Spatio-temporal expression of candidate genes for nectar spur development in Tropaeolum (Tropaeolaceae: Brassicales).

BACKGROUND AND AIMS: Tropaeolaceae (Brassicales) comprise ~100 species native to South and Central America. Tropaeolaceae flowers have a nectar spur, formed by a late expansion and evagination of the fused proximal region of the perianth (i.e. the floral tube). This spur is formed in the domain of the tube oriented towards the inflorescence axis, which corresponds to the adaxial floral region. However, little is known about the molecular mechanisms responsible for the evolution of spurs in Tropaeolaceae. METHODS: In this study, we examined the spatio-temporal expression of genes putatively responsible for differential patterns of cell division between the adaxial and abaxial floral regions in Tropaeolaceae. These genes include previously identified TCP and KNOX transcription factors and the cell division marker HISTONE H4 (HIS4). KEY RESULTS: We found a TCP4 homologue concomitantly expressed with spur initiation and elaboration. Tropaeolaceae possess two TCP4-like (TCP4L) copies, as a result of a Tropaeolaceae-specific duplication. The two copies (TCP4L1 and TCP4L2) in Tropaeolum longifolium show overlapping expression in the epidermis of reproductive apices (inflorescence meristems) and young floral buds, but only TlTCP4L2 shows differential expression in the floral tube at early stages of spur formation, restricted to the adaxial region. This adaxial expression of TlTCP4L2 overlaps with the expression of TlHIS4. Later in development, only TlTCP4L2 is expressed in the nectariferous tissue of the spur. CONCLUSIONS: Based on these results, we hypothesize that Tropaeolaceae TCP4L genes had a plesiomorphic role in epidermal development and that, after gene duplication, TCP4L2 acquired a new function in spur initiation and elaboration. To better understand spur evolution in Tropaeolaceae, it is critical to expand developmental genetic studies to their sister group, the Akaniaceae, which possess simultaneously an independent duplication of TCP4L genes and a spurless floral tube.

Alternate wiring of a KNOXI genetic network underlies differences in leaf development of A. thaliana and C. hirsuta.

Two interrelated problems in biology are understanding the regulatory logic and predictability of morphological evolution. Here, we studied these problems by comparing Arabidopsis thaliana, which has simple leaves, and its relative, Cardamine hirsuta, which has dissected leaves comprising leaflets. By transferring genes between the two species, we provide evidence for an inverse relationship between the pleiotropy of SHOOTMERISTEMLESS (STM) and BREVIPEDICELLUS (BP) homeobox genes and their ability to modify leaf form. We further show that cis-regulatory divergence of BP results in two alternative configurations of the genetic networks controlling leaf development. In C. hirsuta, ChBP is repressed by the microRNA164A (MIR164A)/ChCUP-SHAPED COTYLEDON (ChCUC) module and ChASYMMETRIC LEAVES1 (ChAS1), thus creating cross-talk between MIR164A/CUC and AS1 that does not occur in A. thaliana. These different genetic architectures lead to divergent interactions of network components and growth regulation in each species. We suggest that certain regulatory genes with low pleiotropy are predisposed to readily integrate into or disengage from conserved genetic networks influencing organ geometry, thus rapidly altering their properties and contributing to morphological divergence.

Arabidopsis PRC1 core component AtRING1 regulates stem cell-determining carpel development mainly through repression of class I KNOX genes.

BACKGROUND: Polycomb repressive complex 2 (PRC2)-catalyzed H3K27me3 marks are tightly associated with the WUS-AG negative feedback loop to terminate floral stem cell fate to promote carpel development, but the roles of Polycomb repressive complex 1 (PRC1) in this event remain largely uncharacterized. RESULTS: Here we show conspicuous variability in the morphology and number of carpels among individual flowers in the absence of the PRC1 core components AtRING1a and AtRING1b, which contrasts with the wild-type floral meristem consumed by uniform carpel production in Arabidopsis thaliana. Promoter-driven GUS reporter analysis showed that AtRING1a and AtRING1b display a largely similar expression pattern, except in the case of the exclusively maternal-preferred expression of AtRING1b, but not AtRING1a, in the endosperm. Indeterminate carpel development in the atring1a;atring1b double mutant is due to replum/ovule-to-carpel conversion in association with ectopic expression of class I KNOX (KNOX-I) genes. Moreover, AtRING1a and AtRING1b also play a critical role in ovule development, mainly through promoting the degeneration of non-functional megaspores and proper integument formation. Genetic interaction analysis indicates that the AtRING1a/b-regulated KNOX-I pathway acts largely in a complementary manner with the WUS-AG pathway in controlling floral stem cell maintenance and proper carpel development. CONCLUSIONS: Our study uncovers a novel mechanistic pathway through which AtRING1a and AtRING1b repress KNOX-I expression to terminate floral stem cell activities and establish carpel cell fate identities.

