**4. Non MADS-Box Genes Involved In Floral Organ Identity**

Some non-MADS-box genes are also reported to regulate floral development. Mutants of rice *aberrant panicle organization1* (*apo1*) showed phenotypic resemblance with class C gene mutants. *APO1* mutants convert stamens into lodicules with extra carpels, suggesting that *APO1* positively regulates class C gene functions [121,122]. Moreover, expression of the rice class C gene (*OsMADS3*) was reduced in *apo1* mutants indicating that *APO1* positively regulates *OsMADS3* expression [169]. The Arabidopsis genes *UNUSUAL FLORAL ORGANS (UFO)* and *APO1* are orthologs and both encode F-box proteins. *UFO* activates class B genes [123], suggesting a distinct role for both genes irrespective of their similar biochemical functions. Arabidopsis *FLORICAULA* (*FLO*)/*LEAFY* (*LFY*) and its rice ortholog *RICE FLORICAULA*/*LEAFY* (*RFL*)/*APO2* displayed different yet overlapping functions. For example, *RFL*/*APO2* specifies inflorescence meristem identity through interaction with *APO1* [170], whereas *FLO*/*LFY* specifies floral meristem identity and activates class A, B, and C genes [171].

As described above, carpel identity is defined by class C MADS-box genes, however *YABBY* TFs are also reported to play a major role in carpel identity. The rice mutant *drooping leaf (dl)* has some functional similarity to class C MADS TFs in specifying the carpel identity and mutation in *DL* converts carpels into stamens [80]. These findings support the notion that candidate carpel identity genes in rice (class C and *DL*) redundantly regulate class C gene functions. *DL* and the class B gene *OsMADS16*/*SPW1* antagonize each other and this antagonism is critical to setting boundaries between stamen and carpel identity [80]. The Arabidopsis gene *CRABSCLAW (CRC)* and rice *DL* both encode *YABBY* TFs, although in addition to its function in carpel identity; *CRC* also has a role in nectary development [118]. The expression pattern of *CRC* in homeotic mutants suggests negative regulation by class-(A) and B genes. The wheat *DL* ortholog (*TaDL*), which was identified by homology screening [119] and expression in alloplasmic wheat, was found in true pistils as well as pistil-like stamens, suggesting its role in carpel specification. Moreover, class B genes were not expressed in pistil like stamens indicating that *TaDL* and class B genes are mutually antagonistic [77]. Like rice and wheat, the maize *DL* mutants (*drl1* and *drl2*) have been characterized and cloned. The *drl* mutants displayed ectopic inner-whorl organs in pistillate and staminate florets [120]. Although meristem activity was influenced by the expression of the *Drl* loci, *Drl* transcripts were absent in floral meristems suggesting that *Drl* genes may function autonomously. Sang et al. [125] characterised *CHIMERIC FLORAL ORGANS 1 (CFO1)*, a MADS-box gene, which regulates floral organ identity in rice. Mutants of *CFO1* showed disrupted marginal palea with ectopic but chimeric floral organs. Expression pattern analysis revealed that rice *DL* was ectopically expressed in defective floral organs of *cfo1* flowers, suggesting negative regulation between *CFO1* and *DL* [125].

More recently, Liu et al. [124] reported that *LONG STERILE LEMMA1 (G1)*/*ELONGATED EMPTY GLUME (ELE)* and *OsMADS34*/*PAP2* were associated with rice lemma development and determination of empty glume identity. Mutants of *G1*/*ELE* showed homeotic transformation of empty glumes into lemma like organs. Single and double mutants of *G1*/*ELE* and *OsMADS1*/*OsLHS1* showed redundant roles for both genes in controlling empty glume identity and lemma development. Expression analysis of *G1*/*ELE* in *osmads-1* flowers and *OsMADS1*/*LHS1* in *g-1* flowers indicated that both genes are regulated through independent pathways and do not interact at the transcription level. In *G1*/*ELE* mutant plants, downregulation of empty glume identity genes and ectopic expression of lemma identity genes provides strong evidence that empty glumes are in-fact sterile lemmas. Moreover, Yang et al. [126] identified a single recessive gene *lemma-distortion1 (ld1)* associated with lemma development in rice. This gene encodes a zinc finger protein. Overall these reports suggest that a plethora of non-MADS-box genes are also involved in floral organ identity in eudicots and cereals.
