Characterization and genetic mapping of a mutation (ms35) which prevents anther dehiscence in Arabidopsis thaliana by affecting secondary wall thickening in the endothecium

The male sterile mutant, ms35 , of Arabidopsis thaliana was produced by X-irradiation of seeds. The mutant produces fertile pollen, but is male sterile because the anthers do not dehisce. Anther development in ms35 plants occurs as in wild-type Arabidopsis until shortly after microspores are released from meiotic tetrads. Thereafter, in the wild type, bands of lignified, cellulosic secondary wall thickenings are laid down around the cells of the anther endothecium. In contrast, wall thickenings are not formed in the endothecium of the ms35 mutant. Development of other lignified tissues, for example the vascular tissue of the stamen, occurs normally in ms35 plants. In mutant anthers, as pollen maturation is completed, the stomium is cleaved but the anther wall does not retract to release pollen. The block in anther dehiscence in ms35 plants is specifically correlated with the absence of endothecial wall thickenings. The ms35 mutation represents the first genetic evidence in support of the proposed role of the endothecium in anther dehiscence. The ms35 gene was mapped to the top arm of chromosome 3 ( hy2 -(4.17±2.31 cM)- ms35 -(32.14±5.45 cM)- gl1 ).     

The systematic significance of the endothecium in Begoniaceae

The endothecial thickening patterns in 173 species, representing the three genera of Begoniaceae, were investigated using cleared macerated anthers. Begoniaceae contain taxa with U-shaped thickenings, perforate base plates, entire base plates, tympanate base plates, anticlinal bars, and taxa that lack endothecial thickenings. The degree of correlation between these endothecial classes and accepted taxonomic boundaries varies: some classes are confined to a single taxonomic unit (e.g. absence of thickenings, non-perforate tympanate base plates) while others are present in several taxonomic units (e.g. U-shapes, perforate base plates). This study provides an improved understanding of the diversity of endothecial patterns in a large genus and indicates that in the Begoniaceae the endothecium is of moderate systematic value within and between closely related sections in Begonia and in Symbegonia . We conclude that the endothecium exhibits high levels of homoplasy between distantly related sections of Begonia . In addition, we show that in contrast to past observations of the endothecium in dicotyledons, apical anther dehiscence in the Begoniaceae is not correlated with a loss of endothecial thickening.     

Elucidating the mechanism of poricidal anther dehiscence in Miconia species (Melastomataceae)

Melastomataceae have porate anthers. However, unlike Solanaceae and many monocots, in which the poricidal dehiscence depends on the presence of a mechanical layer (often the endothecium), most members of Melastomataceae have no evident specialized layer related to the poricidal opening. The goal of this study was to characterize the tissues that form the apical pore of the anther in 10 Miconia species, which may help to understand the nearly unknown mechanism of anther dehiscence in this genus, considered to be one of the largest and most diverse New World genera. Before anthesis, the apical pores of all of the species are closed by a uniseriate epidermis, the cells of which lack a cuticle. In contrast, the epidermis of the remainder of the anther is covered by a thick, ornamented cuticle. Among Myrtales, the Melastomataceae form a clade with Alzateaceae, Crypteroniaceae and Penaeaceae, almost all of which have anthers with endothecium lacking wall thickenings. In these families, the endothecium may or may not be present in the mature anther, with degenerating cells in the latter case. Anther dehiscence does not depend on the endothecium as the mechanical layer, and this process is still not well understood. However, in the Miconia species studied here, the cuticle may prevent tissue dehydration, and the pore opening seems to be due to the passive process of dehydration taking place only in the pore region due to the absence of the cuticle.     

