Expression patterns and mutant phenotype of teosinte branched1 correlate with growth suppression in maize and teosinte

The evolution of domesticated maize from its wild ancestor teosinte is a dramatic example of the effect of human selection on agricultural crops. Maize has one dominant axis of growth, whereas teosinte is highly branched. The axillary branches in maize are short and feminized whereas the axillary branches of teosinte are long and end in a male inflorescence under normal growth conditions. Previous QTL and molecular analysis suggested that the teosinte branched1 (tb1) gene of maize contributed to the architectural difference between maize and teosinte. tb1 mutants of maize resemble teosinte in their overall architecture. We analyzed the tb1 mutant phenotype in more detail and showed that the highly branched phenotype was due to the presence of secondary and tertiary axillary branching, as well as to an increase in the length of each node, rather than to an increase in the number of nodes. Double-mutant analysis with anther ear1 and tassel seed2 revealed that the sex of the axillary inflorescence was not correlated with its length. RNA in situ hybridization showed that tb1 was expressed in maize axillary meristems and in stamens of ear primordia, consistent with a function of suppressing growth of these tissues. Expression in teosinte inflorescence development suggests a role in pedicellate spikelet suppression. Our results provide support for a role for tb1 in growth suppression and reveal the specific tissues where suppression may occur.     

Suppression of sorghum axillary bud outgrowth by shade, phyB and defoliation signalling pathways

In recent years, several genetic components of vegetative axillary bud development have been defined in both monocots and eudicots, but our understanding of environmental inputs on branching remains limited. Recent work in sorghum ( Sorghum bicolor ) has revealed a role for phytochrome B (phyB) in the control of axillary bud outgrowth through the regulation of Teosinte Branched1 ( TB1 ) gene. In maize ( Zea mays ), TB1 is a dosage-dependent inhibitor of axillary meristem progression, and the expression level of TB1 is a sensitive measure of axillary branch development. To further explore the mechanistic basis of branching, the expression of branching and cell cycle-related genes were examined in phyB-1 and wild-type sorghum axillary buds following treatment with low-red : far-red light and defoliation. Although defoliation inhibited bud outgrowth, it did not influence the expression of sorghum TB1 ( SbTB1 ), whereas changes in SbMAX2 expression, a homolog of the Arabidopsis ( Arabidopsis thaliana ) branching inhibitor MAX2 , were associated with the regulation of bud outgrowth by both light and defoliation. The expression of several cell cycle-related genes was also decreased dramatically in buds repressed by defoliation, but not by phyB deficiency. The data suggest that there are at least two distinct molecular pathways that respond to light and endogenous signals to regulate axillary bud outgrowth.     

Overexpression of a petunia zinc-finger gene alters cytokinin metabolism and plant forms

Shoot branching and plant height are among the key factors that define the overall architecture of plants. We found that overexpression of a cDNA for a zinc-finger protein of petunia, designated Lateral shoot-Inducing Factor (LIF), in transgenic petunia plants resulted in a dramatic increase in lateral shoots and reduced plant height. LIF overexpression also caused a decrease in the number of cells in the stem, leaf, and flower, accompanied by enlargement of cells. trans-Zeatin was decreased while N6-(Delta2-isopentenyl)adenine was increased in the leaves of LIF-overexpressed petunia. Most of the riboside, ribotide, and glucoside forms were also increased. Expression analysis using a LIF::GUS fusion gene and RT-PCR suggested that LIF is specifically expressed around the bases of axillary buds and weakly in basal part of flowers in wild-type petunia. GFP-LIF-GUS fusion proteins were translocated into the nucleus when transiently expressed in onion epidermal cells. LIF overexpression resulted in enhanced branching also in tobacco and Arabidopsis, indicating the conservation of the response to LIF overexpression among dicotyledonous plants. On the basis of these results we discuss about possible functions of LIF.     

