Departamento de Botânica, Av. Prof. Moraes Rêgo s/n, Cidade Universitária, 50632-970 Recife, PE, Brasil

^IIUniversidade de São Paulo, Instituto de Biociências, Departamento de Botânica, Rua do Matão, Travessa 14. Cidade Universitária, 05422-970 São Paulo, SP, Brasil

ABSTRACT

The meristematic endodermis in adventitious roots of Richterago species originates in one of the fundamental meristem cells, which undergo sucessive anticlinal and periclinal divisions to build the inner cortex. The meristematic endodermis or proendodermis remains as a meristematic layer until its differentiation into endodermis, with Casparian strip. When sieve elements differentiate, endodermic secretory canals of esquizogenous origin are present at the region adjacent to primary phloem. Articulated laticifers, with cells perforated at both terminal and transversal walls, also occur during initial phases of secondary development. Presence of inulin as reserve carbohydrate in the inner cortex and vascular tissue may be related to abiotic factors, as an adaptive strategy of these species.

Key words: Asteraceae, laticifers, meristematic endodermis, Richterago, secretory canals

RESUMO

Nas raízes adventícias de espécies de Richterago, a endoderme meristemática origina-se a partir de uma das células do meristema fundamental, que irá sofrer sucessivas divisões anticlinais e periclinais para formar o córtex interno, permanecendo como uma camada meristemática até sua diferenciação em endoderme com as estrias de Caspary. Quando os elementos crivados já se encontram diferenciados, canais secretores endodérmicos, de origem esquizógena, podem ser visualizados nas regiões adjacentes ao floema primário. Laticíferos articulados ocorrem na fase inicial do desenvolvimento secundário, cujas células apresentam pontoações nas paredes terminais e transversais. A presença de inulina no córtex interno e no cilindro vascular, como carboidrato de reserva, pode estar relacionada com fatores abióticos, sendo uma estratégia adaptativa das espécies.

Palavras-chave: Asteraceae, canais secretores, endoderme meristemática, laticíferos, Richterago

Introduction

The occurrence of a meristematic phase of endodermis in Asteraceae was first demonstrated by Williams (1947). That author demonstrated that the cellular layer, which surrounds the plerome (pericycle), acts as a cambium generating the cortex, which later will differentiate into endodermis. Subsequent to Williams's paper, a considerable amount of work (Hurst 1956 apud Van Fleet 1961, Van Fleet 1961, Mueller 1991, Seago Júnior et al. 1999, Seago Júnior et al. 2000) recognised meristematic endodermis in the root´s of other vascular plants.

From the second half of the 19^th century, many authors have studied secretory structures in Asteraceae (Tetley 1925). The intercellular canals and laticifers are features recognized in this family (Col 1899, 1901, 1903, 1904, Tetley 1925, Williams 1947, 1954, Metcalf & Chalk 1950, Esau 1960, Fahn 1974, Heywood 1978, Bremer 1994). In roots, secretory canals may occur in the inner cortex (Col 1899, Triebel 1885, Solereder 1908, Tetley 1925, Williams 1947, 1954, Lersten & Curtis 1986, Luque et al. 1997), pericycle (Grotta 1944, Hoehne et al. 1952) and phloem (Metcalf & Chalk 1950, Grotta 1944). Nevertheless, no studies so far have focused on root secretory canals species of the tribe Mutisieae.

The genus Richterago Kuntze belongs to the tribe Mutisieae and is endemic to Brazil. The present work focuses on morphoanatomic features of its adventitious roots, with emphasis on the occurrence of meristematic endodermis and secretory structures.

Material and methods

Eleven species of Richterago were studied: R. stenophylla Cabrera, R. angustifolia (Gardner) Cabrera, R. arenaria (Baker) Roque, R. radiata (Vell.) Cabrera, R. polymorpha (Less.) Cabrera, R. conduplicata Roque, R. hatschbachii Zardini, R. polyphylla (Baker) Roque, R. riparia Roque, R. amplexifolia and R. lanata Roque, all collected at Serra do Cipó, Santana do Riacho country (Minas Gerais-Brazil). Material is deposited in the herbarium of Universidade de São Paulo, SPF (Roque et al. 347, Roque et al. 352, Melo et al. 19, Melo & Vitta 4, Melo et al. 15, Melo & Vitta 10, Roque et al. 345, Roque et al. 450, Roque & Hervêncio 493, Melo & Vitta 8 and Melo et al. 12).

The primary root apex and adventitious roots were obtained from wild-collected young plants fixed in FGAA (Lersten & Curtis 1988). After dehydration in graded ethanol series, the material was preserved in 70% ethanol and embedded in paraffin (Johansen 1940). The material was double-stained with Astra blue and basic fucsin, following Kraus et al. (1998). Dissociation technique with hydrogen peroxide and acetic acid (Franklin 1945) was used to analyze the laticifers.

