Sarah J. Liljegren

SHATTERPROOF MADS-box genes control seed dispersal in Arabidopsis

The fruit, which mediates the maturation and dispersal of seeds, is a complex structure unique to flowering plants. Seed dispersal in plants such as Arabidopsis occurs by a process called fruit dehiscence, or pod shatter. Few studies have focused on identifying genes that regulate this process, in spite of the agronomic value of controlling seed dispersal in crop plants such as canola. Here we show that the closely related SHATTERPROOF (SHP1) and SHATTERPROOF2 (SHP2 ) MADS-box genes are required for fruit dehiscence in Arabidopsis. Moreover, SHP1 and SHP2 are functionally redundant, as neither single mutant displays a novel phenotype. Our studies of shp1 shp2 fruit, and of plants constitutively expressing SHP1 and SHP2, show that these two genes control dehiscence zone differentiation and promote the lignification of adjacent cells. Our results indicate that further analysis of the molecular events underlying fruit dehiscence may allow genetic manipulation of pod shatter in crop plants.

Assessing the redundancy of MADS-box genes during carpel and ovule development.

Carpels are essential for sexual plant reproduction because they house the ovules and subsequently develop into fruits that protect, nourish and ultimately disperse the seeds. The AGAMOUS (AG) gene is necessary for plant sexual reproduction because stamens and carpels are absent from ag mutant flowers. However, the fact that sepals are converted into carpelloid organs in certain mutant backgrounds even in the absence of AG activity indicates that an AG-independent carpel-development pathway exists. AG is a member of a monophyletic clade of MADS-box genes that includes SHATTERPROOF1 (SHP1), SHP2 and SEEDSTICK (STK), indicating that these four genes might share partly redundant activities. Here we show that the SHP genes are responsible for AG-independent carpel development. We also show that the STK gene is required for normal development of the funiculus, an umbilical-cord-like structure that connects the developing seed to the fruit, and for dispersal of the seeds when the fruit matures. We further show that all four members of the AG clade are required for specifying the identity of ovules, the landmark invention during the course of vascular plant evolution that enabled seed plants to become the most successful group of land plants.

Interactions among APETALA1 , LEAFY , and TERMINAL FLOWER1 Specify Meristem Fate

Upon floral induction, the primary shoot meristem of an Arabidopsis plant begins to produce flower meristems rather than leaf primordia on its flanks. Assignment of floral fate to lateral meristems is primarily due to the cooperative activity of the flower meristem identity genes LEAFY (LFY), APETALA1 (AP1), and CAULIFLOWER. We present evidence here that AP1 expression in lateral meristems is activated by at least two independent pathways, one of which is regulated by LFY. In lfy mutants, the onset of AP1 expression is delayed, indicating that LFY is formally a positive regulator of AP1. We have found that AP1, in turn, can positively regulate LFY, because LFY is expressed prematurely in the converted floral meristems of plants constitutively expressing AP1. Shoot meristems maintain an identity distinct from that of flower meristems, in part through the action of genes such as TERMINAL FLOWER1 (TFL1), which bars AP1 and LFY expression from the inflorescence shoot meristem. We show here that this negative regulation can be mutual because TFL1 expression is downregulated in plants constitutively expressing AP1. Therefore, the normally sharp phase transition between the production of leaves with associated shoots and formation of the flowers, which occurs upon floral induction, is promoted by positive feedback interactions between LFY and AP1, together with negative interactions of these two genes with TFL1.

Control of Fruit Patterning in Arabidopsis by INDEHISCENT

The Arabidopsis seedpod opens through a spring-loaded mechanism known as pod shatter, which is essential for dispersal of the seeds. Here, we identify INDEHISCENT (IND), an atypical bHLH protein, that is necessary for fruit opening and is involved in patterning each of the three fruit cell types required for seed dispersal. Previous studies suggested that FRUITFULL (FUL), a member of the MADS-domain transcription factor family, is required for fruit growth since ful mutant fruit fail to undergo the dramatic enlargement that normally occurs after fertilization. Here we show, however, that FUL is not directly required for fruit elongation and instead is required to prevent ectopic activity of IND. Our molecular and genetic studies suggest a model for the regulatory interactions among the genes that control fruit development and the mechanism that results in the expression of IND in a narrow stripe of cells.

