Molecular Control of Carpel Development in the Grass Family

Abstract

Carpel is the ovule-bearing female reproductive organ of flowering plants and is required to ensure its protection, an efficient fertilization, and the development of diversified types of fruits, thereby it is a vital element of most food crops. The origin and morphological changes of the carpel are key to the evolution and adaption of angiosperms. Progresses have been made in elucidating the developmental mechanisms of carpel establishment in the model eudicot plant Arabidopsis thaliana, while little and fragmentary information is known in grasses, a family that includes many important crops such as rice (Oryza sativa), maize (Zea mays), barley (Hordeum vulgare), and wheat (Triticum aestivum). Here, we highlight recent advances in understanding the mechanisms underlying potential pathways of carpel development in grasses, including carpel identity determination, morphogenesis, and floral meristem determinacy. The known role of transcription factors, hormones, and miRNAs during grass carpel formation is summarized and compared with the extensively studied eudicot model plant Arabidopsis. The genetic and molecular aspects of carpel development that are conserved or diverged between grasses and eudicots are therefore discussed.

Keywords: carpel; carpel identity; grass; meristem determinacy; miRNA; plant hormones.

FIGURE 1

Schematic draws of Arabidopsis, rice, and maize florets and gynoecia. (A–C) Flower and spikelet structure of Arabidopsis (A), rice (B), and maize (C). (D) The pistil of Arabidopsis can be divided into ovary, style, stigma, and the ovules derive from a placenta (E) Rice syncarpic ovary harbors two styles and feathery stigmas, (F) while maize has a higher extent of fusion into one silk. Like in all grasses, only one ovule is developed in the pistil of rice and maize.

FIGURE 2

Carpel specification in dicot and monocot model plants Arabidopsis and rice. The expression domains of C-, D-, and E-class genes; SUP; and CRC, DL in carpel and ovule of Arabidopsis (A) and rice (B) are shown with blue, brown, purple, yellow, and red colors, respectively. Genetic interactions within these carpel identity regulators and other homeotic genes are also indicated. Direct regulations have been shown in Arabidopsis, whereas in rice a direct binding of OsMADS6 has only been shown to the second intron of OsMADS58 (Li et al., 2011a).

FIGURE 3

Proposed model to illustrate the genetic interaction between rice floral homeotic genes in FMD in whorl 4. OsMADS3 and OsMADS58 are possibly activated by E-function genes OsMADS1 and OsMADS6 and also APO1, the expression of DL may be repressed by OsMADS6 and OsMADS13. In turn, DL may indirectly regulate OsMADS13 in a positive way. The onset of OsMADS13 expression in the final stage of the FM leads its transition to ovule primordium.