Senescence-induced expression of cytokinin reverses pistil abortion during maize flower development

Maize is a monoecious species that produces imperfect (unisexual), highly derived flowers called florets. Within the spikelet, the basic repeating unit of the maize inflorescence, the spikelet meristem gives rise to an upper and a lower floret. Although initially bisexual, floret unisexuality is established through selective organ elimination. In addition, the lower floret of each ear spikelet is aborted early in its development, leaving the upper floret to mature as the only pistillate floret. Expression from the cytokinin-synthesizing isopentenyl transferase (IPT) enzyme under the control of the Arabidopsis senescence-inducible promoter SAG (senescence associated gene)12 was observed during early maize floret development. Moreover, the lower floret was rescued from abortion, resulting in two functional florets per spikelet. The pistil in each floret was fertile, but the spikelet produced just one kernel composed of a fused endosperm with two viable embryos. The two embryos were genetically distinct, indicating that they had arisen from independent fertilization events. These results suggest that cytokinin can determine pistil cell fate during maize floret development.     

Developmental Events Associated with the Critical Stage for Sex Determination in Wild-Rice Florets

The developmental events of florets and a critical stage for sex determination in two wild-rice populations (Zizania palustris cv. Franklin and Zizania palustris cv. K-2Pi) have been identified. Formation of bisexual florets precedes the development of both male and female florets. Developmental indicators, established by measuring the length of florets and panicles, indicate that the critical stage for sex determination occurs when floret and panicle lengths are 1-2 mm and 3 cm, respectively. The stage of floret development at which sex determination occurs is the same in the two investigated wild-rice populations. Organ suppression in bisexual florets is an essential step for sex determination during the formation of unisexual florets. Histological examination of suppressed stamens or pistils in unisexual florets of wild-rice indicates that cell death does not occur during sex determination. In addition, the length of anthers and pistils in bisexual florets indicates that floral development in the transition zone is normal when compared with the male florets in the male spikelets and female florets in the female spikelets.     

Development of Male Flowers in Zizania aquatica (North American Wild-Rice; Gramineae)

We investigated the histology and developmental morphology of flowers of wild-rice (Zizania aquatica), a member of grass subfamily Oryzoideae, to determine whether male flowers in this species develop in a manner similar to those in the subfamily Panicoideae, a group that includes many species with unisexual flowers. Zizania has evolved unisexual flowers from hermaphrodite ancestors and is only distantly related to the Panicoideae; the origins of unisexual flowers are independent in the two taxa. Ontogenetic evidence indicates that many species within the subfamily Panicoideae develop male flowers by a process similar to that already described for maize (Zea mays), a panicoid grass. Unisexual male flowers in maize initiate both the stamen (androecium) and the pistil (gynoecium), but the gynoecium aborts early in development through the death of the subepidermal cells. Cell death in gynoecia of maize is known to be controlled by the product of the gene tasselseed2 (ts2), and an orthologue of ts2 has been shown to have the same effect in the sister genus Tripsacum. It seems likely that ts2 orthologues mediate cell death throughout the Panicoideae, but the phylogenetic range of the cell death mechanism is not known. In this study we show that male flowers of Z. aquatica show neither the distinctive pattern of cell death nor the ontogenetic timing of abortion that are characteristic of male flower formation in studied species of Panicoideae. This indicates that these unisexual flowers may be produced by an entirely different mechanism from that employed by the Panicoideae. Either ts2 does not control sex expression in Zizania, or it is deployed at a different time, and possibly in different tissues, with a different histological result. Our results indicate that the independent origins of male flowers in Gramineae apparently do not have a common system of genetic control.     

Unisexual flowers as a robust synapomorphy in Cariceae (Cyperaceae)? Evidence for bisexual flowers in Schoenoxiphium

Cariceae, the largest tribe within Cyperaceae, comprises about 2000 species in five genera. Cariceae is usually considered to be distinct from other Cyperaceae by the presence of exclusively unisexual flowers and by the arrangement of the pistillate flowers in single-flowered spikelets that are enclosed by the flask-like spikelet prophyll (utricle or perigynium). The nature of several morphological features of the Cariceae inflorescence remains controversial. The staminate reproductive units, as well as earlier reported bisexual reproductive units in Schoenoxiphium have been considered to be reduced partial inflorescences, or flowers. Aims of this study are to test both interpretations, based on a floral ontogenetic investigation. Moreover, for the first time, detailed SEM micrographs are presented of the inflorescence and floral development and of bisexual flowers in Schoenoxiphium. We propose that ‘inhibition of bisexuality’ is a more robust synapomorphy in Cariceae than ‘presence of only unisexual flowers’.     

