Heterochronic mutations affecting shoot development in maize

Three semidominant, nonallelic mutations of maize, Teopod 1 (Tp1), Teopod 2 (Tp2) and Teopod 3 (Tp3), have a profound effect on both vegetative and reproductive development. Although each mutation is phenotypically distinct, they all (1) increase the number of vegetative phytomers; (2) increase the number of phytomers producing ears, tillers and prop roots; (3) increase the number of leaves bearing epidermal wax; (4) decrease the size of leaves and internodes; (5) decrease the size of both the ear and tassel; and (6) transform reproductive structures into vegetative ones. The analysis presented here suggests that this phenotype reflects the prolonged expression of a juvenile, vegetative developmental program which overlaps with the reproductive developmental program. The expression of these mutations is different in each of the four inbred backgrounds used in this study. Tp1 and Tp2 have similar phenotypes and are more highly expressed in the A632 and Oh51a inbred backgrounds than in W23 and Mo17. Tp3 has less extreme effects than either of these mutations and has the opposite modification pattern; i.e., it is more highly expressed in W23 and Mo17 than in A632 and Oh51a. The expression of Tp1 and Tp2 in the presence of varying doses of their wild-type alleles indicate that both are gain-of-function mutations. The phenotypes of Tp1 and Tp2 and the nature of their response to variation in gene dose suggest that they control related, but nonidentical functions. The developmental and evolutionary implications of the heterochronic phenotype of these mutations is discussed.     

A rice heterochronic mutant, mori1, is defective in the juvenile-adult phase change.

We have identified five recessive allelic mutations, mori1-1 to mori1-5, which drastically modify the shoot architecture of rice. The most remarkable feature of mori1 plants is a rapid production of small leaves and short branches. The mori1 plants are about 5 cm in height even 7 months after sowing. No reproductive growth was attained in mori1 plants even if inductive short-day treatment was applied. Leaves of mori1 at any position were very small and the size and shape were comparable to those of the wild-type 2nd leaf. The stem of mori1 7 months after sowing did not differentiate node and internode and had randomly oriented vascular bundles, which were characteristic of the basal part of the wild-type stem where 2nd and 3rd leaves were inserted. These structural characteristics indicate that mori1 maintains the 2nd-leaf stage (juvenile phase) of the wild type. The short plastochron and high cell division activity in the shoot apical meristem further confirmed the juvenility of mori1, corresponding to the 2nd-leaf-differentiation stage in the wild-type embryo. Furthermore, the apparent photosynthetic rate in mori1 leaves was low as in the wild-type 2nd leaf. Thus, mori1 is a heterochronic mutation that suppresses the induction of adult phase and the termination of the juvenile phase. Therefore, MORI1 plays an important role in the juvenile-adult phase change. The importance of heterochronic mutations in modifying shoot architecture is discussed.     

A recessive heterochronic mutation, plastochron1, shortens the plastochron and elongates the vegetative phase in rice

We describe two recessive alleles of a rice heterochronic gene, plastochron1-1 (pla1-1) and pla1-2, that reduce the length of the plastochron to approximately half that of the wild type. Because the onset of the reproductive phase in pla1 was not temporally affected, the number of leaves produced in the vegetative phase was nearly twice that produced in the wild type. Panicle development was severely disturbed in pla1 mutants. In pla1-1, many primordia of primary rachis branches were converted into vegetative shoots. These ectopic shoots repeated the initiation of panicle development and the conversion of primary rachis branches into shoots. In the weak allele pla1-2, however, only the basal one or two primordia developed as vegetative shoots, and the remaining primordia developed to produce a truncated panicle. These results indicate that both vegetative and reproductive programs are expressed simultaneously during the reproductive phase of pla1; however, the degree varied depending on the strength of the allele. Accordingly, pla1 is a heterochronic mutation that extends the vegetative period. The shoot apical meristem of pla1 was larger than that of the wild type, although the shape was not modified. An in situ hybridization experiment using the histone H4 gene as a probe revealed that cell divisions are accelerated in the pla1 meristem. The PLA1 gene is considered to regulate the duration of the vegetative phase by controlling the rate of leaf production in the meristem.     

Maize mutants and variants altering developmental time and their heterochronic interactions.

