Molecular and genetic analyses of the silky1 gene reveal conservation in floral organ specification between eudicots and monocots.

The degree to which the eudicot-based ABC model of flower organ identity applies to the other major subclass of angrosperms, the monocots, has yet to be fully explored. We cloned silky1 (si1), a male sterile mutant of Zea mays that has homeotic conversions of stamens into carpels and lodicules into palea/lemma-like structures. Our studies indicate that si1 is a monocot B function MADS box gene. Moreover, the si1 zag1 double mutant produces a striking spikelet phenotype where normal glumes enclose reiterated palea/lemma-like organs. These studies indicate that B function gene activity is conserved among monocots as well as eudicots. In addition, they provide compelling developmental evidence for recognizing lodicules as modified petals and, possibly, palea and lemma as modified sepals.

The blooming of grass flower development.

The past half decade has provided a wealth of information concerning the molecular and genetic control of floral organ and meristem identity in dicotyledonous plants. Comparatively little is understood about these processes in grass species in spite of the importance that these species play in human agriculture. The isolation of grass genes that are homologous to dicot floral homeotic genes in combination with recent advances in reverse genetic technology and improvements in cereal transformation opens the door for understanding molecular mechanisms of grass flower development. Such information will also focus attention on the evolutionary relationships between grass and dicot flowers and the degree to which the developmental pathways leading to reproductive organ development in divergent angiosperms have utilized conserved mechanisms.

Diversification of C-function activity in maize flower development.

The Arabidopsis gene AGAMOUS is required for male and female reproductive organ development and for floral determinacy. Reverse genetics allowed the isolation of a transposon-induced mutation in ZAG1, the maize homolog of AGAMOUS. ZAG1 mutants exhibited a loss of determinacy, but the identity of reproductive organs was largely unaffected. This suggested a redundancy in maize sex organ specification that led to the identification and cloning of a second AGAMOUS homolog, ZMM2, that has a pattern of expression distinct from that of ZAG1. C-function organ identity in maize (as defined by the A, B, C model of floral organ development) may therefore be orchestrated by two closely related genes, ZAG1 and ZMM2, with overlapping but nonidentical activities.