Of a MAPK Signaling Pathway Regulating Cell Differentiation

The roles of MAPKKKs other than NPK1 in cell differentiation are becoming established (Table 1). The yoda mutants show distinctive changes in the pattern of cell division during embryogenesis, and the responsible gene encodes a MAPKKK. (Lukowitz et al. 2004). The cell division patterns of Arabidopsis embryos are well documented and stereotyped (J├╝rgens and Mayer 1994). After fertilization, a zygote elongates longitudinally and divides asymmetrically to produce apical and basal cells. The zygotes of yoda mutants are defective in this longitudinal elongation and divide into two cells of nearly the same sizes (Lukowitz et al. 2004). Furthermore, some basal cells of yoda mutants show longitudinal division planes, which are never observed in the wild-type Arabidopsis embryo. These defects in the cell division pattern are related to the lack in most yoda mutants of a suspensor, which is derived from basal cells and is not incorporated in the embryo, suggesting the importance of YODA in determining the fate of extra-embryonic cells.

The yoda mutation also affects the development of stomata. Since stomata are required for gas exchange and transpiration, which are essential for the survival of land plants, the number and distribution of stomata are regulated genetically. In the yoda mutant, the number of stomata increases and their distribution is affected (Bergmann et al. 2004). Although stomata are separated from each other by more than one pavement cell in wild-type plants,

Table 1 MAP kinase pathways in plant development

MAPKKK MAPKK MAPK

Table 1 MAP kinase pathways in plant development

MAPKKK MAPKK MAPK

Zygote development

YODA1

Stomatal development

YODA2

MKK4/53

MPK3/63

Root hair elongation

SIMKK6

SIMK4

Ethylene signaling

CTR15

SIMK, MKK36

Auxin signaling

NPK17

MKK78

1 Lukowitz et al. 2004

2 Bergmann et al. 2004

3 Wang et al. 2007

4 Samaj et al. 2002

5 Kieber et al. 1993

6 Ouaked et al. 2003

7 Kovtun et al. 1998

8 Dai et al. 2006

1 Lukowitz et al. 2004

2 Bergmann et al. 2004

3 Wang et al. 2007

4 Samaj et al. 2002

5 Kieber et al. 1993

6 Ouaked et al. 2003

7 Kovtun et al. 1998

8 Dai et al. 2006

clustered stomata are observed in yoda mutants. These phenotypes are explained by the loss of proper differentiation of the meristemoids into guard mother cells. Considering the abnormal cell divisions of zygotes of the yoda mutant, YODA may be an essential component of asymmetrical cell division in plant development.

In the context of stomatal development, two MAPKs and MAPKKs downstream of YODA have been identified recently. Mutations in both Arabidopsis MAPKs, namely MPK3 and MPK6, result in numerous stomata and no pavement cells in the epidermis of the cotyledons (Wang et al. 2007). Repression of both MKK4 and MKK5 MAPKKs also increases the number of stomata, as in mpk3/mpk6 double mutants (Wang et al. 2007). These results demonstrate that the activation of the MAPKK upstream of MPK3 and MPK6 inhibits stomata development and represses the phenotypes caused by the yoda mutation. Thus, the MAP kinase pathway, consisting of YODA, MKK4/5 and MPK3/6, regulates the differentiation of stomata. Since clustered stomata are also found in cytokinetic mutants (Nishihama et al. 2001, 2002; Soyano et al. 2003), it might be intriguing to examine whether there is a direct relationship between the components responsible for cytokinesis and asymmetrical cell division.

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