The following stains were used for histochemical tests: Sudan IV (Sass 1951); 10% ferric chloride (Johansen 1940) and lugol (Berlyn & Mikche 1976), for lipidic substances, phenolic substances and starch, respectively. The technique described by Purvis et al. (1964) was utilized to crystallize inulin with 100% ethanol for 48 hours.

Results

The rootcap generated by calyptrogen was observed at the region of subapical meristem in adventitious roots (figures 1-2). Protoderm, ground meristem and procambium originated in the promeristem. Ground meristem generated the proendodermis or meristematic endodermis through longitudinal (tangential) divisions, creating many rows.

On transverse section of the meristematic region, the meristematic endodermis is distinguishable from the pericycle, due to morphoanatomical difference between cells (figures 3-5). A thickening on the contact walls between the meristematic endodermis and the pericycle cells was also observed (figure 6). Meristematic activity of the inner layer of the cortex shows cytoplasmic gradient characterized by the vacuole enlargement in the derivative cells, if compared to cells closer to the meristematic endodermis (figure 3). Over subsequent stages, cells resultanting from the meristematic endodermis will be arranged radially, forming the inner cortex (figures 3, 4).

At this stage, secretory canals are seen at innermost layers of the cortex, originated by an esquizogenous process, and restricted to regions adjacent to the phloem (figure 5). After meristematic activity of the proendodermis, the innermost layer of the cortex will differentiate in endodermis with deposition of Casparian strips in anticlinal walls. In innermost canals, endodermal cells with Casparian strips constitutes two of the four cells underlying the canal. The contents of secretory canals react positively to phenolic substances, carbohydrates and lipids.

During initial phase of differentiation, sieve tube elements are clearly observed on transverse section (figure 5). At subsequent stages, protoxylem differentiation initiates.

The radial arrangement of the cells generated by meristematic endodermis is shown in figure 4. Intercellular spaces are present closer to the exodermis, forming cavities. In Richterago arenaria and R. stenophylla, sclereids were present within the entire cortical region (figure 7). At the outer cortical region, cells contaning lipidic droplets were observed (figure 8). The pericycle is located internally to the endodermis and it may be biseriated or triseriated inwardly (figure 9). The radial arrangement of innermost layers of the cortex becomes less evident at advanced stages of the secondary structure (figures 10, 11).

Articulated laticifers at the cortex with perforation fields on transverse and terminal walls were observed (figures 12-14). Inulin content is observed in the inner cortex cells, including endodermis (figure 15), in the pericycle and in xylem cells. Phloem parenchyma cells also contain inulin. Inulin crystals, when analyzed under polarized light, form Malta crosses (figures 16, 17).

Discussion

Williams (1947) discussed the occurrence of a cellular layer which surrounds the tissue he called plerome which later differentiates into endodermis. The author states this layer acts as a cambium, generating every tissue between endodermis and hypodermis in the primary body of the roots. This meristematic phase of the endodermis was called proendodermis by Hurst (1956 apud Van Fleet 1961), even though Van Fleet (l.c.) and Williams (1947) had previously refered to it as "meristematic endodermis". The term proendodermis has been used by several authors (Mueller 1991, Seago Júnior et al. 1999, Seago Jr. & Scholey 1999, Seago Júnior et al. 2000), who described it as a layer derived from an initial cell of the ground meristem, located near the procambium and which would differentiate into endodermis. These features have been found in species of Richterago, especially the meristematic role of the inner layer of the cortex, which differentiates into endodermis.

The organization of secretory structures in Asteraceae has been extensively studied by Col (1899, 1901, 1903, 1904). Three types of organization in secretory structures of Asteraceae have been recognized by that author: anastomosed laticifers, secretory canals and isolated cells, which secrete latex. Col also recognized two distinct types of secretory canals: the secretory canal itself, which is always formed near the endodermis, and secretory purses, which are wider and shorter than the canals, and have their cavity surrounded by secretory cells. These purses differ from the canals only in size.

Tetley (1925) described two types of secretory canals: endodermic and non-endodermic. According to him, endodermal secretory canals may be surrounded by four epithelial cells. Nevertheless, in most cases, these cells keep dividing originating 10 epithelial cells, as in the case of Buphthalmum speciosum (Tetley 1925). Endodermal secretory canals are present in the region of cells derived from meristematic endodermis in Richterago species, but the epithelial cells do not divide and the canal is limited by four cells only.

