Lessons from the vegetative shoot apex - Intercellular Communication in Plants - page 161

expression of AS1 , the signals responsible for the patterned expression of these genes
have yet to be identified. Recent work has suggested that auxin might be involved
in this process (see Section 6.7).
An intriguing feature of the KN1 protein is its ability to move between cells of the
shoot apex (Jackson et al. , 1994; Kim et al. , 2002). Similarly, STM and BP/KNAT1
proteins also move between cell layers of the Arabidopsis apex, suggesting that
this is a general property of KNOX proteins and is important for their function
(Kim et al. , 2003). Further analysis of KNOX movement showed that these proteins
form steep concentration gradients over several cell layers (Kim et al. , 2003). These
transcription factors might therefore be involved in short-range signalling in specific
regions of the meristem. The presence of developmentally regulated symplastic
networks within the meristem lends weight to this hypothesis (Rinne & van der
Schoot, 1998; Gisel et al. , 1999). Interestingly, movement is not limited to KNOX
proteins as RNA of the tomato KN1 orthologue, LeT6 ,was recently shown to move
over much larger distances through the phloem (Kim et al. , 2001). However, the
relevance of this movement for KNOX function has yet to be resolved.

6.6 Interactions between KNOX genes and hormones regulate

meristem activity

Classic studies have shown that phytohormones play an important role in the meris-
tem. For instance, a high cytokinin-to-auxin ratio is required to generate shoot meris-
tems from callus. When levels of endogenous cytokinin are reduced, plants display
a number of defects associated with an aberrant meristem, including a reduction
in meristem size (Werner et al. , 2001). The formation of ectopic shoots on leaves
with elevated cytokinins levels is consistent with this hormone-promoting meristem
activity (Estruch et al. , 1991). Interestingly, ectopic KNOX gene expression was also
detected in leaves with elevated cytokinins levels, suggesting that one function of
cytokinins is to promote KNOX gene expression (Rupp et al. , 1999). However, other
work points to a different relationship, with ectopic KNOX gene expression causing
an accumulation of cytokinins (Tamaoki et al. , 1997; Ori et al. , 1999; Frugis et al. ,
2001). Taken together, these results show that cytokinins and KNOX genes promote
each other's accumulation within the meristem (see Fig. 6.3).
KNOX genes also function to repress biosynthesis of gibberellin (GA), a hor-
mone associated with cell differentiation. Misexpression of the tobacco KNOX gene
NTH15 in leaves results in direct repression Ntc12 ,akey GA biosynthetic gene
(Sakamoto et al. , 2001). Thus, KNOX genes may prevent GA accumulation in the
meristem by repressing genes involved in GA biosynthesis. Several lines of ev-
idence support this model. Firstly, expression of Ntc12 and NTH15 is mutually
exclusive, with NTC12 transcript accumulating in organs and NTH15 in the meris-
tem (Tamaoki et al. , 1997; Tanaka-Ueguchi et al. , 1998; Sakamoto et al. , 2001).
And secondly, exogenous application of GA or increasing the level of GA sig-
nalling is sufficient to overcome the affects of KNOX gene misexpression in leaves