It is useful to envision two fundamentally different ways by which the timing of plant development is regulated: developmental stage-transition mechanisms and time-to-flowering mechanisms. The existence of both mechanisms is indicated by the behavior of various mutants. Shoot stage transitions are defined by dominant mutants representing at least four different genes; each mutant retards transitions from juvenile shoot stages to more adult shoot stages. In addition, dominant leaf stage-transition mutants in at least seven different genes have similar phenotypes, but the leaf rather than the shoot is the focus (and at least two of these genes encode homeodomain proteins.) One mutant, Hairy sheath frayed 1-O (Hsf1-O) simultaneously affects shoot and leaf; this mutant's behavior initiated our interest in plant heterochronism. The second type of timekeeping involves time-to-flowering. As with most plant but not animal species, cultivars of the maize species vary greatly for the time-to-flowering quantitative trait: between 6 and 14 weeks is common. It is via the 'slipping time frames' interaction that takes place between stage-transition mutants and time-to-flowering genetic backgrounds that unexpected and radical phenotypes occur. We see a reservoir of previously unsuspected morphological possibilities among the few heterochronic genotypes we have constructed, possibilities that may mimic the sort of variation needed to fuel macroevolution without having to posit (as done by Goldschmidt) any special macromutational mechanisms.     

The effect of brassinosteroids on the morphology,development and yield of field-grown maize

The response of two field-grown inbred lines of maize (Zea mays L.) and their F1 hybrid to the application of 10⁻⁸–10⁻¹⁴ M solutions of 24-epibrassinolide or synthetic androstane analogue of castasterone in V3/4 and V6/7 developmental stages was followed during the vegetative and early reproductive phases of plant development. Brassinosteroids (BRs) significantly affected (either positively or negatively, depending on the genotype and the developmental stage they were applied) the height of plants during the early weeks after their application, but not the final plant height nor the number of leaves. Spraying of plants with BRs in V3/4 developmental stage usually also increased the length of the 7th to 10th leaf, whereas the application in V6/7 developmental stage had the opposite effect. The beginning of the reproductive phase of plant development and the course of flowering was strongly influenced by the application of BRs. Treatment of plants in V3/4 stage delayed and treatment of plants in V6/7 stage advanced the dates of anthesis and silking, regardless of the type of BR used, its concentration or plant genotype. The influence of BRs on the development of the secondary ear was the least pronounced in the F1 hybrid; in both inbred lines it strongly depended on the concentrations of BRs used. Various yield parameters were also affected by treatment of plants with BRs, but this effect depended on the developmental stage during which the application of BRs occured, the plant genotype, the type of BR and its concentration.     

Biomechanical analysis of the Rolled (RLD) leaf phenotype of maize

The pleiotropic effects of the Rld1-O/+ mutation of Zea mays (Poaceae) on leaf phenotype include a suppression of normal transverse unrolling, a reversed top/bottom epidermal polarity, and an apparently straighter longitudinal shape. According to engineering shell theory, there might be mechanical coupling between transverse and longitudinal habit, i.e., the leaf rolling itself might produce the longitudinal straightening. We tested this possibility with quantitative curvature measurements and mechanical uncoupling experiments. The contributions of elastic bending under self weight, mechanical coupling, and rest state of leaf parts to the longitudinal and transverse habit were assessed in Rld1-O/+ mutants and a population of sibling +/+ segregants. Elastic bending and curvature coupling are shown to be relatively unimportant. The Rld1-O/+ mutation is shown to alter not only the unrolling process, but also the developmental longitudinal curving in the growing leaf, leading to a straighter midrib and a rolled lamina. The Rld1-O/+ mutant is thus a suitable model to study the relation between tissue polarity and differential curvature development in the maize leaf. Since on the abaxial side of the leaf, more abundant sclerenchyma is found in +/+ than in Rld1-O/+, a gradient in sclerification may contribute to the development of midrib curvature.     

Heterochronic expression of sexual reproductive programs during apomictic development in Tripsacum

Some angiosperms reproduce by apomixis, a natural way of cloning through seeds. Apomictic plants bypass both meiosis and egg cell fertilization, producing progeny that are genetic replicas of the mother plant. In this report, we analyze reproductive development in Tripsacum dactyloides, an apomictic relative of maize, and in experimental apomictic hybrids between maize and Tripsacum. We show that apomictic reproduction is characterized by an alteration of developmental timing of both sporogenesis and early embryo development. The absence of female meiosis in apomictic Tripsacum results from an early termination of female meiosis. Similarly, parthenogenetic development of a maternal embryo in apomicts results from precocious induction of early embryogenesis events. We also show that male meiosis in apomicts is characterized by comparable asynchronous expression of developmental stages. Apomixis thus results in an array of possible phenotypes, including wild-type sexual development. Overall, our observations suggest that apomixis in Tripsacum is a heterochronic phenotype; i.e., it relies on a deregulation of the timing of reproductive events, rather than on the alteration of a specific component of the reproductive pathway.     