CLADISTIC ANALYSIS OF PATTERNS OF ENDOTHECIAL THICKENINGS IN THE POALES/RESTIONALES

The three-dimensional structure of the endothecial thickenings in the anthers was investigated in 87 species from 70 genera, chosen to provide representative coverage of the families Cyperaceae, Restionaceae, Anarthriaceae, Ecdeiocoleaceae, Centrolepidaceae, Joinvilleaceae, Flagellariaceae, Poaceae, Xyridaceae, and Eriocaulaceae. There is complex variation in the patterns of endothecial thickening: the Eriocaulaceae, Flagellariaceae, and most Poaceae have thickenings with a complete baseplate; the Cyperaceae and most of the Restionaceae are characterized by helical thickenings; some genera in the Bambusoideae have annular thickenings; and U-shaped thickenings occur in the Xyridaceae and Eriocaulaceae and in some Poaceae and Restionaceae. Joinvillea and Ecdeiocolea have unique thickening types. Endothecial characters were subjected to cladistic analysis. Including endothecial characters in an existing cladogram of the group indicates that there is no single, well-corroborated cladogram available for the Poales/Restionales.     

ENDOTHECIUM IN IRIDACEAE AND ITS SYSTEMATIC IMPLICATIONS

An examination of the endothecial thickenings in 44 species of Iridaceae, selected from the four subfamilies and all major tribes, provides useful information about generic and tribal relationships in the family. U-shaped thickenings occur in Nivenioideae and Iridoideae—Sisyrinchieae, the latter the least specialized tribe of its subfamily. The occurrence of helical thickenings in all members examined of Iridiodeae tribes Irideae, Mariceae, and Tigridieae (a putatively monophyletic group) and Ixioideae is consistent with the recognition of these two lines as distinct taxa based on anatomical, morphological, phytochemical, and in the case of Ixioideae, palynological criteria. Baseplate thickenings are restricted to Patersonia. However, the shrubby Cape genera—Nivenia, Klattia, and Witsenia—have U-shaped thickenings which show a tendency for the bars on the inner periclinal cell walls to anastomose, suggesting a trend towards the baseplate condition in Patersonia. This accords with a suggested relationship between these genera, based on anatomical and flavonoid similarities. The pattern of variation in endothecial thickenings in Iridaceae is consistent with the phylogeny proposed by Goldblatt (1990). The distribution of thickening types within the family does not make it possible to polarize this character, but the most parsimonious interpretation assumes that U-shapes are basic. However, in at least some other monocotyledonous families the pattern suggests that U-shaped thickenings are derived from helices.     

PATTERNS OF ENDOTHECIAL WALL THICKENINGS IN ARACEAE: SUBFAMILIES POTHOIDEAE AND MONSTEROIDEAE

A survey of the patterns of endothecial wall thickenings in 106 representative species from 20 genera in the Pothoideae and Monsteroideae was made using cleared anthers, sections and macerations. The wide variety of wall thickenings that is present is based on an annular-helical pattern. Variations in thickenings are related to differences in cell shape, cell orientation, intergradation between helical and annular patterns, pitch of helices, presence of branched thickenings, and various types of discontinuities in thickenings. Notable exceptions to the annular-helical pattern include Culcasia, which lacks a differentiated endothecial layer with thickenings, and Acorus, which has a peculiar stellate pattern that is unique in the family. No single pattern consistently characterizes either subfamily, although continuous helices are common in the Monsteroideae, and rare in the endothecium of Pothoideae (except Anadendrum). Monsteroideae frequently exhibit a series of slanted separate thickenings on anticlinal walls, which is absent from Pothoideae except in Heteropsis. The slanted pattern is considered a variation on a rectangular helix, involving discontinuities of thickenings on the periclinal walls. Some monsteroid genera show considerably more interspecific variation (Rhaphidophora) than others (Monstera). Endothecial thickenings constitute an anatomical character that is useful in the systematic study of Araceae; present results support other anatomical studies in identifying Culcasia and Acorus as highly divergent genera in the Pothoideae.     