Papers that demonstrate the endodermis participation in the formation of secretory canals include Solereder (1908), Tetley (1925), Guttemberg (1968) and Luque et al. (1997). According to Solereder (l.c.), endodermic cells divide forming biseriated region and secretory canals through an esquizogenous process. Guttenberg (1968) describes the ontogeny of secretory canals in Asteraceae, from one endodermic mother cell, which undergoes divisions forming two cell layers. This fact was confirmed by Luque et al. (1997) on Lychnophora studied species.

A survey of works on species of the family Asteraceae (table 1) shows that the 42 studied species include good representation of the subfamilies Asteroideae and Cichorioideae, but no records of the subfamily Barnadesioideae. So far, the only reference to secretory canals in Mutisieae are for the genera Richterago (present paper) and Ianthopappus (MelodePinna 2000). Secretory canals occur on the inner cortex and phloem, but they may also occur internally to the endodermis, associated with the pericycle, and have been recorded in two species of the tribe Heliantheae (Grotta 1944, Hoehne et al. 1952). In Spilanthes acmella, Grotta (1944) observed that secretory canals appear in the parenchyma of secondary phloem as the root grows.

Some authors have suggested that secretory canals and primary phloem constitute a morphological unity in which the canals help the sieve tube elements on the conduction of metabolites (Solereder 1908, Guttenberg 1968, Williams 1947, 1954). Secretory canals in the primary phloem have not been observed in Richterago.

Tetley (1925) assumed that the contents of the canals come from the phloem, because they are formed when the first sieve elements are already distinguishable. That author also suggests that secretion is generated in the phloem and passes through the radial walls of the cell which subsequently differentiates in endodermis. The three species of Lychnophora studied by Luque et al. (1997) lack indication that secretory canals were aiding phloem on the conduction of metabolites. Nevertheless, histochemical tests revealed contents of the same nature in endodermal canals and phloem cells. Physiological studies must proceed in association with histochemical and ultrastructural studies in order to elucidate a possible functional relationship between these canals and the phloem.

According to Solereder (1908), latex-secreting cells may be associated with phloem, endodermis or fibers and, when within the cortex, may be either isolated or in linear series. Datta & Iqbal (1994) suggested that articulated laticifers may be simply interconnected by plasmodesmata; either through perforations on terminal walls or by their complete absorption. For Asteraceae, Vertrees & Mahlberg (1975, 1978) described the presence of articulated laticifers with perforations on terminal walls in Cichorium intybus. Richterago species have articulated laticifers in linear series not associated with the endodermis. These laticifers have perforations on both terminal and transversal walls, in contrast to the condition described by Vertrees & Mahlberg (1975, 1978).

The carbohydrate reserve detected through histochemical tests in the roots of Richterago is only inulin. Starch grains and other polysaccharides have not been detected. Some works have emphasized the occurrence of inulin in species of the family Asteraceae (Figueiredo-Ribeiro et al. 1986, Blocklebank & Hendry 1989, Felipe & Dale 1990, Isejima & Figueiredo-Ribeiro 1991, Tertuliano & Figueiredo-Ribeiro 1993). According to Tertuliano & Figueiredo-Ribeiro (1993), in species of Asteraceae, crystals of inulin occur in cells from the parenchyma associated or not with vascular bundles. The authors do not report inulin in species of the tribe Mutisieae, although they have detected its presence in two species of Gochnatia (G. barrosii and G. pulchra).

Inulin is associated with the secondary xylem and, in minor amounts, with the medullary parenchyma of Viguiera discolor (Isejima & Figueiredo-Ribeiro 1991). In Microseris lanceolata (Incoll et al. 1989) and Helianthus tuberosus (Soja et al. 1989), inulin is associated with the secondary phloem. The Richterago species studied in this work have the same pattern of inulin distribution recorded in species of the tribes Heliantheae and Eupatorieae, i.e., cortical parenchyma, pericycle and axial parenchyma of xylem and phloem.

According to some authors (Blocklebank & Hendry 1989, Felipe & Dale 1990, Isejima & Figueiredo-Ribeiro 1991), fructans have functions other than that of energy source or reserve carbohydrate, and are also related to environmental stress tolerance, especially in savannas, as an adaptive strategy to the dry season. Richterago species studied occur in the campos rupestres, where abiotic factors like fire, low water availability in the soil and high temperatures all may affect the metabolism of these plants, and so inulin reserve might represent an adaptive strategy.

Acknowledgments - The authors thank Coordenadoria de Aperfeiçoamento de Pesquisa (Capes) and Fundação de Amparo à Pesquisa do Estado de São Paulo (Fapesp / Proc. 1997/0493-2) for funding; We are grateful to M.E.M. Estelita for discussions about laticifers.

(received: December 13, 2000; accepted: August 14, 2002)

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Publication Dates

• Publication in this collection
  17 Nov 2003
• Date of issue
  Mar 2003

History

• Received
  13 Dec 2000
• Accepted
  14 Aug 2002