Class II tassel seed mutations provide evidence for multiple types of inflorescence meristems in maize (Poaceae)

Abstract

The tassel seed mutations ts4 and Ts6 of maize cause irregular branching in its inflorescences, tassels, and ears, in addition to feminization of the tassel due to the failure to abort pistils. A comparison of the development of mutant and wild-type tassels and ears using scanning electron microscopy reveals that at least four reproductive meristem types can be identified in maize: the inflorescence meristem, the spikelet pair meristem, the spikelet meristem, and the floret meristem. ts4 and Ts6 mutations affect the fate of specific reproductive meristems in both tassels and ears. ts4 mutants fail to form spikelet meristems from spikelet pair meristems. Ts6 mutants are delayed in the conversion of certain spikelet meristems into floret meristems. Once floret meristems are established in both of these mutants, they form florets that appear normal but fail to undergo pistil abortion in the tassel. The abnormal branching associated with each mutant is suppressed at the base of ears, permitting the formation of normal, fertile spikelets. The classification of the different types of reproductive meristems will be useful in interpretation of gene expression patterns in maize. It also provides a framework for understanding meristem functions that can be varied to diversify inflorescence architectures in the Gramineae.

Experimental Analysis of Tassel Development in the Maize Mutant Tassel Seed 6

“…In addition, patterns of expression of the maize gene Knotted (Kn), which is expressed in meristems but not in their determinate lateral derivatives (Jackson et al, 1994), was examined in Ts6 mutant tassels using in situ hybridization. Kn expression patterns are consistent with the model of inflorescence development (Irish, 1997) in which Ts6 inflorescences produce extra florets by a delay in determinacy of spikelet meristems.…”

“…Each of these meristems undergoes a precisely regulated pattern of organ initiation (Bennett, 1948;Cheng et al, 1982;Veit et al, 1993;Irish, 1997) that results in the formation of four florets, arranged in pairs of two-flowered spikelets, in the following manner. The meristems that are initiated on the developing inflorescence, called spikelet pair meristems, develop into two spikelet meristems, except for those at the base of tassels, which develop into branches.…”

“…In wild-type maize, each spikelet meristem gives rise to only two floret meristems (Fig. 4A), whereas in Ts6 inflorescences, determinacy of spikelet meristems appears to be delayed (Irish, 1997), resulting in the formation of multiple floret meristems in each spikelet (Fig. 4B).…”

“…An SEM study comparing the development of tassels and ears from both mutants to wild type has suggested a model (Fig. 1) for the developmental events that underlie inflorescence development in normal maize (Irish, 1997). This model proposes that each primordium initiated on a developing inflorescence progresses through a sequence of meristem states: spikelet pair meristem, then spikelet meristem, then floret meristem.…”

“…According to this model, spikelet pair meristems of ts4 inflorescences remain indeterminate and their derivatives do not progress to the next meristem state, but instead exhibit spikelet pair activity, also. This results in branched branches that form along the length of ts4 tassels instead of only at the base (Irish, 1997). Ts6 is delayed in spikelet meristem determinacy or may accelerate the initiation of floral meristems.…”