Comparative microsporogenesis and anther development of selected species from Magnoliaceae

Stamen development and microsporogenesis of four species from Magnoliaceae was investigated in order to provide additional data from this family. Stamen bases were found to be wide and short, without morphological differentiation in Magnolia moto, M. paenetalauma and Woonyoungia septentrionalis. In contrast, stamens are distinctly differentiated into anther and filament regions in Michelia crassipes. The orientation of dehiscence is introrse, introrse‐latrorse and latrorse in M. moto, M. paenetalauma and M. crassipes, respectively. The vascular bundles range from three to five (M. moto, M. paenetalauma) to one (M. crassipes). The amount of the connective tissue has been reduced from three to two times of the sporogenous tissue in M. moto and M. paenetalauma. The two parts are nearly equal in M. crassipess. In W. septentrionalis, the orientation of dehiscence, the vascular bundles and the size of the connective tissue vary in different parts of the floral receptacle. The endothecium and endothecial‐like cells form a ring that encloses the entire anther. The middle layer cells originate from both the outer and inner secondary parietal layers, and start to degenerate gradually at the microspore interphase stage or meiosis stage. The tapetum is of the secretory type, derived from the inner secondary parietal cells. The mature anther wall is composed of one epidermal, one endothecial, three to four middle layer(s) and one glandular tapetum. Only one epidermis, one endothecium, and the remnants of the middle layer and tapetum are left before anther dehiscence. Microspore tetrads appear as isobilateral, tetrahedral, decussate and T‐shaped, produced by a modified simultaneous microsporogenesis, which have evolved from the common ancestor of all Magnoliaceae. Our results support an ancestral state with stamens with non‐marginal sporangia and the amount of sterile tissue exceeding the amount of sporogenous tissue, and evolutionary trends toward equalization of the amount of fertile and sterile tissue on the stamen.     

中国合耳菊属(菊科-千里光族)两新记录种

首次报道了红脉合耳菊[Synotis rufinervis(DC.)C.JeffreyY.L.Chen]和须弥合耳菊[S.kunthiana(Wall.ex DC.)C.JeffreyY.L.Chen](菊科-千里光族)在中国的分布记录。两种植物都在中国西藏南部有分布。提供了它们的详细形态描述、形态特征图及其在中国的地理分布,同时报道了两种植物的花药领构型和花药内壁细胞增厚方式。     

Parasenecio chenopodiifolius (Compositae–Senecioneae) is a Synotis and conspecific with S. otophylla based on evidence from morphology,cytology and ITS/ETS sequence data

Observations on living plants and herbarium specimens have convinced us that Synotis otophylla (Compositae–Senecioneae) from south Xizang (Tibet), China, must be merged with the Himalayan Parasenecio chenopodiifolius. However, is the plant a Synotis or a Parasenecio? We have examined floral micro‐morphological and cytological characters of P. chenopodiifolius, and performed molecular phylogenetic analyses based on ITS and ETS sequence data. The anther collar of this taxon is balusterform, and the endothecial tissue cell wall thickenings are radial. These conform to the floral micro‐characters of Synotis but differ from those of Parasenecio, which has subcylindrical anther collars, and polarized endothecial tissue cell wall thickenings. Chromosome counts indicate the taxon has 2n = 40, a number common in Synotis, whereas Parasenecio has 2n = 60. Our phylogenetic analyses based on ITS/ETS sequences also place P. chenopodiifolius in Synotis. We make the new combination S. chenopodiifolia and synonymize S. otophylla.     

Floral micromorphology and its systematic implications in the genus Sinosenecio (Senecioneae-Asteraceae)

Several floral microcharacters (configuration of stigmatic areas on the inner surface of the style branch, shape of the anther apical appendage, anther size, and configuration of the endothecial thickenings and of the filament collar) in 63 populations representing 35 species and one variety of Sinosenecio (Senecioneae-Asteraceae) were investigated. The floral micromorphological data obtained are highly consistent with evidence from molecular systematics and cytology, strongly suggesting a polyphyletic nature of the currently circumscribed Sinosenecio and the necessity of a taxonomic change at generic level.     

