L1Expressed WOXClass Homeobox Genes and Recruitment of Cells into Organs

Another class of mutants which may have some bearing on the genetic regulation of cell division planes in the L1 cell layer of meristems and organ primordia is that composed of the pressed flower (prs) and pretty few seeds2 (pfs2) mutants of Arabidopsis (Matsumoto and Okada 2001; Park et al. 2005) and the double narrow sheath1 (ns1)/narrow sheath2 (ns2) mutant of maize (Scanlon et al. 1996; Scanlon 2000). These mutants all affect the recruitment of cells into developing organs, and the cells affected are principally (but possibly not exclusively) epidermal. Mutants in PRS affect recruitment of cells into the lateral sepals during early flower primordium development, and later recruitment into the margins of the remaining adaxial and abaxial sepals (Matsumoto and Okada 2001). Additionally, prs mutants fail to recruit most of the cells which would form the stipules and other minor basal structures of the wild-type leaf (Nardmann et al. 2004). The ns1/ns2 mutant of maize, shows an analogous deletion of lateral regions of the basal leaf domain, leading to a marked reduction in the width of the leaf sheath. PRS, NS1 and NS2 all encode proteins of the WUSCHEL-related homeobox (WOX) class (Haecker et al. 2004) and are expressed in restricted region of the L1 in meristems where cells are to be recruited to the lateral domains of flower or organ pri-mordia, and in the marginal domains of young organ primordia (Nardmann et al. 2004). Ectopic expression of PRS leads to the ectopic outgrowth of epidermal bilayers on organ surfaces (Matsumoto and Okada 2001).

PFS, another WOX protein, may play a similar role to that of NS1/2 and PRS, but in recruiting L1 cells from the ovule chalaza into incipient integument primordia, a role that it appears to share with WUSCHEL itself (Mat-sumoto and Okada 2001; Gross-Hardt et al. 2002). WUS expression, like that of NS1/2, is largely restricted to the L1 cell layer of the developing ovule pri-mordium prior to integument initiation (J. Goodrich, unpublished results). Local ectopic expression of WUS in the zone of integument initiation under the control of the AINTEGUMENTA (ANT) promoter is reported to lead to the initiation of ectopic integuments, an effect reminiscent of that following the ectopic expression of PRS (Gross-Hardt et al. 2002).

Interestingly, clonal analysis of ns mutants indicated that NS function was required not in the L1, where NS1 and NS2 are expressed, but in a small population of underlying L2 cells. NS function was also found to be non-cell autonomous within the NS domain (that is deleted in ns mutants) (Scanlon 2000). It seems possible then, that the functions of WOX genes in allowing L1 cells to enter organ primordia may be complex and involve interactions between L1 and underlying cells. One possible explanation is that WOX function is somehow required either to block production or perception of an L2 derived signal, which normally prevents L1 cells from proliferating independently of L2 growth. This interpretation is consistent with the effects of ectopically expressing PRS and WUS. How this function interacts with that of other genes involved in organ initiation, such as ANT, remains to be clarified.

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