Evidence for distinct roles of the SEPALLATA gene LEAFY HULL STERILE1 in Eleusine indica and Megathyrsus maximus (Poaceae)

LEAFY HULL STERILE1 (LHS1) is an MIKC-type MADS-box gene in the SEPALLATA class. Expression patterns of LHS1 homologs vary among species of grasses, and may be involved in determining palea and lemma morphology, specifying the terminal floret of the spikelet, and sex determination. Here we present LHS1 expression data from Eleusine indica (subfamily Chloridoideae) and Megathyrsus maximus (subfamily Panicoideae) to provide further insights into the hypothesized roles of the gene. E. indica has spikelets with three to eight florets that mature acropetally; E. indica LHS1 (EiLHS1) is expressed in the palea and lemma of all florets. In contrast, M. maximus has spikelets with two florets that mature basipetally; M. maximus LHS1 (MmLHS1) is expressed in the palea and lemma of the distal floret only. These data are consistent with the hypothesis that LHS1 plays a role in determining palea and lemma morphology and specifies the terminal floret of basipetally maturing grass spikelets. However, LHS1 expression does not correlate with floret sex expression; MmLHS1 is restricted to the bisexual distal floret, whereas EiLHS1 is expressed in both sterile and bisexual floret meristems. Phylogenetic analyses reconstruct a complex pattern of LHS1 expression evolution in grasses. LHS1 expression within the gynoecium has apparently been lost twice, once before diversification of a major clade within tribe Paniceae, and once in subfamily Chloridoideae. These data suggest that LHS1 has multiple roles during spikelet development and may have played a role in the diversification of spikelet morphology.     

Floret-specific differences in gene expression and support for the hypothesis that tapetal degeneration of Zea mays L. Occurs via programmed cell death

The maize (Zea mays) spikelet consists of two florets, each of which contains three developmentally synchronized anthers. Morphologically, the anthers in the upper and lower florets proceed through apparently similar developmental programs. To test for global differences in gene expression and to identify genes that are coordinately regulated during maize anther development, RNA samples isolated from upper and lower floret anthers at six developmental stages were hybridized to eDNA rnicroarrays. Approximately 9% of the tested genes exhibited statistically significant differences in expression between anthers in the upper and lower florets. This finding indicates that several basic biological processes are differentially regulated between upper and lower floret anthers, including metabolism, protein synthesis and signal transduction. Genes that are coordinately regulated across anther development were identified v/a cluster analysis.Analysis of these results identified stage-specific, early in development, late in development and bi-phasic expression profiles. Quantitative RT-PCR analysis revealed that four genes whose homologs in other plant species are involved in programmed cell death are up-regulated just prior to the time the tapetum begins to visibly degenerate (i.e., the mid-microspore stage). This finding supports the hypothesis that developmentally normal tapetal degeneration occurs via programmed cell death.     

Analysis of inflorescence organogenesis in eastern gamagrass, Tripsacum dactyloides (Poaceae): the wild type and the gynomonoecious gsf1 mutant

Inflorescence organogenesis of a wild-type and a gynomonoecious (pistillate) mutant in Tripsacum dactyloides was studied using scanning electron microscopy. SEM (scanning electron microscope) analysis indicated that wild-type T. dactyloides (Eastern gamagrass) expressed a pattern of inflorescence organogenesis that is observed in other members of the subtribe Tripsacinae (Zea: maize and teosinte), family Poaceae. Branch primordia are initiated acropetally along the rachis of wild-type inflorescences in a distichous arrangement. Branch primordia at the base of some inflorescences develop into long branches, which themselves produce an acropetal series of distichous spikelet pair primordia. All other branch primordia function as spikelet pair primordia and bifurcate into pedicellate and sessile spikelet primordia. In all wild-type inflorescences development of the pedicellate spikelets is arrested in the proximal portion of the rachis, and these spikelets abort, leaving two rows of solitary sessile spikelets. Organogenesis of spikelets and florets in wild-type inflorescences is similar to that previously described in maize and the teosintes. Our analysis of gsf1 mutant inflorescences reveals a pattern of development similar to that of the wild type, but differs from the wild type in retaining (1) the pistillate condition in paired spikelets along the distal portion of the rachis and (2) the lower floret in sessile spikelets in the proximal region of the rachis. The gsf1 mutation blocks gynoecial tissue abortion in both the paired-spikelet and the unpaired-spikelet zone. This study supports the hypothesis that both femaleness and maleness in Zea and Tripsacum inflorescences are derived from a common developmental pathway. The pattern of inflorescence development is not inconsistent with the view that the maize ear was derived from a Tripsacum genomic background.     