Arabidopsis MYB26/MALE STERILE35 regulates secondary thickening in the endothecium and is essential for anther dehiscence

The Arabidopsis thaliana MYB26/MALE STERILE35 (MS35) gene is critical for the development of secondary thickening in the anther endothecium and subsequent dehiscence. MYB26 is localized to the nucleus and regulates endothecial development and secondary thickening in a cell-specific manner in the anther. MYB26 expression is seen in anthers and also in the style and nectaries, although there is no effect on female fertility in the ms35 mutant. MYB26 expression in anthers occurs early during endothecial development, with maximal expression during pollen mitosis I and bicellular stages, indicating a regulatory role in specifying early endothecial cell development. Overexpression of MYB26 results in ectopic secondary thickening in both Arabidopsis and tobacco (Nicotiana tabacum) plants, predominantly within the epidermal tissues. MYB26 regulates a number of genes linked to secondary thickening, including IRREGULAR XYLEM1 (IRX1), IRX3, IRX8, and IRX12. Changes in expression were also detected in two NAC domain genes, NAC SECONDARY WALL-PROMOTING FACTOR1 (NST1) and NST2, which have been linked to secondary thickening in the anther endothecium. These data indicate that MYB26 regulates NST1 and NST2 expression and in turn controls the process of secondary thickening. Therefore, MYB26 appears to function in a regulatory role involved in determining endothecial cell development within the anther and acts upstream of the lignin biosynthesis pathway.     

Contribution to the flower structure of Sararanga (Pandanaceae)

In Sararanga , the fruit is a berry as in Freycinetia. The testa comprises a lignifled outer integument with several cell layers, and an unlignified inner integument with two cell layers. Abortive fruits are frequent; they correspond to normal fruits that do not have carpels and sometimes have a lateral process that suggests an abortive carpel. The staminate flowers have a pistillode as in Freycinetia. The anther walls have 1–3 cell layers with endothecial thickenings, one layer in the distal part, 2–3 layers in the proximal part, as in Pandanus. Thus, within the family Pandanaceae, Sararanga has an intermediate position between Pandanus and Freycinetia. Generally speaking, there is a gradient in the vascularization of the bracts on the inflorescences: upper bracts are unvascularized, lower bracts vascularized. Anatomy suggests that the cupules are a perianth.     

橡胶树花药和小孢子发生与发育过程的观察

应用石蜡切片法．观察橡胶树的实生树和RRIM600、GT-1品系的花药壁以及小孢子的发生和发育过程,得到如下结果：1．实生树的花药壁通常由四层细胞组成,发育形式为双子叶型。药室内壁细胞在发育后期进行径向条纹加厚．至花药开裂时仍保留着原生质体。中层由一层或不规则的两层细胞组成,在小孢子单核期消失。绒毡层细胞具单棱或双核,属分泌型,至花粉发育到三细胞时消失。小孢子母细胞减数分裂为同时型。成熟花粉粒具三十细胞。精细胞椭圆形,在光镜下不能区分出细胞质鞘和核仁。所观察的实生树雄花,多数发育正常,很少有空秕的花粉。2．RRIMB00品系的花药和小孢子发生与发育和实生树相似,但至后期只有少数花粉发育正常,多数成为大小不等的败育花粉;此外也有一些败育的雄花。3．GT-1的花药在小孢子母细胞减数分裂时,绒毡层细胞的体积开始异常增大并液泡化．小孢子在四分体内解体或分离后成为空秕花粉。     

HEAT INJURY DURING FLORAL DEVELOPMENT IN COWPEA (VIGNA UNGUICULATA,FABACEAE)

High night temperatures during floral development induce male sterility in cowpea (Vigna unguiculata [L.] Walp.). The objectives of this study were to determine: the possible causes of the male sterility; the stage of floral development when damage due to heat stress occurs; and whether specific tissues are damaged during the period of sensitivity to heat. Plants were grown under controlled temperatures in both greenhouses and growth chambers in separate experiments. Floral development was normal under a night temperature of 20 C, whereas flowers developed under high night temperature (30 C) set no pods due to low pollen viability and anther indehiscence. Anthers developed under 33/30 C day/night temperatures did not exhibit endothecial formation, whereas anthers developed under 33/20 C day/night temperatures exhibited normal development of the endothecial layer. Reciprocal transfers of plants between chambers with high or optimum night temperature demonstrated that the stage of floral development most sensitive to heat stress occurs 9 to 7 d before anthesis. Anthers developed under either optimal or high night temperatures were compared cytologically. Development was similar through meiosis, but after tetrad release, which occurred 8 d before anthesis, the tapetal layer degenerated prematurely under high night temperature. Premature degeneration of the tapetal layer and lack of endothecial development may be responsible for the low pollen viability, low anther dehiscence, and low pod set under high night temperatures.     