Targeted disruption in Arabidopsis

Homologous recombination has been used for two decades to target insertions into cloned genes in bacteria and yeast, and more recently has become a routine method of gene inactivation in mammals. Arabidopsis is one of several multicellular model organisms (along with Drosophila, Caenorhabditis and zebrafish) in which mechanisms controlling development have been studied. Previously, traditional genetic methods have been used, as targeted disruption by homologous recombination has not been successful in any of these organisms. We have now successfully disrupted the AGL5 MADS-box gene in Arabidopsisby homologous recombination, providing a useful tool for future analyses.

MADS-box gene expression in lateral primordia, meristems and differentiated tissues of Arabidopsis thaliana roots

Abstract. Although MADS-box genes involved in flower and fruit development have been well characterized, the function of MADS-box genes expressed in vegetative structures has yet to be explored. At least seven members of this family are grouped in clades of genes that are preferentially expressed in roots of Arabidopsis thaliana (L.) Heynh.. We report here the cloning of the AGL21 MADS-box gene, which belongs to the ANR1 clade, and the mRNA in situ expression patterns of this and two other root MADS-box genes. AGL17 appears to be a lateral root cap marker in the root tip, and towards the elongation zone this gene is expressed in the epidermal cells. AGL21 is highly expressed in lateral root primordia and it has a punctate expression pattern in the primary root meristem. AGL12 also has a punctate expression pattern in the primary root meristem. AGL12 and AGL21 are also expressed in the central cylinder of differentiated roots and both are expressed in developing embryos. This study, combined with previous phylogenetic analyses, indicates that these MADS-box genes may play distinct regulatory roles during root development.

Genetic control of shoot and flower meristem behavior

Flowers and shoots are derived from specialized groups of stem cells termed meristems. Recent studies in Arabidopsis have identified factors that contribute to meristem structure and identity, such as CLAVATA1, CLAVATA3, and SHOOTMERISTEMLESS, which act in both shoot and flower meristems, as well as LEAFY and APETALA1 which specifically determine a floral fate.

The Role of MADS-Box Genes in the Control of Flower and Fruit Development in Arabidopsis

Arabidopsis flowers and fruit are typical of the more than three thousand species of Brassicaceae and have been the subject of intensive genetic and molecular studies. Among the many genes that have been identified that control various aspects of flower and fruit development, the MADS-box family has been shown to play central roles. MADS-box genes encode putative transcriptional regulators that play regulatory roles not only in diverse plant species, but also in fungal and animal development. The first Arabidopsis MADS-box gene to be cloned was AGAMOUS in 1990, and in the ensuing years, dozens of related genes have been cloned and functionally characterized (Yanofsky et al., 1990; Riechmann and Meyerowitz, 1997; Theissen, 2000). Three general lessons have been learned from these functional studies. The first, is that MADS-box genes play diverse roles in plant development, ranging from the control of flowering time, meristem identity, organ identity, fruit development, and they also appear to play roles during embryo, ovule, seed, root, stem and leaf development. A second lesson that we have learned is that MADS-box genes frequently play multiple roles during development. For example, the FRUITFULL gene is involved in leaf development as well as in fruit development. A third lesson is that functional redundancy is prevalent among MADS-box genes. In some cases, single mutants are indistinguishable from the wild type, whereas double mutants carrying mutations in two closely related MADS-box genes display striking phenotypic abnormalities. In this manuscript, we will focus on recent examples from our laboratory that highlight these three basic conclusions of MADS-box gene function.