Effects of exogenous hormones on floret development and grain setin wheat

At specific stages during floret development, solutions of IAA,GA₃, zeatin and ABA were injected into the leaf sheath around theyoung spike of wheat (Triticum aestivum L.) to study theregulating effects of exogenous hormones on floret development. Zeatin promotedfloret development and significantly increased the number of fertile florets aswell as grain set, especially at the stage of anther-lobe formation. Zeatinalsoincreased the sugar concentrations in spikes at anthesis. In contrast, IAA,GA₃ and ABA inhibited floret development, with different patternsforeach of the hormones. IAA inhibited the development of the whole spike and allflorets in the spikelets such that grain loss occurred in all positions in thespikelets. GA₃ increased the number of fertile florets per spike,butdecreased grain set of the third floret in each spikelet, especially whenapplied at terminal spikelet formation. ABA inhibited floret development, anddecreased the number of fertile florets and grain set at almost all developmentstages, except at anther-lobe formation. The inhibitory effect of ABA wasmainlyon the first and third florets in each spikelet.     

Inflorescence development in a new teosinte: Zea nicaraguensis (Poaceae)

Inflorescence development in a newly discovered teosinte, Zea nicaraguensis (Poaceae), from Nicaragua has been investigated using scanning electron microscopy (SEM). The SEM examination revealed that the pattern of both male and female inflorescence development was similar to previously described inflorescence in other Zea taxa. Branch primordia were initiated acropetally in a distichous pattern along the rachis of male and female inflorescences. Spikelet pair primordia bifurcated into pedicellate and sessile spikelet primordia. Predictably, pedicellate spikelet development was arrested and aborted in the female teosinte inflorescence. Organogenesis of functional spikelets and florets was similar to that previously described in maize and teosintes. The results were consistent with our hypothesis that both femininity and masculinity share a common mechanism of inflorescence development in Zea and Tripsacum and are in accord with a putative common mechanism of sex determination in the Andropogoneae. Interestingly, this population of teosinte, unique in its ability to grow in water-logged soils, showed a stable pattern of early inflorescence development. Our results also revealed the uncharacteristic presence of inflorescence polystichy in this population of Zea nicaraguensis. We propose this novel phenotypic variation raises the possibility that a domestic evolution of polystichy in maize was enabled by an occasional polystichous phenotypic in teosinte.     

BRANCHED SILKLESS mediates the transition from spikelet to floral meristem during Zea mays ear development

The molecular and genetic control of inflorescence and flower development has been studied in great detail in model dicotyledonous plants such as Arabidopsis and Antirrhinum . In contrast, little is known about these important developmental steps in monocotyledonous species. Here we report the analysis of the Zea mays mutant branched silkless1–2 (bd1–2) , allelic to bd1 , which we have used as a tool to study the transition from spikelet to floret development in maize. Floret development is blocked in the female inflorescence (the ear) of bd1–2 plants, whereas florets develop almost normally in the male inflorescence (the tassel). Detailed phenotypic analyses indicate that in bd1–2 mutants ear inflorescence formation initiates normally, however, the spikelet meristems do not proceed to form floret meristems. The ear spikelets, at anthesis, contain various numbers of spikelet-like meristems and glume-like structures. Furthermore, growth of branches from the base of the ear is often observed. Expression analyses show that the floral-specific MADS box genes Zea mays AGAMOUS1 ( ZAG1 ), ZAG2 and Zea mays MADS 2 ( ZMM2 ) are not expressed in ear florets in bd1–2 mutants, whereas their expression in tassel florets is similar to that of wild type. Taken together, these data indicate that the development from spikelet to floret meristem is differentially controlled in the ear and tassel in the monoecious grass species Zea mays , and that BRANCHED SILKLESS plays an important role in regulating the transition from spikelet meristem to floral meristem during the development of the female inflorescence of maize.     

Autophagy regulated by day length determines the number of fertile florets in wheat

SUMMARY: The wheat spikelet meristem differentiates into up to 12 floret primordia, but many of them fail to reach the fertile floret stage at anthesis. We combined microarray, biochemical and anatomical studies to investigate floret development in wheat plants grown in the field under short or long days (short days extended with low-fluence light) after all the spikelets had already differentiated. Long days accelerated spike and floret development and greening, and the expression of genes involved in photosynthesis, photoprotection and carbohydrate metabolism. These changes started while the spike was in the light-depleted environment created by the surrounding leaf sheaths. Cell division ceased in the tissues of distal florets, which interrupted their normal developmental progression and initiated autophagy, thus decreasing the number of fertile florets at anthesis. A massive decrease in the expression of genes involved in cell proliferation, a decrease in soluble carbohydrate levels, and an increase in the expression of genes involved in programmed cell death accompanied anatomical signs of cell death, and these effects were stronger under long days. We propose a model in which developmentally generated sugar starvation triggers floret autophagy, and long days intensify these processes due to the increased carbohydrate consumption caused by the accelerated plant development.     

