INNER NO OUTER regulates abaxial- adaxial patterning in Arabidopsis ovules

Abstract

Consisting of only four morphological parts, the Arabidopsis ovule is a relatively simple structure that lends itself to the study of genetic regulation of pattern formation and organogenesis in plants (for review, see Gasser and Robinson-Beers 1993;Reiser and Fischer 1993;Angenent and Colombo 1996;Gasser et al. 1998;Schneitz et al. 1998b). Although initiating as a radially symmetrical primordium, the developing ovule subsequently exhibits differences between the side toward the apex of the carpel (adaxial) and the side toward the base of the carpel (abaxial)-resulting in a bilaterally symmetrical structure. Asymmetric development is most apparent in the outer integument (one of two sheathing structures) and in the funiculus (supporting stalk). The outer integument grows extensively only on the abaxial side of the ovule, and the funiculus curves in the abaxial direction. Two genetic loci involved in asymmetric development of the outer integument have been described. In addition to effects on flower development (Schultz et al. 1991;Bowman et al. 1992), the SUPERMAN (SUP) gene is essential for suppressing adaxial growth of the outer integument (Gaiser et al. 1995), and mutations in this gene lead to nearly equal outer integument growth on both sides of the ovule. In contrast, inner no outer (ino) mutations can lead to an absence of outer integument growth on both sides of the ovule primordium, implicating INO as a positive regulator of integument growth or a determinant of polarity (Gaiser et al. 1995;Baker et al. 1997;Schneitz et al. 1997).In angiosperms, bilateral symmetry is also a common characteristic of leaves, floral organs, and often whole flowers. In these structures, bilateral symmetry results from significant differences in development between the adaxial (toward the shoot apex) and abaxial (away from the shoot apex) sides. Recently, several genes involved in establishing these abaxial-adaxial patterns have been identified. The cycloidea (cyc) and dichotoma (dich) genes of Antirrhinum majus are expressed on the adaxial sides of flowers, where they specify adaxial floral development (Luo et al. 1996). In contrast, phantastica (phan), which appears to be essential for identity of the adaxial leaf surface in this same species, is expressed throughout the leaf (Waites et al. 1998), and must, therefore, require additional asymmetrically distributed factors for its activity.In Arabidopsis thaliana, vegetative structures of the phabulosa-1d mutant are radially symmetrical, apparently as a result of adaxialization, and PHABULOSA may be one determinant of abaxial cell fate in this species (McConnell and Barton 1998). A recently described family of Arabidopsis genes, the YABBY genes, encoding putative transcription factors (Bowman and Smyth 1999;Kumaran et al. 1999;Sawa et al. 1999;Siegfried et al. 1999), participate in determination of abaxial identity in a variety of organs.

Regulation of Ovule Development

“…This and the expression of INO in the outer layer of the outer integument suggests that INO not only participates in establishing the polarity of the ovule primordium but also appears to provide abaxial identity in the outer integument (Villanueva et al, 1999;Meister et al, 2002). The requirement of INO for outer integument growth suggests that an organ boundary resulting from the juxtaposition of adaxial and abaxial domains, such as that required for leaf blade outgrowth, is required for outer integument growth (Waites and Hudson, 1995;Siegfried et al, 1999).…”

“…INO expression initiates only toward the gynobasal side of the ovule, where the outer integument initiates and exhibits greatest growth ( Figures 1E to 1G). Throughout ovule development, this pattern of expression is maintained (Villanueva et al, 1999). The maintenance of the expression pattern is dependent on the putative zinc finger transcription factor SUPERMAN (SUP) (Meister et al, 2002).…”

“…Expression is only on the gynobasal side of the ovule, in the proximal half of the region of cells that will give rise to the outer integument ( Figure 1E) (Villanueva et al, 1999;Meister et al, 2002). The sequential initiation of integument growth, with the inner integument emerging first, may indicate that the initiation of outer integument growth, and hence the position of the initial expression of INO, may be dependent on the position of the inner integument.…”

“…Thus, ANT and INO may constitute an autoregulatory loop in which each reinforces the expression of the other ( Figure 5). INO has been shown to positively affect its own expression; both INO transcript accumulation and a P-INO:GUS reporter gene showed reduced expression in an ino mutant background (Villanueva et al, 1999;Meister et al, 2002). The autoregulation of INO may be direct, or the autoactivation of INO could be a secondary effect of the INO activation of ANT expression.…”

“…Strong ino mutants completely lack an outer integument but otherwise are phenotypically wild type ( Figure 4G) (Gaiser et al, 1995;Baker et al, 1997;Schneitz et al, 1997). Therefore, INO is required specifically for outer integument formation (Bowman and Smyth, 1999;Villanueva et al, 1999).…”