The spikelet is the specific unit of the grass inflorescence, and produces a variable number of florets (1–40) depending on whether the species displays determinate or indeterminate growth. Spikelet morphogenesis therefore plays an important role in crop yield; for example in wheat (Triticum aestivum) inflorescences with the indeterminate spikelet, the spikelet meristem is continuously active and produces a plethora of lateral floral meristems, leading to the formation of variable numbers of florets. In inflorescences of rice (Oryza sativa) and maize (Zea mays) with the determinate spikelet, the spikelet meristem produces the fixed numbers of lateral floral meristems, after which its fate is terminated as a terminal floral meristem, resulting in the formation of one or two florets. In rice, MFS1, SNB, EG1/DF1, TOB1 and OsMADS22 regulates the spikelet determinacy or indeterminacy (Lee et al., 2007; Ren et al., 2013). The loss of function of any of these genes induces the production of two complete florets or extra lemmas (secondary florets) within a single spikelet. In this study, we described a new mutant allele of FON4, named fon4‐7.

In the previous studies, the fon4 mutant produced increased inner floral organs and elongated sterile lemmas with the indeterminate identity (Chu et al., 2006), whereas we confirmed that the fon4‐7 mutant showed two florets and induced the reversion of sterile lemmas into lemmas (lateral florets). We discuss two possibilities for the formation of multi‐floret spikelet in rice, which may facilitate the breeding of rice cultivars with increased grain number per rice panicle.

Rice strictly produces a pair of sterile lemmas and one floret per spikelet, each containing one lemma and one palea, two lodicules adjacent to the lemma, six stamens and a pistil (Figure 1A1–A5).