Leaf shape and anatomy as indicators of phase change in the grasses: comparison of maize, rice, and bluegrass

Leaf morphology and anatomy during vegetative phase change was compared in bluegrass, rice, and maize. Maize juvenile leaves are coated with epicuticular wax, lack specialized cells, such as trichomes and bulliform cells, and epidermal cell walls stain a uniform purple color. Adult maize leaves are pubescent, lack epicuticular waxes, and have crenulated epidermal cell walls that stain purple and blue. All bluegrass and rice blades are pubescent, coated with epicuticular waxes, and show purple and blue wall staining. In all three grasses, blade width steadily increases at each node until a threshold size is achieved several nodes before reproductive competence is acquired. Blade-to-sheath length showed a similar trend of continuous change followed by discontinuous change prior to reproduction. Analysis of leaf development demonstrated that maize primordia initiate more rapidly relative to blade and sheath growth than do either bluegrass or rice. We conclude that leaf shape, as defined by blade width and blade-to-sheath ratio, is a reliable indicator of phase, whereas anatomy is not a universal indicator of phase change in the grasses. We speculate that different growth patterns among these grasses may be attributed to changes in the timing of embryonic and postembryonic development.     

Heterochrony, cannibalism, and the evolution of viviparity in Salamandra salamandra

The way in which novelties that lead to macroevolutionary events originate is a major question in evolutionary biology, and one that can be addressed using the fire salamander (Salamandra salamandra) as a model system. It is exceptional among amphibians in displaying intraspecific diversity of reproductive strategies. In S. salamandra, two distinct modes of reproduction co-occur: the common mode, ovoviviparity (females giving birth to many small larvae), and a phylogenetically derived reproductive strategy, viviparity (females producing only a few large, fully metamorphosed juveniles, which are nourished maternally). We examine the relationship between heterochronic modifications of the ontogeny and the evolution of the new reproductive mode in the fire salamander. The in vitro development of embryos of ovoviviparous and viviparous salamanders from fertilization to metamorphosis is compared, highlighting the key events that distinguish the two modes of reproduction. We identify the heterochronic events that, together with the intrauterine cannibalistic behavior, characterize the derived viviparous reproductive strategy. The ways in which evolutionary novelties can arise by modification of developmental programs can be studied in S. salamandra. Moreover, the variation in reproductive modes and the associated variation of sequences of development occur in neighboring, conspecific populations. Thus, S. salamandra is a unique biological system in which evolutionary developmental research questions can be addressed at the level of populations.     

The liguleless2 gene of maize functions during the transition from the vegetative to the reproductive shoot apex

During a maize plant's (Zea mays) development, the shoot apical meristem (SAM) generates an apex that proceeds through different phases: juvenile vegetative, adult vegetative and reproductive. During each phase the structures produced are distinguishable from structures produced during the other phases. In this paper, we demonstrate that the LIGULELESS2 (LG2) function is required for an accurate vegetative to reproductive phase transition. The maize gene liguleless2 (lg2) has been shown to encode a basic-leucine zipper (bZIP) protein and to function in narrowing the region from which the ligule and auricle develop in a typical maize leaf. Here we show that lg2 mutant plants can have reduced long tassel branches, extra vegetative leaves and extra husk leaves when compared to wild-type siblings. This indicates a role for the lg2 gene in the vegetative to reproductive phase transition of the shoot apex. We also discuss a potential role for the lg2 gene in general phase transition processes.     

Gibberellins promote vegetative phase change and reproductive maturity in maize.

Postembryonic shoot development in maize (Zea mays L.) is divided into a juvenile vegetative phase, an adult vegetative phase, and a reproductive phase that differ in the expression of many morphological traits. A reduction in the endogenous levels of bioactive gibberellins (GAs) conditioned by any one of the dwarf1, dwarf3, dwarf5, or another ear1 mutations in maize delays the transition from juvenile vegetative to adult vegetative development and from adult vegetative to reproductive development. Mutant plants cease producing juvenile traits (e.g. epicuticular wax) and begin producing adult traits (e.g. epidermal hairs) later than wild-type plants. They also cease producing leaves and begin producing reproductive structures later than wild-type plants. These mutations greatly enhance most aspects of the phenotype of Teopod1 and Teopod2, suggesting that GAs suppress part but not all of the Teopod phenotype. Application of GA3 to Teopod2 mutants and Teopod1, dwarf3 double mutants confirms this result. We conclude that GAs act in conjunction with several other factors to promote both vegetative and reproductive maturation but affect different developmental phases unequally. Furthermore, the GAs that regulate vegetative and reproductive maturation, like those responsible for stem elongation, are downstream of GA20 in the GA biosynthetic pathway.     

