Vol.:(0123456789) 1 3 Plant Reproduction https://doi.org/10.1007/s00497-018-0345-2 REVIEW Seed coats as an alternative molecular factory: thinking outside the box Edith Francoz 1  · Loïc Lepiniec 1  · Helen M. North 1 Received: 10 May 2018 / Accepted: 13 July 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Key message Seed coats as commodities. Abstract Seed coats play important roles in the protection of the embryo from biological attack and physical damage by the environment as well as dispersion strategies. A signifcant part of the energy devoted by the mother plant to seed production is channeled into the production of the cell layers and metabolites that surround the embryo. Nevertheless, in crop species these are often discarded post-harvest and are a wasted resource that could be processed to yield co-products. The production of novel compounds from existing metabolites is also a possibility. A number of macromolecules are already accumulated in these maternal layers that could be exploited in industrial applications either directly or via green chemistry, notably favonoids, lignin, lignan, polysaccharides, lipid polyesters and waxes. Here, we summarize our knowledge of the in planta biosynthesis pathways of these macromolecules and their molecular regulation as well as potential applications. We also outline recent work aimed at providing further tools for increasing yields of existing molecules or the development of novel biotech approaches, as well as trial studies aimed at exploiting this underused resource. Keywords Biotechnology · Pericarp · Polysaccharides · Polyphenols · Seed coat · Waxes Introduction The improvement of seed yield tends to focus on the accu- mulation of reserves in the tissues that develop directly from the double fertilization: endosperm and embryo. Neverthe- less, the maternal tissues surrounding the embryo constitute a non-negligible part of the seed that can be separated from other tissues and, with the exception of cotton, represent an untapped resource for increasing yield or the production of valuable co-products. The seed coat originates from the maternal integument layers that form the ovule (Fig. 1; Coen and Magnani 2018). In certain species, including major crops such as wheat, maize and rice, the dry seed remains associated after har- vest or dissemination with pericarp from the ovaries (Fig. 1). This is termed an achene, except when the pericarp is fused to the seed coat, where it is referred to as a caryopsis or grain. This pericarp can also undergo developmental pro- grams to generate elaborate structures such as wings for dispersal or hard nutshells for protection. In crop cereals, these maternal tissues represent a by-product that can be exploited in animal feed or as a food additive for dietary fber, antioxidants and nutraceuticals (Sharif et al. 2014; Abdel-Aal et al. 2018; Smuda et al. 2018). Surprisingly little targeted improvement has, however, been performed consid- ering the nutritional and health benefts associated with their consumption. In this review the external maternal tissues that remain associated with the mature dry embryo will be considered as potential biotechnological targets. For reasons of space, however, the species considered will be limited to the genetic models Arabidopsis thaliana (Arabidopsis) and Medicago truncatula with reference to equivalent studies in the crops wheat, rice, barley, oilseed rape, camelina, soy- bean, fax and maize where relevant. As the maternal layers surrounding the embryo undergo programmed cell death prior to seed desiccation and disper- sal, the production and accumulation of desired molecules take place during seed development on the mother plant (Sano et al. 2016). Nevertheless, these must be reconciled A contribution to the special issue ‘Seed Biology’. Communicated by L. Lepiniec, H. North, G. Ingram. * Helen M. North helen.north@inra.fr 1 Institut Jean-Pierre Bourgin, INRA, AgroParisTech, CNRS, Université Paris-Saclay, 78000 Versailles, France