OsARID3, an AT-rich Interaction Domain-containing protein, is required for shoot meristem development in rice.

The shoot apical meristem (SAM) produces all of the plant's aerial organs. The SAM is established either during embryogenesis or experimentally in in vitro tissue culture. Although several factors including the Class I KNOTTED1-LIKE HOMEOBOX (KNOXI) proteins, auxin, and cytokinin are known to play essential roles in SAM development, the underlying mechanisms of SAM formation and maintenance are still largely not understood. Herein we demonstrate that OsARID3, a member of the rice (Oryza sativa) AT-rich Interaction Domain (ARID) family, is required for SAM development. Disruption of OsARID3 leads to a defective SAM, early seedling lethality, and impaired capacity of in vitro shoot regeneration. We show that the expression levels of several KNOXI genes and the biosynthetic genes for auxin and cytokinin are significantly altered in the Osarid3 mutant calli. Moreover, we determine that auxin concentrations are increased, whereas cytokinin levels are decreased, in Osarid3 calli. Furthermore, chromatin immunoprecipitation results demonstrate that OsARID3 binds directly to the KNOXI gene OSH71, the auxin biosynthetic genes OsYUC1 and OsYUC6, and the cytokinin biosynthetic genes OsIPT2 and OsIPT7. We also show through electrophoretic mobility shift assays that OsARID3 specifically binds to the AT-rich DNA sequences of the identified target genes. We conclude that OsARID3 is an AT-rich specific DNA-binding protein and that it plays a major role in SAM development in rice.

SEUSS and SEUSS-LIKE 2 coordinate auxin distribution and KNOXI activity during embryogenesis.

In Arabidopsis, SEUSS (SEU) and SEUSS-LIKE 2 (SLK2) are components of the LEUNIG (LUG) repressor complex that coordinates various aspects of post-embryonic development. The complex also plays a critical role during embryogenesis, as seu slk2 double mutants have small, narrow cotyledons and lack a shoot apical meristem (SAM). Here we show that seu slk2 double mutant embryos exhibit delayed cotyledon outgrowth and that this is associated with altered PIN-FORMED1 (PIN1) expression and localisation during the early stages of embryogenesis. These observations suggest that SEU and SLK2 promote the transition to bilateral symmetry by modulating auxin distribution in the embryonic shoot. This study also shows that loss of SAM formation in seu slk2 mutants is associated with reduced expression of the class I KNOX (KNOXI) genes SHOOTMERISTEMLESS (STM), BREVIPEDICELLUS and KNAT2. Furthermore, elevating STM expression in seu slk2 mutant embryos was sufficient to restore SAM formation but not post-embryonic activity, while both SAM formation and activity were rescued when SLK2 expression was restored in either the cotyledons or boundary regions. These results demonstrate that SEU and SLK2 function redundantly to promote embryonic shoot development and likely act through a non-cell autonomous pathway to promote KNOXI activity.

Developmental Analysis of Compound Leaf Development in Arachis hypogaea.

Leaves are the primary photosynthetic structures, while photosynthesis is the direct motivation of crop yield formation. As a legume plant, peanut (Arachis hypogaea) is one of the most economically essential crops as well as an important source of edible oil and protein. The leaves of A. hypogaea are in the tetrafoliate form, which is different from the trifoliate leaf pattern of Medicago truncatula, a model legume species. In A. hypogaea, an even-pinnate leaf with a pair of proximal and distal leaflets was developed; however, only a single terminal leaflet and a pair of lateral leaflets were formed in the odd-pinnate leaf in M. truncatula. In this study, the development of compound leaf in A. hypogaea was investigated. Transcriptomic profiles revealed that the common and unique differentially expressed genes were identified in a proximal leaflet and a distal leaflet, which provided a research route to understand the leaf development in A. hypogaea. Then, a naturally occurring mutant line with leaf developmental defects in A. hypogaea was obtained, which displayed a pentafoliate form with an extra terminal leaflet. The characterization of the mutant indicated that cytokinin and class I KNOTTED-LIKE HOMEOBOX were involved in the control of compound leaf pattern in A. hypogaea. These results expand our knowledge and provide insights into the molecular mechanism underlying the formation of different compound leaf patterns among species.