Figure 6.

BEL1 and SPL Integrate Hormonal Signaling in Ovules.

(A) In wild-type ovules, cytokinin activates WUS, which promotes the expression of SPL in the nucellus (nu) (Sieber et al., 2004) and BEL1 in the chalaza (chal). PIN1 is activated in the nucellus by SPL and repressed in the chalaza by BEL1, which in turn represses WUS (Brambilla et al., 2007). f, funiculus.

(B) In spl mutant ovules, PIN1 is not expressed, leading to a premature block of female gametophyte development and phenocopies the pin1-5 mutant.

(C) In the bel1 mutant, PIN1 is upregulated and is also expressed in the chalaza region, where normally it is not present.

(D) In finger-like bel1 spl double mutant ovules, PIN1 is not expressed in the ovule.

(E) The application of exogenous NPA to the bel1 mutant triggers the formation of finger-like ovules, because the inhibition of the auxin flux by NPA treatment avoids the formation of the aberrant structures typical for bel1 ovules.