A COMPARISON OF CLEISTOGAMOUS AND CHASMOGAMOUS FLORAL DEVELOPMENT IN COLLOMIA GRANDIFLORA DOUGL. EX LINDL. (POLEMONIACEAE)

An individual of Collomia grandiflora typically produces both closed or cleistogamous (CL) and open or chasmogamous (CH) flowers. The developmental origin of these dimorphic floral types within a plant was investigated using histological techniques, allometric relationships, and scanning electron microscopy. Prior to archesporal cell stage in the anthers, CL and CH meristems are indistinguishable. In the CL anther, an absence of ventral locule cell differentiation together with a shorter period of time between archesporial cell differentiation and meiosis in the two dorsal locules results in accelerated anther dehiscence and a smaller mature anther size and pollen grain number. Divergence between the CL and CH patterns of corolla development is coincident with microspore mitosis in the CH anther. At this point, there is an increase in growth in corolla length relative to growth in calyx length in the CH flower which does not occur in the CL flower. Calyx and ovary development are similar in the two floral forms; however, ovary expansion due to fertilization occurs earlier in the CL flower as a result of precocious anther development and stigma receptivity. The hypothesis that anther differentiation may trigger organ growth rate changes and differentiation events in the flower and hypothetical roles for abscisic acid and gibberellin in modifying floral development in C. grandiflora are discussed.     

DEVELOPMENTAL AND ANATOMICAL CHARACTERISTICS OF THIN-HULL MUTANTS OF CARTHAMUS TINCTORIUS [COMPOSITAE]

Histological observations were made on normal and mutant strains of safflower in order to compare the development of the “fibrous” tissues among the various strains. The fibers of the vascular bundles of normal and F₁'s from crosses between normal and thin-hull mutant types had well-developed secondary cell walls, but they appeared reduced in mutant types. The anthers of all types showed a similar pattern of differentiation and maturation up to the final stages of tapetal breakdown when secondary walls and rib formations appeared in the connective regions and endothecial cells of the normal, striped and F₁ types. These formations were absent from the thin-hull mutants. In both types dehiscence took place along a longitudinal fissure at the junction of the pollen sacs of one lobe of the anther. The anther flaps of normal types opened abruptly, thus effectively bringing the pollen into contact with the elongating style. Those of the mutants collapsed in place preventing the release of pollen. Hulls of the mutant strains were thin because cells were not sclerified during differentiation of the pericarp. Striped hulls resulted from the additional localization of secretory canals in the pericarp of the striped mutant.     

Systematic significance of cell inclusions in Haemodoraceae and allied families: silica bodies and tapetal raphides

This paper presents the first record of silica deposits in tissues of Haemodoraceae and adds new records of tapetal raphides in this family. Within the order Commelinales, silica is present in leaves of three families (Hanguanacaeae, Haemodoraceae and Commelinaceae), but entirely absent from the other two (Pontederiaceae and Philydraceae). Presence or absence of characteristic cell inclusions may have systematic potential in commelinid monocotyledons, although the existing topology indicates de novo gains and losses in individual families. Silica sand was observed in leaves of five out of nine genera examined of Haemodoraceae, predominantly in vascular bundle sheath cells and epidermal cells. Within Haemodoraceae, silica is limited to subfamily Conostylidoideae. The occurrence of silica in Phlebocarya supports an earlier transfer of this genus from Haemodoroideae to Conostylidoideae. The presence of raphides (calcium oxalate crystals) in the anther tapetum represents a rare character, only reported in a few monocot families of the order Commelinales, and possibly representing a mechanism for regulation of cytoplasmic free calcium levels. Tapetal raphides were observed here in Anigozanthus and Conostylis (both Haemodoraceae), and Tradescantia (Commelinaceae), thus supplementing two earlier records in Haemodoraceae, Philydraceae and Commelinaceae.