In higher plants the female germline is formed from the megaspore mother cell (MMC), a single cell in the pre-meiotic ovule. Previously, it was reported that mutants in RNA dependent DNA methylation - RdDM - pathway might be involved in restricting female germline to a single nucellus cell. We show that the DRM methyltransferase double mutant drm1drm2 also presents also presents ectopic enlarged cells, consistent with supernumerary MMC-like cells. In wild-type ovules MMC differentiation required SPOROCYTELESS/NOZZLE (SPL/NZZ), as demonstrated by the spl/nzz mutant failing to develop an MMC. We address the poorly understood upstream regulation of SPL/NZZ in ovules, showing that the RdDM pathway is important to restrict SPL/NZZ expression. In ago9, rdr6 and drm1drm2 mutants, SPL/NZZ is ectopically expressed, suggesting that the mutiple MMC-like cells observed might be due to the ectopic expression of SPL/NZZ. We show that the ovule identity gene SEEDSTICK, directly regulates the AGO9 and RDR6 expression in the ovule and therefore indirectly SPL/NZZ expression. A model is presented describing the network required to restrict SPL/NZZ expression to specify a single MMC.

Grain production in cereal crops depends on the stable formation of male and female gametes in the flower. In most angiosperms, the female gamete is produced from a germline located deep within the ovary, protected by several layers of maternal tissue, including the ovary wall, ovule integuments and nucellus. In the field, germline formation and floret fertility are major determinants of yield potential, contributing to traits such as seed number, weight and size. As such, stimuli affecting the timing and duration of reproductive phases, as well as the viability, size and number of cells within reproductive organs can significantly impact yield. One key stimulant is the phytohormone auxin, which influences growth and morphogenesis of female tissues during gynoecium development, gametophyte formation, and endosperm cellularization. In this review we consider the role of the auxin signaling pathway during ovule and seed development, first in the context of Arabidopsis and then in the cereals. We summarize the gene families involved and highlight distinct expression patterns that suggest a range of roles in reproductive cell specification and fate. This is discussed in terms of seed production and how targeted modification of different tissues might facilitate improvements.

Maintaining and enhancing grain production in cereal crops is a key priority for global research efforts. The formation of floral organs impacts the number and quality of grain produced, and is an important component of cereal yield. The grain is derived predominantly from the ovule, a multifunctional tissue located in the ovary of the flower that specifies and nurtures the female germline, produces a female gametophyte, and supports embryo and endosperm development after fertilisation. Grain cannot form without successful production and fertilisation of the female gametophyte, and the stages of floral development encompassing gametophyte formation are particularly sensitive to environmental fluctuations. A deeper fundamental understanding of female reproductive development from a tissue‐ and cell‐type‐specific perspective may provide opportunities to sustain and increase grain yields. In this article, we consider flower and ovule development, with a particular focus on pre‐fertilisation stages in cereals and their role in stress tolerance and downstream grain formation.

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