Abstract
Dry fruits consist of two types, dehiscent and indehiscent, whereby the fruit is splitting open or remains closed at maturity, respectively. The seed, the dispersal unit (DU) of dehiscent fruits, is composed of three major parts, the embryo and the food reserve, encapsulated by the maternally-derived organ, the seed coat. Indehiscent fruit constitutes the DU in which the embryo is covered by two protective layers (PLs), the seed coat and the fruit coat. In grasses, the caryopsis, a one-seeded fruit, can be further enclosed by the floral bracts to generate two types of DUs, florets and spikelets. All protective layers enclosing the embryo undergo programmed cell death (PCD) at maturation and are thought to provide mainly a physical shield for embryo protection and a means for dispersal. In this review article, I wish to highlight the elaborate function of these dead organs enclosing the embryo as unique storage structures for beneficial substances and discuss their potential role in seed biology and ecology.
Highlights
The seed is the fundamental unit of dispersal in higher plants where the embryo is encapsulated by maternally-derived dead coverings until the seed germinates and resumes growth
Besides providing a physical shield for embryo protection and a means for dispersal, DOEEs function as storage structures for nutrition, proteins, and other regulatory substances (Figure 2) that act together as an organized unit to ensure the survival of the embryo during seed storage in the soil, and its proper growth and establishment in the habitat
Stress conditions that are expected to increase in severity, over large areas of the world, due to climate change modify the embryo properties and the properties of the DOEEs, which in turn impact seed performance and fate, soil properties, and plant population dynamics and diversity [78,87,88]
Summary
The seed is the fundamental unit of dispersal in higher plants where the embryo is encapsulated by maternally-derived dead coverings until the seed germinates and resumes growth. The seed is the DU of dry dehiscent fruits (e.g., Arabidopsis thaliana), whereby the embryo is covered by a single layer, the seed coat (Figure 1A). The fruit represents the DU of dry indehiscent fruit (e.g., Medicago spp.) and the embryo is covered by two layers, the seed coat and the fruit coat (the pericarp and its accessories) (Figure 1B). The seed is essentially composed of an embryo, a food reserve (endosperm, cotyledons, hypocotyl, perisperm) and various numbers of dead layers (seed coat, pericarps, floral bracts) commonly thought to provide a physical shield for embryo protection and a means for dispersal [2,3]. I will elaborate on commonly-ignored, maternally-derived organs, namely DOEEs, as integral parts of the dispersal unit (DU) and highlight their function as a unique storage entity providing nutrition as well as regulatory substances that could affect seed biology and ecology
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