Gregor Schmitz

The tomato Blind gene encodes a MYB transcription factor that controls the formation of lateral meristems

The multitude of forms observed in flowering plants is largely because of their ability to establish new axes of growth during postembryonic development. This process is initiated by the formation of secondary meristems that develop into vegetative or reproductive branches. In the blind and torosa mutants of tomato, initiation of lateral meristems is blocked during shoot and inflorescence development, leading to a strong reduction in the number of lateral axes. In this study, it is shown that blind and torosa are allelic. The Blind gene has been isolated by positional cloning, and it was found that the mutant phenotype is caused by a loss of function of an R2R3 class Myb gene. RNA interference-induced blind phenocopies confirmed the identity of the isolated gene. Double mutant analysis shows that Blind acts in a novel pathway different from the one to which the previously identified Lateral suppressor gene belongs. The findings reported add a new class of transcription factors to the group of genes controlling lateral meristem initiation and reveal a previously uncharacterized function of R2R3 Myb genes.

Blind Homologous R2R3 Myb Genes Control the Pattern of Lateral Meristem Initiation in Arabidopsis

In seed plants, shoot branching is initiated during postembryonic development by the formation of secondary meristems. These new meristems, which are established between the stem and leaf primordia, develop into vegetative branches or flowers. Thus, the number of axillary meristems has a major impact on plant architecture and reproductive success. This study describes the genetic control of axillary meristem formation in Arabidopsis thaliana by a group of three R2R3 Myb genes, which are homologous to the tomato (Solanum lycopersicum) Blind gene and were designated REGULATORS OF AXILLARY MERISTEMS (RAX). rax mutants show new phenotypes that are characterized by defects in lateral bud formation in overlapping zones along the shoot axis. RAX genes are partially redundant in function and allow a fine-tuning of secondary axis formation. As revealed by monitoring of SHOOT MERISTEMLESS transcript accumulation, the RAX genes control a very early step of axillary meristem initiation. The RAX1 and RAX3 expression domains specifically mark a cell group in the center of the leaf axil from which the axillary meristem develops. Double mutant combinations of lateral suppressor and rax1-3 as well as expression studies suggest that at least two pathways control the initiation of axillary meristems in Arabidopsis.

Comparative sequence analysis reveals extensive microcolinearity in the lateral suppressor regions of the tomato, Arabidopsis, and Capsella genomes.

A 57-kb region of tomato chromosome 7 harboring five different genes was compared with the sequence of the Arabidopsis genome to search for microsynteny between the genomes of these two species. For all five genes, homologous sequences could be identified in a 30-kb region located on Arabidopsis chromosome 1. Only two inversion events distinguish the arrangement of the five genes in tomato from that in Arabidopsis. Inversions were not detected when the arrangement of the five Arabidopsis genes was compared with the arrangement in the orthologous region of Capsella, a plant closely related to Arabidopsis. These results provide evidence for microcolinearity between closely and distantly related dicotyledonous species. The degree of microcolinearity found can be exploited to localize orthologous genes in Arabidopsis and tomato in an unambiguous way.

PROCERA encodes a DELLA protein that mediates control of dissected leaf form in tomato

Leaves of seed plants can be described as simple, where the leaf blade is entire, or dissected, where the blade is divided into distinct leaflets. Mechanisms that define leaflet number and position are poorly understood and their elucidation presents an attractive opportunity to understand mechanisms controlling organ shape in plants. In tomato (Solanum lycopersicum), a plant with dissected leaves, KNOTTED1-like homeodomain proteins (KNOX) are positive regulators of leaflet formation. Conversely, the hormone gibberellin (GA) can antagonise the effects of KNOX overexpression and reduce leaflet number, suggesting that GA may be a negative regulator of leaflet formation. However, when and how GA acts on leaf development is unknown. The reduced leaflet number phenotype of the tomato mutant procera (pro) mimics that of plants to which GA has been applied during leaf development, suggesting that PRO may define a GA signalling component required to promote leaflet formation. Here we show that PRO encodes a DELLA-type growth repressor that probably mediates GA-reversible growth restraint. We demonstrate that PRO is required to promote leaflet initiation during early stages of growth of leaf primordia and conversely that reduced GA biosynthesis increases the capability of the tomato leaf to produce leaflets in response to elevated KNOX activity. We propose that, in tomato, DELLA activity regulates leaflet number by defining the correct timing for leaflet initiation.

