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SHATTERPROOF1 (SHP1) and SHP2, previously called AGL1 and AGL5 respectively, are closely related MADS-box genes that are members of a monophyletic clade that also includes AGAMOUS and AGL11.  SHP1 and SHP2 are both expressed in similar patterns during carpel and fruit development.

SHP1 sequence

SHP1 Map position

Although shp1  and shp2 single mutants cannot be distinguished from the wild type, shp1 shp2 double mutants fail to disperse their seeds normally.  Specifically, shp1 shp2 double mutants fail to differentiate a dehiscence zone at the valve/replum boundary where cell separation normally occurs late in fruit development to allow for seed dispersal.  Thus, the SHATTERPROOF genes are functionally redundant and are required for dehiscence zone differentiation during fruit development. SHP2 sequence

SHP2 Map position

Differentiation of the dehiscence zone at the valve/replum boundary in wild-type fruit (left) leads to valve separation from the replum and the dispersal of seeds.  In shp1 shp2 double mutants (right), valves to not separate from the replum because the dehiscence zone fails to differentiate.
Selected References:

Flanagan, C.A., Hu, Y., and Ma, H. (1996) Specific expression of AGL1 MADS-box gene suggests regulatory functions in Arabidopsis gynoecium and ovule development. Plant J. 10:343-353.

Kempin, S.A, Liljegren, S.J., Block, L.M, Rounsley, S.D., Lam, E., and Yanofsky, M.F.(1997) Targeted disruption in Arabidopsis. Nature 389:802.

Sarah Liljegren, Gary  Ditta, Yuval Eshed, Beth Savidge, John Bowman and Martin Yanofsky (2000) SHATTERPROOF MADS-box genes control seed dispersal in Arabidopsis. Nature 404:766-770

Ma, H., Yanofsky, M.F., and Meyerowitz, E. M. (1991). AGL1-AGL6, an Arabidopsis gene family with similarity to floral homeotic and transcription factor genes. Genes Dev. 5, 484495.

Savidge, B., Rounsley, S.D., and Yanofsky, M.F. (1995) Temporal relationship between the transcription of two Arabidopsis MADS-box genes and the organ identity genes. Plant Cell 7:721-733