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.
Summary
In Arabidopsis, the closely related APETALA1 (AP1) and CAULIFLOWER (CAL) MADS‐box genes share overlapping roles in promoting flower meristem identity. Later in flower development, the AP1 ...gene is required for normal development of sepals and petals. Studies of MADS‐domain proteins in diverse species have shown that they often function as heterodimers or in larger ternary complexes, suggesting that additional proteins may interact with AP1 and CAL during flower development. To identify proteins that may interact with AP1 and CAL, we used the yeast two‐hybrid assay. Among the five MADS‐box genes identified in this screen, the SEPALLATA3 (SEP3) gene was chosen for further study. Mutations in the SEP3 gene, as well as SEP3 antisense plants that have a reduction in SEP3 RNA, display phenotypes that closely resemble intermediate alleles of AP1. Furthermore, the early flowering phenotype of plants constitutively expressing AP1 is significantly enhanced by constitutive SEP3 expression. Taken together, these studies suggest that SEP3 interacts with AP1 to promote normal flower development.
Local hormone maxima are essential for the development of multicellular structures and organs. For example, steroid hormones accumulate in specific cell types of the animal fetus to induce sexual ...differentiation and concentration peaks of the plant hormone auxin direct organ initiation and mediate tissue patterning. Here we provide an example of a regulated local hormone minimum required during organogenesis. Our results demonstrate that formation of a local auxin minimum is necessary for specification of the valve margin separation layer where Arabidopsis fruit opening takes place. Consequently, ectopic production of auxin, specifically in valve margin cells, leads to a complete loss of proper cell fate determination. The valve margin identity factor INDEHISCENT (IND) is responsible for forming the auxin minimum by coordinating auxin efflux in separation-layer cells. We propose that the simplicity of formation and maintenance make local hormone minima particularly well suited to specify a small number of cells such as the stripes at the valve margins.
Plant meristems carry pools of continuously active stem cells, whose activity is controlled by developmental and environmental signals. After stem cell division, daughter cells that exit the stem ...cell domain acquire transit amplifying cell identity before they are incorporated into organs and differentiate. In this study, we used an integrated approach to elucidate the role of
(
) genes in regulating developmental trajectories of shoot stem cells in
. Our work reveals that
function stabilizes cell fate in distinct zones of the shoot meristem thereby controlling the spatio-temporal dynamics of stem cell differentiation. Importantly, this activity is concomitant with the local modulation of cellular responses to cytokinin and auxin, two key phytohormones regulating cell behaviour. Mechanistically, we show that HEC factors transcriptionally control and physically interact with MONOPTEROS (MP), a key regulator of auxin signalling, and modulate the autocatalytic stabilization of auxin signalling output.
Summary
The Arabidopsis thaliana MERISTEM LAYER 1 (ATML1) gene is expressed in the epidermis of developing embryos and shoot meristems. To identify regulatory sequences necessary for ...epidermis‐specific expression, three fusions of overlapping ATML1 genomic sequences to the GUS reporter gene were introduced into Arabidopsis plants. All fusion genes conferred epidermis‐specific expression of both GUS mRNA and protein activity but varied in both the timing and relative levels of expression, suggesting partial redundancy of ATML1 regulatory elements. This study defines L1‐specific regulatory sequences that are sufficient to direct foreign gene expression in a layer‐specific manner.
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.
Cell separation, or abscission, is a highly specialized process in plants that facilitates remodeling of their architecture and reproductive success. Because few genes are known to be essential for ...organ abscission, we conducted a screen for mutations that alter floral organ shedding in Arabidopsis. Nine recessive mutations that block shedding were found to disrupt the function of an ADP-ribosylation factor-GTPase-activating protein (ARF-GAP) we have named NEVERSHED (NEV). As predicted by its homology to the yeast Age2 ARF-GAP and transcriptional profile, NEV influences other aspects of plant development, including fruit growth. Co-localization experiments carried out with NEV-specific antiserum and a set of plant endomembrane markers revealed that NEV localizes to the trans-Golgi network and endosomes in Arabidopsis root epidermal cells. Interestingly, transmission electron micrographs of abscission zone regions from wild-type and nev flowers reveal defects in the structure of the Golgi apparatus and extensive accumulation of vesicles adjacent to the cell walls. Our results suggest that NEV ARF-GAP activity at the trans-Golgi network and distinct endosomal compartments is required for the proper trafficking of cargo molecules required for cell separation.
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.
Fruit morphogenesis is a process unique to flowering plants, and yet little is known about its developmental control. Following fertilization, fruits typically undergo a dramatic enlargement that is ...accompanied by differentiation of numerous distinct cell types. We have identified a mutation in Arabidopsis called fruitfull (ful-1), which abolishes elongation of the silique after fertilization. The ful-1 mutation is caused by the insertion of a DsE transposable enhancer trap element into the 5' untranslated leader of the AGL8 MADS-box gene. beta-glucuronidase (GUS) reporter gene expression in the enhancer trap line is observed specifically in all cell layers of the valve tissue, but not in the replum, the septum or the seeds, and faithfully mimics RNA in situ hybridization data reported previously. The lack of coordinated growth of the fruit tissues leads to crowded seeds, a failure of dehiscence and, frequently, the premature rupture of the carpel valves. The primary defect of ful-1 fruits is within the valves, whose cells fail to elongate and differentiate. Stomata, which are frequent along the epidermis of wild-type valves, are completely eliminated in the ful mutant valves. In addition to the effect on fruit development, ful cauline leaves are broader than those of wild type and show a reduction in the number of internal cell layers. These data suggest that AGL8/FUL regulates the transcription of genes required for cellular differentiation during fruit and leaf development.
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.
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