Regulating Proxima-Distal and Adaxial-Abaxial Patterning during early Ovule Development.
Ovules are the major female reproductive organs of higher plants. Within the ovule the egg cell is produced and, upon fertilisation, the embryo develops during seed development. Arabidopsis ovules provide an excellent system to study pattern formation and aspects of organ polarity establishment in plants (Schneitz et al., 2001, Gasser et.al., 1998). Ovules develop from the placenta inside gynoecia as finger-like protrusions. Three proximal-distal elements, the funiculus, chalaza and nucellus, can be distinguished early on (Esau, 1977; Schneitz et al., 1995). Within the distally-located nucellus meiosis takes place in the megaspore mother cell. Only one of the four megaspores survives and develops into the multicellular, haploid female gametophyte, the embyro sac. One of the cells of the embryo sac is the egg cell proper. The chalaza is located beneath the nucellus and is characterised by the two integuments, the progenitors of the seed coat, inititating from its flanks. The funiculus is a stalk-like structure, which harbors the vascular strand and connects the ovule to the placenta.
After the primordium completed its proximal-distal extension the inner and outer integuments develop in a sequential fashion. First, the inner integument initiates as a collar around the chalaza. Second, the outer integument initiates but first on the gynbasal side of the primordium, which is the side towards the base of the gynoecium. Outer integument initiation at the chalaza is the first morphological evidence that adaxial-abaxial polarity establishment had has occurred and that the radially symmetrically primordium had has switched to bilateral development (Baker et al., 1997).
Some progress has recently been made towards the understanding of the molecular mechanism underlying the coordination of proximal-distal and adaxial-abaxial pattern formation. At least two genes, NOZZLE (NZZ) and INNER NO OUTER (INO), appear to be essential for this aspect of pattern formation. Before the integuments initiate, transcription of INO can be detected in a few cells on the abaxial side of the ovule. Developing ovules lacking INO function only form the inner integument and thus appear radialized (Balasubramanian and Schneitz, 2000; Balasubramanian and Schneitz, 2002; Meister et al., 2002; Villanueva et al., 1999). INO belongs to the YABBY family of putative plant-specific transcription factors (Villanueva et al., 1999). Additional members include FIL and YABBY3 (YAB3) (Bowman, 2000). YABBY genes are associated with the regulation of abaxial cell fate by a mechanism which presently is not understood (reviewed in Bowman et al., 2000; Bowman and Smyth, 1999; Eshed et al., 1999; Sawa et al., 1999; Siegfried et al., 1999; Villanueva et al., 1999). There is evidence coming from in vitro binding studies, that FIL protein dimers bind to DNA through their high mobility group (HMG)-like (YABBY) domains in a non-sequence-specific fashion (Kanaya et al., 2002).
We have recently identified the NZZ gene (Schiefthaler et al., 1999), also known as SPOROCYTELESS (SPL) (Yang et al., 1999). NZZ encodes a novel protein which is likely to be a putative transcriptional regulator. Extensive genetic evidence indicates that NZZ coordinates development of the proximal-distal and adaxial-abaxial axes to allow proximal-distal axis formation before the initiation of the adaxial-abaxial axis and outer integument development in the chalaza (Balasubramanian and Schneitz, 2000; Balasubramanian and Schneitz, 2002). NZZ appears to control INO expression in a temporal fashion via its negative regulation of the positive auto-regulatory loop of INO transcription (Balasubramanian and Schneitz, 2000; Balasubramanian and Schneitz, 2002; Meister et.al., 2002). Biochemical in vitro studies indicate that the NZZ protein can directly bind to INO, thereby inhibiting INO function and attenuating the INO feedback loop (Sieber et.al., 2004). We are presently continuing to elucidate the corresponding molecular mechanism.
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