Genetic analysis of mutations that alter cell fates in maize leaves: Dominant Liguleless mutations

The three major components of the maize leaf are the blade, the sheath, and at their junction, the ligular region. Each exhibits specific cell types and organization. Four dominant Liguleless (Lg) mutations (Lg3-O, Lg4-O, Lg*347, and Lg*9167) in at least three different genes cause a similar morphological phenotype in leaves, although each mutation affects a distinct domain of the blade. Mutant leaves display regions of altered cell fate in the blade, occompanied by elimination of ligule and auricle at their wild-type positions and development of ligule and auricle in the blade at the borders of the altered regions. The affected blade cells are transformed into sheath-like cells, as determined by morphological and genetic tests. Lg4-O expressivity is highly dependent on genetic background. For example, two different backgrounds may specify converse patterns of phenotypic expression. Lg4-O expressivity is also affected by the heterochronic mutation Teopod2 (Tp2). Gene dosage experiments indicate that Lg4-O is a neomorph. Interactions between recessive lg mutations (which eliminate ligular structures) and the dominant Lg mutations suggest that the lg⁺ genes act after the Lg mutations. Lg3-O and Lg4-O act semidominantly, and interact with each other and with other mutations in the Knotted1 (Kn1)-like family (a family in which dominant mutant alleles cause blade to sheath transformation phenotypes). These interactions suggest that the above Kn1-like mutations may function similarly in the leaf. We discuss the similarities between the Lg mutations and the other mutations of the Kn1-like family, which led us to postulate that lg3 and lg4 are members of a growing family of kn1-like (knox) homeobox genes that are identified by dominant mutant alleles causing leaf transformation phenotypes. We also propose that certain key characteristics of this family of dominant neomorphic mutations are important for generating meaningful morphological changes during evolution. © 1996 Wiley-Liss, Inc.     

Leafy head2, which encodes a putative RNA-binding protein, regulates shoot development of rice

During vegetative development, higher plants continuously form new leaves in regular spatial and temporal patterns. Mutants with abnormal leaf developmental patterns not only provide a great insight into understanding the regulatory mechanism of plant architecture, but also enrich the ways to its modification by which crop yield could be improved. Here, we reported the characterization of the rice leafy-head2 （lhd2） mutant that exhibits shortened plastochron, dwarfism, reduced tiller number, and failure of phase transition from vegetative to reproductive growth. Anatomical and histological study revealed that the rapid emergence of leaves in lhd2 was resulted from the rapid initiation of leaf primordia whereas the reduced tiller number was a consequence of the suppression of the tiller bud outgrowth. The molecular and genetic analysis showed that LHD2 encodes a putative RNA binding protein with 67% similarity to maize TEl. Comparison of genome-scale expression profiles between wild-type and lhd2 plants suggested that LHD2 may regulate rice shoot development through KNOXand hormone-related genes. The similar phenotypes caused by LHD2 mutation and the conserved expression pattern of LHD2 indicated a conserved mechanism in controlling the temporal leaf initiation in grass.     

激光共聚焦显微镜观察玉米茎尖形态学变化

以玉米光敏感自交系CML288和不敏感自交系黄早4为实验材料,采用长日照15 h、短日照9 h的不同光周期处理,利用激光扫描共聚焦显微镜(laser scanning confocal microscope, LCSM)观察了不同叶龄期玉米茎尖分生组织的形态学变化.结果表明,短日照能促进玉米开花,促进茎端分生组织向生殖生长转化,黄早4和CML288分别在6叶期和7叶期完成茎尖分生组织的生殖转化;而长日照则明显延迟开花,延迟茎尖分生组织向生殖生长转化,黄早4和CML288分别在8叶期和11叶期完成茎尖分生组织的生殖转化;因此光周期诱导玉米开花因光照条件和品种有一定差异,短日照条件下,光敏感和不敏感的玉米自交系开花提前,花期更接近,而长日照条件下光敏感玉米自交系开花延迟要比不敏感自交系明显得多.     

NARROW SHEATH1 functions from two meristematic foci during founder-cell recruitment in maize leaf development

The narrow sheath duplicate genes (ns1 and ns2) perform redundant functions during maize leaf development. Plants homozygous for mutations in both ns genes fail to develop wild-type leaf tissue in a lateral domain that includes the leaf margin. Previous studies indicated that the NS gene product(s) functions during recruitment of leaf founder-cells in a lateral, meristematic domain that contributes to leaf margin development. A mosaic analysis was performed in which the ns1-O mutation was exposed in hemizygous, clonal sectors in a genetic background already homozygous for ns2-O. Analyses of mutant, sectored plants demonstrate that NS1 function is required in L2-derived tissue layers for development of the narrow sheath leaf domain. NS1 function is not required for development of the central region of maize leaves. Furthermore, the presence of the non-mutant ns1 gene outside the narrow sheath domain cannot compensate for the absence of the non-mutant gene within the narrow sheath domain. NS1 acts non-cell autonomously within the narrow sheath-margin domain and directs recruitment of marginal, leaf founder cells from two discrete foci in the maize meristem. Loss of NS1 function during later stages of leaf development results in no phenotypic consequences. These data support our model for NS function during founder-cell recruitment in the maize meristem.