表观遗传调控植物分枝/分蘖研究进展

分蘖是禾本科植物特有的分枝类型, 是影响作物产量的关键因素之一。分枝/分蘖数由叶腋处侧生分生组织的数量和侧芽的活性共同决定。表观遗传修饰调控植物生长发育的各个方面, 但是如何调控植物的分枝/分蘖数还未见系统报道。该综述归纳了表观遗传调控侧生分生组织的形成和侧芽向外生长两个方面, 并展望了表观遗传在调控植物分枝/分蘖中的研究方向, 以期为通过表观遗传修饰改良作物品种的育种途径提供理论指导。     

Trehalose 6‐phosphate is involved in triggering axillary bud outgrowth in garden pea (Pisum sativum L.)

Trehalose 6‐phosphate (Tre6P) is a signal of sucrose availability in plants, and has been implicated in the regulation of shoot branching by the abnormal branching phenotypes of Arabidopsis (Arabidopsis thaliana) and maize (Zea mays) mutants with altered Tre6P metabolism. Decapitation of garden pea (Pisum sativum) plants has been proposed to release the dormancy of axillary buds lower down the stem due to changes in sucrose supply, and we hypothesized that this response is mediated by Tre6P. Decapitation led to a rapid and sustained rise in Tre6P levels in axillary buds, coinciding with the onset of bud outgrowth. This response was suppressed by simultaneous defoliation that restricts the supply of sucrose to axillary buds in decapitated plants. Decapitation also led to a rise in amino acid levels in buds, but a fall in phosphoenolpyruvate and 2‐oxoglutarate. Supplying sucrose to stem node explants in vitro triggered a concentration‐dependent increase in the Tre6P content of the buds that was highly correlated with their rate of outgrowth. These data show that changes in bud Tre6P levels are correlated with initiation of bud outgrowth following decapitation, suggesting that Tre6P is involved in the release of bud dormancy by sucrose. Tre6P might also be linked to a reconfiguration of carbon and nitrogen metabolism to support the subsequent growth of the bud into a new shoot.     

barren inflorescence2 Encodes a co-ortholog of the PINOID serine/threonine kinase and is required for organogenesis during inflorescence and vegetative development in maize

Organogenesis in plants is controlled by meristems. Axillary meristems, which give rise to branches and flowers, play a critical role in plant architecture and reproduction. Maize (Zea mays) and rice (Oryza sativa) have additional types of axillary meristems in the inflorescence compared to Arabidopsis (Arabidopsis thaliana) and thus provide an excellent model system to study axillary meristem initiation. Previously, we characterized the barren inflorescence2 (bif2) mutant in maize and showed that bif2 plays a key role in axillary meristem and lateral primordia initiation in the inflorescence. In this article, we cloned bif2 by transposon tagging. Isolation of bif2-like genes from seven other grasses, along with phylogenetic analysis, showed that bif2 is a co-ortholog of PINOID (PID), which regulates auxin transport in Arabidopsis. Expression analysis showed that bif2 is expressed in all axillary meristems and lateral primordia during inflorescence and vegetative development in maize and rice. Further phenotypic analysis of bif2 mutants in maize illustrates additional roles of bif2 during vegetative development. We propose that bif2/PID sequence and expression are conserved between grasses and Arabidopsis, attesting to the important role they play in development. We provide further support that bif2, and by analogy PID, is required for initiation of both axillary meristems and lateral primordia.     

Tillering in the sugary1 sweet corn is maintained by overriding the teosinte branched1 repressive signal

The evolution of apical dominance in maize during domestication from teosinte is associated with higher expression from the teosinte branched1 (tb1) gene that inhibits tiller bud outgrowth. Unlike many standard maize varieties, the sweet corn inbred P39 that carries a mutation in a starch biosynthesis gene sugary1 produces multiple tillers and providing an opportunity to explore the diversification of the tb1 signal in maize. Through gene expression analysis, we show that tiller buds in P39 continue to grow by overriding the high expression level of tb1 that arrests bud outgrowth in maize inbred B73. In addition, we demonstrate that while B73 is largely non-responsive to shade, both P39 and teosinte respond through tb1-independent and tb1-dependent molecular mechanisms, respectively, leading to inhibition of tiller bud outgrowth.     

