MADS-box gene evolution beyond flowers: expression in pollen, endosperm, guard cells, roots and trichomes.

MADS-box genes encode transcriptional regulators involved in diverse aspects of plant development. Here we describe the cloning and mRNA spatio-temporal expression patterns of five new MADS-box genes from Arabidopsis: AGL16, AGL18, AGL19, AGL27 and AGL31. These genes will probably become important molecular tools for both evolutionary and functional analyses of vegetative structures. We mapped our data and previous expression patterns onto a new MADS-box phylogeny. These analyses suggest that the evolution of the MADS-box family has involved a rapid and simultaneous functional diversification in vegetative as well as reproductive structures. The hypothetical ancestral genes had broader expression patterns than more derived ones, which have been co-opted for putative specialized functions as suggested by their expression patterns. AGL27 and AGL31, which are closely related to the recently described flowering-time gene FLC (previously AGL25), are expressed in most plant tissues. AGL19 is specifically expressed in the outer layers of the root meristem (lateral root cap and epidermis) and in the central cylinder cells of mature roots. AGL18, which is most similar in sequence to the embryo-expressed AGL15 gene, is expressed in the endosperm and in developing male and female gametophytes, suggesting a role for AGL18 that is distinct from previously characterized MADS-box genes. Finally, AGL16 RNA accumulates in leaf guard cells and trichomes. Our new phylogeny reveals seven new monophyletic clades of MADS-box sequences not specific to flowers, suggesting that complex regulatory networks involving several MADS-box genes, similar to those that control flower development, underlie development of vegetative structures.

Negative regulation of the SHATTERPROOF genes by FRUITFULL during Arabidopsis fruit development.

The terminal step of fruit development in Arabidopsis involves valve separation from the replum, allowing seed dispersal. This process requires the activities of the SHATTERPROOF MADS-box genes, which promote dehiscence zone differentiation at the valve/replum boundary. Here we show that the FRUITFULL MADS-box gene, which is necessary for fruit valve differentiation, is a negative regulator of SHATTERPROOF expression and that constitutive expression of FRUITFULL is sufficient to prevent formation of the dehiscence zone. Our studies suggest that ectopic expression of FRUITFULL may directly allow the control of pod shatter in oilseed crops such as canola.

An ancestral MADS-box gene duplication occurred before the divergence of plants and animals.

Changes in genes encoding transcriptional regulators can alter development and are important components of the molecular mechanisms of morphological evolution. MADS-box genes encode transcriptional regulators of diverse and important biological functions. In plants, MADS-box genes regulate flower, fruit, leaf, and root development. Recent sequencing efforts in Arabidopsis have allowed a nearly complete sampling of the MADS-box gene family from a single plant, something that was lacking in previous phylogenetic studies. To test the long-suspected parallel between the evolution of the MADS-box gene family and the evolution of plant form, a polarized gene phylogeny is necessary. Here we suggest that a gene duplication ancestral to the divergence of plants and animals gave rise to two main lineages of MADS-box genes: TypeI and TypeII. We locate the root of the eukaryotic MADS-box gene family between these two lineages. A novel monophyletic group of plant MADS domains (AGL34 like) seems to be more closely related to previously identified animal SRF-like MADS domains to form TypeI lineage. Most other plant sequences form a clear monophyletic group with animal MEF2-like domains to form TypeII lineage. Only plant TypeII members have a K domain that is downstream of the MADS domain in most plant members previously identified. This suggests that the K domain evolved after the duplication that gave rise to the two lineages. Finally, a group of intermediate plant sequences could be the result of recombination events. These analyses may guide the search for MADS-box sequences in basal eukaryotes and the phylogenetic placement of new genes from other plant species.

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.

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 influorescence 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.

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.