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The plant ovule is a fundamentally important organ that is the direct progenitor of the seed. It is one of the last structures to form in the flower and contains relatively few tissues, but undergoes complex developmental transitions that are essential for reproduction. Ovule number and flower fertility are important factors influencing yield, yet studies have identified challenges in trying to increase one without compromising the other. Recent findings in Arabidopsis and cereal crops highlight regulatory pathways that contribute to this yield constraint. Here, we consider the basis for variation in ovule number and development, with a particular focus on hormones and transcriptional regulators that constitute promising targets for the optimisation of reproductive traits and yield.

Background and AimsIs there selection minimizing the costs of ovule production? Such selection should lead to a smaller ovule size in relation to seed size and, at the same time, smaller variation in ovule size within plants, the latter because the minimum structures and resources for functioning of ovules should be the same among ovules. Additionally, within species, ovule size should not depend on the plant’s resource status.MethodsTo confirm these predictions, we examined ovule and seed production for a variety of species.Key ResultsAmong the 27 species studied, we found a significant negative dependence of the species mean of the coefficient of variation for plant ovule size on the ratio of the mean species seed size/mean species ovule size. Thus, the smaller the ovule size as compared with seed size, the smaller the degree of variation in ovule size. Among the 49 species studied, only two species showed significant positive dependence of mean ovule size on plant size. Although larger plants should have greater resources for ovule production, selection has not enhanced the production of large ovules in most species.ConclusionsThese results suggest that there is selection minimizing the costs of ovule production.