Suppressor of sessile spikelets1 functions in the ramosa pathway controlling meristem determinacy in maize

The spikelet, which is a short branch bearing the florets, is the fundamental unit of grass inflorescence architecture. In most grasses, spikelets are borne singly on the inflorescence. However, paired spikelets are characteristic of the Andropogoneae, a tribe of 1,000 species including maize (Zea mays). The Suppressor of sessile spikelets1 (Sos1) mutant of maize produces single instead of paired spikelets in the inflorescence. Therefore, the sos1 gene may have been involved in the evolution of paired spikelets. In this article, we show that Sos1 is a semidominant, antimorph mutation. Sos1 mutants have fewer branches and spikelets for two reasons: (1) fewer spikelet pair meristems are produced due to defects in inflorescence meristem size and (2) the spikelet pair meristems that are produced make one instead of two spikelet meristems. The interaction of Sos1 with the ramosa mutants, which produce more branches and spikelets, was investigated. The results show that Sos1 has an epistatic interaction with ramosa1 (ra1), a synergistic interaction with ra2, and an additive interaction with ra3. Moreover, ra1 mRNA levels are reduced in Sos1 mutants, while ra2 and ra3 mRNA levels are unaffected. Based on these genetic and expression studies, we propose that sos1 functions in the ra1 branch of the ramosa pathway controlling meristem determinacy.     

Unisexual flower, spikelet, and inflorescence development in monoecious/dioecious Bouteloua dimorpha (Poaceae, Chloridoideae)

Unisexual flowers have evolved repeatedly in the angiosperms. In Poaceae, multiple transitions from bisexual to unisexual flowers are hypothesized. There appear to be at least three distinct developmental mechanisms for unisexual flower formation as found in members of three subfamilies (Ehrhartoideae, Panicoideae, Pharoideae). In this study, unisexual flower development is described for the first time in subfamily Chloridoideae, as exemplified by Bouteloua dimorpha. Scanning electron microscopy (SEM) and anatomy were used to characterize the development of male (staminate) and female (pistillate) flowers, spikelets, and inflorescences. We found the developmental pathway for staminate flowers in B. dimorpha to be distinct from that described in the other three subfamilies, showing gynoecial arrest occurs at a different stage with possible loss of some cellular contents. However, pistillate flowers of B. dimorpha had some similarity to those described in other unisexual-flowered grasses, with filament and anther differentiation in abortive stamens. Comparing our findings with previous reports, unisexual flowers seem to have evolved independently in the four examined grass subfamilies. This analysis suggests the action of different genetic mechanisms, which are consistent with previous observations that floral unisexuality is a homoplasious condition in angiosperms.     

A Study of Floret Development in Wheat (Triticum aestivum L.)

Plants of wheat (Triticum aestlvum L.) cv. Aotea were grownat high or low nitrogen levels and in a natural photoperiodor continuous light. Starting 17–21 days from the double-ridgestage, eight plants from each treatment were sampled every 3days until anthesis, and the two basal, the sixth, and the terminalspikelets were sectioned longitudinally. A developmental scorewas assigned to each floret and rates of development calculated.Continuous light hastened development but reduced the numberof spikelets per ear, while high nitrogen delayed developmentbut increased spikelet numbers. The number of florets initiatedin each spikelet varied within narrow limits, but grain settingdepended strongly on spikelet position and on treatment. Althoughflorets were initiated in acropetal succession, the rate ofdevelopment tended to increase up to floret 4 but then declinedmarkedly. As a result grain setting was confined to basal floretpositions, although the two basal spikelets developed so slowlythat they contributed relatively little to grain yield. Distalflorets degenerated almost simultaneously at or before ear emergence,but those in intermediate positions continued to develop untilafter fertilization in the lower florets. It is argued thatthe spikelet is an integrated system in which correlative mechanismsplay a part throughout the development of the florets.     

菅属植物的地理分布

菅属(Themeda Forssk.)是禾本科高粱族(Poaceae:Andropogoneae)中佛焰苞物种的代表类群之一,在高粱族占据关键的系统演化位置,具有高度的形态和生态多样性。通过野外调查和查阅标本及文献,菅属约有27种植物,旧世界均有分布,新世界3种隶属于菅组。中国有13种,分布在西南至华南各省(区),云南干热河谷地区有10种。研究表明中国云南及印度北部是菅属的分布中心和多样性中心,中国云南及印度北部是否为菅属的起源地尚需确证。