Shoot Branching and Leaf Dissection in Tomato Are Regulated by Homologous Gene Modules

This work shows that axillary meristem initiation and leaflet initiation in tomato share common regulators that act in boundary establishment, including the NAM/CUC ortholog Goblet and the MYB domain transcription factors Blind and Potato leaf, which are orthologs of the Arabidopsis branching regulator RAX1. Aerial plant architecture is predominantly determined by shoot branching and leaf morphology, which are governed by apparently unrelated developmental processes, axillary meristem formation, and leaf dissection. Here, we show that in tomato (Solanum lycopersicum), these processes share essential functions in boundary establishment. Potato leaf (C), a key regulator of leaf dissection, was identified to be the closest paralog of the shoot branching regulator Blind (Bl). Comparative genomics revealed that these two R2R3 MYB genes are orthologs of the Arabidopsis thaliana branching regulator REGULATOR OF AXILLARY MERISTEMS1 (RAX1). Expression studies and complementation analyses indicate that these genes have undergone sub- or neofunctionalization due to promoter differentiation. C acts in a pathway independent of other identified leaf dissection regulators. Furthermore, the known leaf complexity regulator Goblet (Gob) is crucial for axillary meristem initiation and acts in parallel to C and Bl. Finally, RNA in situ hybridization revealed that the branching regulator Lateral suppressor (Ls) is also expressed in leaves. All four boundary genes, C, Bl, Gob, and Ls, may act by suppressing growth, as indicated by gain-of-function plants. Thus, leaf architecture and shoot architecture rely on a conserved mechanism of boundary formation preceding the initiation of leaflets and axillary meristems.

The bHLH protein ROX acts in concert with RAX1 and LAS to modulate axillary meristem formation in Arabidopsis

During post-embryonic shoot development, new meristems are initiated in the axils of leaves. They produce secondary axes of growth that determine morphological plasticity and reproductive efficiency in higher plants. In this study, we describe the role of the bHLH-protein-encoding Arabidopsis gene REGULATOR OF AXILLARY MERISTEM FORMATION (ROX), which is the ortholog of the branching regulators LAX PANICLE1 (LAX1) in rice and barren stalk1 (ba1) in maize. rox mutants display compromised axillary bud formation during vegetative shoot development, and combination of rox mutants with mutations in RAX1 and LAS, two key regulators of axillary meristem initiation, enhances their branching defects. In contrast to lax1 and ba1, flower development is unaffected in rox mutants. Over-expression of ROX leads to formation of accessory side shoots. ROX mRNA accumulates at the adaxial boundary of leaf and flower primordia. However, in the vegetative phase, axillary meristems initiate after ROX expression has terminated, suggesting an indirect role for ROX in meristem formation. During vegetative development, ROX expression is dependent on RAX1 and LAS activity, and all three genes act in concert to modulate axillary meristem formation.

A novel transposon tagging element for obtaining gain-of-function mutants based on a self-stabilizing Ac derivative

A novel tagging system AcREH, designed for obtaining gain-of-function mutations, was prepared on the basis of a self-stabilizing Ac transposon derivative. The transposable element, DsAT, was constructed in a way that it can activate transcription of neighboring genes by two 35S promoters and/or by four tandem repeats of the enhancer fragment of this promoter. DsAT revealed somatic excision in the first generation of the tobacco transformants. The element exhibited germinal excision to the next generation, as demonstrated by PCR and Southern hybridization analysis. In spite of the structure of the element, which may inhibit the expression of the transposase gene, the frequency of germinal excision was comparable to or higher than those so far reported, suggesting the applicability of the element for gene tagging.

Trifoliate encodes an MYB transcription factor that modulates leaf and shoot architecture in tomato

Leaf morphology and the pattern of shoot branching determine to a large extent the growth habit of seed plants. Until recently, the developmental processes that led to the establishment of these morphological structures seemed unrelated. Here, we show that the tomato Trifoliate (Tf) gene plays a crucial role in both processes, affecting the formation of leaflets in the compound tomato leaf and the initiation of axillary meristems in the leaf axil. Tf encodes a myeloblastosis oncoprotein (MYB)-like transcription factor related to the Arabidopsis thaliana LATERAL ORGAN FUSION1 (LOF1) and LOF2 proteins. Tf is expressed in the leaf margin, where leaflets are formed, and in the leaf axil, where axillary meristems initiate. During tomato ontogeny, expression of Tf in young leaf primordia increases, correlating with a rise in leaf dissection (heteroblasty). Formation of leaflets and initiation of axillary meristems can be traced back to groups of pluripotent cells. Tf function is required to inhibit differentiation of these cells and thereby to maintain their morphogenetic competence, a fundamental process in plant development. KNOTTED1-LIKE proteins, which are known regulators in tomato leaf dissection, require Tf activity to exert their function in the basal part of the leaf. Similarly, the plant hormone auxin needs Tf activity to initiate the formation of lateral leaflets. Thus, leaf dissection and shoot branching rely on a conserved mechanism that regulates the morphogenetic competence of cells at the leaf margin and in the leaf axil.

Genetic and evolutionary perspectives on the interplay between plant immunity and development

There is now ample evidence that plant development, responses to abiotic environments, and immune responses are tightly intertwined in their physiology. Thus optimization of the immune system during evolution will occur in coordination with that of plant development. Two alternative and possibly complementary forces are at play: genetic constraints due to the pleiotropic action of players in both systems, and coevolution, if developmental changes modulate the cost-benefit balance of immunity. A current challenge is to elucidate the ecological forces driving evolution of quantitative variation for defense at molecular level. The analysis of natural co-variation for developmental and immunity traits in Arabidopsis thaliana promises to bring important insights.