Hormonal control of shoot branching

Shoot branching is the process by which axillary buds, located on the axil of a leaf, develop and form new flowers or branches. The process by which a dormant bud activates and becomes an actively growing branch is complex and very finely tuned. Bud outgrowth is regulated by the interaction of environmental signals and endogenous ones, such as plant hormones. Thus these interacting factors have a major effect on shoot system architecture. Hormones known to have a major influence are auxin, cytokinin, and a novel, as yet chemically undefined, hormone. Auxin is actively transported basipetally in the shoot and inhibits bud outgrowth. By contrast, cytokinins travel acropetally and promote bud outgrowth. The novel hormone also moves acropetally but it inhibits bud outgrowth. The aim of this review is to integrate what is known about the hormonal control of shoot branching in Arabidopsis, focusing on these three hormones and their interactions.     

The Relationship between auxin transport and maize branching

Maize (Zea mays) plants make different types of vegetative or reproductive branches during development. Branches develop from axillary meristems produced on the flanks of the vegetative or inflorescence shoot apical meristem. Among these branches are the spikelets, short grass-specific structures, produced by determinate axillary spikelet-pair and spikelet meristems. We investigated the mechanism of branching in maize by making transgenic plants expressing a native expressed endogenous auxin efflux transporter (ZmPIN1a) fused to yellow fluorescent protein and a synthetic auxin-responsive promoter (DR5rev) driving red fluorescent protein. By imaging these plants, we found that all maize branching events during vegetative and reproductive development appear to be regulated by the creation of auxin response maxima through the activity of polar auxin transporters. We also found that the auxin transporter ZmPIN1a is functional, as it can rescue the polar auxin transport defects of the Arabidopsis (Arabidopsis thaliana) pin1-3 mutant. Based on this and on the groundbreaking analysis in Arabidopsis and other species, we conclude that branching mechanisms are conserved and can, in addition, explain the formation of axillary meristems (spikelet-pair and spikelet meristems) that are unique to grasses. We also found that BARREN STALK1 is required for the creation of auxin response maxima at the flanks of the inflorescence meristem, suggesting a role in the initiation of polar auxin transport for axillary meristem formation. Based on our results, we propose a general model for branching during maize inflorescence development.     

Teosinte Branched1 and the Origin of Maize: Evidence for Epistasis and the Evolution of Dominance

Two quantitative trait loci (QTL) controlling differences in plant and inflorescence architecture between maize and its progenitor (teosinte) were analyzed. Complementation tests indicate that one of these, which is on chromosome arm 1L, is the locus for the maize mutant teosinte branched1 (tb1). This QTL has effects on inflorescence sex and the number and length of internodes in the lateral branches and inflorescences. This QTL has strong phenotypic effects in teosinte background but reduced effects in maize background. The second QTL, which is on chromosome arm 3L, affects the same traits as the QTL on 1L. We identify two candidate loci for this QTL. The effects of this QTL on several traits are reduced in both maize and teosinte background as compared to a maize-teosinte F(2) population. Genetic background appears to affect gene action for both QTL. Analysis of a population in which both QTL were segregating revealed that they interact epistatically. Together, these two QTL substantially transform both plant and inflorescence architecture. We propose that tb1 is involved in the teosinte plant's response to local environment to produce either long or short branches and that maize evolution involved a change at this locus to produce short branches under all environments.     

Photoregulation of growth and branching of plum shoots: Physiological action of two photosystems

Summary Plum shoot proliferation was investigated in terms of two distinct processes: axillary bud differentiation and axillary shoot development. Results showed that light quality influenced bud differentiation and interacted with apical dominance in determining shoot outgrowth, resulting in a differentiated structure of shoot clusters and type of branching. Results suggested that blue light, acting through its photoreceptor, increased the number of axillary buds differentiated from apical meristem, but did not remove the apical dominance. Red light removed apical dominance, while reducing the formation of axillary buds; both events appeared to be dependent on the putative amount of phytochrome active form, and independent of light photon fluence rate. On the contrary, blue light action appeared to be dependent on photon fluence rate. In addition, apparent blue-red interactions related to photomorphogenic events fit an antagonistic model for branching regulated by light via cryptochrome and phytochrome photoreceptors. Our results show that the dynamics of shoot cluster development is the product of two events: the formation of new axillary buds and their release from apical dominance.