Flowers are the most complex structures of plants. Studies of Arabidopsis thaliana, which has typical eudicot flowers, have been fundamental in advancing the structural and molecular understanding of flower development. The main processes and stages of Arabidopsis flower development are summarized to provide a framework in which to interpret the detailed molecular genetic studies of genes assigned functions during flower development and is extended to recent genomics studies uncovering the key regulatory modules involved. Computational models have been used to study the concerted action and dynamics of the gene regulatory module that underlies patterning of the Arabidopsis inflorescence meristem and specification of the primordial cell types during early stages of flower development. This includes the gene combinations that specify sepal, petal, stamen and carpel identity, and genes that interact with them. As a dynamic gene regulatory network this module has been shown to converge to stable multigenic profiles that depend upon the overall network topology and are thus robust, which can explain the canalization of flower organ determination and the overall conservation of the basic flower plan among eudicots. Comparative and evolutionary approaches derived from Arabidopsis studies pave the way to studying the molecular basis of diverse floral morphologies.

Heavy metal hyperaccumulation in plants is an intriguing and poorly understood phenomenon. Transmembrane metal transporters are assumed to play a key role in this process. We describe the cloning and isolation of three zinc transporter cDNAs from the Zn hyperaccumulator Thlaspi caerulescens. The ZTP1 gene is highly similar to the Arabidopsis ZAT gene. Of the other two, one is most probably an allele of the recently cloned ZNT1 gene from T. caerulescens (Pence et al; Proceedings of the National Academy of Science USA 97, 4956–4960, 2000). The second, called ZNT2, is a close homologue of ZNT1. All three zinc transporter genes show increased expression in T. caerulescens compared with the non‐hyperaccumulator congener T. arvense, suggesting an important role in heavy metal hyperaccumulation. ZNT1 and ZNT2 are predominantly expressed in roots and ZTP1 is mainly expressed in leaves but also in roots. In T. arvense, ZNT1 and ZNT2 are exclusively expressed under conditions of Zn deficiency. Their expression in T. caerulescens is barely Zn‐responsive, suggesting that Zn hyperaccumulation might rely on a decreased Zn‐induced transcriptional downregulation of these genes. ZTP1 expression was higher in plants from calamine soil than in plants from serpentine or normal soil. The calamine plants were also the most Zn tolerant, suggesting that high ZTP1 expression might contribute to Zn tolerance.

Angiosperms form the largest group of land plants and display an astonishing diversity of floral structures. The development of flowers greatly contributed to the evolutionary success of the angiosperms as they guarantee efficient reproduction with the help of either biotic or abiotic vectors. The female reproductive part of the flower is the gynoecium (also called pistil). Ovules arise from meristematic tissue within the gynoecium. Upon fertilization, these ovules develop into seeds while the gynoecium turns into a fruit. Gene regulatory networks involving transcription factors and hormonal communication regulate ovule primordium initiation, spacing on the placenta, and development. Ovule number and gynoecium size are usually correlated and several genetic factors that impact these traits have been identified. Understanding and fine-tuning the gene regulatory networks influencing ovule number and pistil length open up strategies for crop yield improvement, which is pivotal in light of a rapidly growing world population. In this review, we present an overview of the current knowledge of the genes and hormones involved in determining ovule number and gynoecium size. We propose a model for the gene regulatory network that guides the developmental processes that determine seed yield.

Cell wall modifications are of pivotal importance during plant development. Among cell wall components, xyloglucans are the major hemicellulose polysaccharide in primary cell walls of dicots and non graminaceous monocots. They can connect the cellulose microfibril surface to affect cell wall mechanical properties. Changes in xyloglucan structure are known to play an important role regulating cell growth. Therefore, the degradation of xyloglucan is an important modification that alters the cell wall. The α-XYLOSIDASE1 (XYL1) gene encodes the only α-xylosidase acting on xyloglucans in Arabidopsis thaliana. Here, we show that mutation of XYL1 strongly influences seed size, seed germination, and fruit elongation. We found that the expression of XYL1 is directly regulated in developing seeds and fruit by the MADS-box transcription factor SEEDSTICK (STK). We demonstrate that XYL1 complements the stk smaller seed phenotype. Finally, by Atomic Force Microscopy (AFM), we investigate the role of XYL1 activity in maintaining cell stiffness and growth, confirming the importance of cell wall modulation in shaping organs.

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