Abstract
Plants have evolved a variety of dispersal units whereby the embryo is enclosed by various dead protective layers derived from maternal organs of the reproductive system including seed coats (integuments), pericarps (ovary wall, e.g., indehiscent dry fruits) as well as floral bracts (e.g., glumes) in grasses. Commonly, dead organs enclosing embryos (DOEEs) are assumed to provide a physical shield for embryo protection and means for dispersal in the ecosystem. In this review article, we highlight recent studies showing that DOEEs of various species across families also have the capability for long-term storage of various substances including active proteins (hydrolases and ROS detoxifying enzymes), nutrients and metabolites that have the potential to support the embryo during storage in the soil and assist in germination and seedling establishment. We discuss a possible role for DOEEs as natural coatings capable of “engineering” the seed microenvironment for the benefit of the embryo, the seedling and the growing plant.
Highlights
The seed is the fundamental unit of dispersal in higher plants and is at the focal point of consumers, farmers, seed companies and seed banks
Dry fruits consist of two groups: dehiscent, in which the fruit is splitting open at maturity to allow for seed dispersal, and indehiscent, whereby the fruit is not opened at maturity and constitutes the dispersal unit (Figure 1)
Two additional types of dispersal units are common in Poaceae species: a floret, in which the caryopsis is covered by the lemma and palea, and a spikelet, whereby the floret is further covered by the glumes (Figure 1)
Summary
The seed is the fundamental unit of dispersal in higher plants and is at the focal point of consumers, farmers, seed companies and seed banks. Seeds, the dispersal unit of dry dehiscent fruits, have a single protective layer (the seed coat) enclosing the embryo while in fruits the embryo is covered by two protective layers, the seed coat and the fruit coat (the pericarp and its accessories). In various Poaceae species, the basic dispersal unit constitutes of a unique type of dry fruit in which the seed coat and the pericarp are fused together to form the caryopsis. Two additional types of dispersal units are common in Poaceae species: a floret, in which the caryopsis is covered by the lemma and palea, and a spikelet, whereby the floret is further covered by the glumes (Figure 1). All protective layers enclosing the embryo, namely, seed coats, pericarps, lemmas, paleas and glumes, are maternally-derived and undergo programmed cell death (PCD) at maturity. Ilanygeras,ssreess,ptehcetibvaeslyic. dInispgerrassasleus,ntiht ecobnasstictudteispaeursnailquneittycpoenosftidturtyesfruaituinniqwuheicthyptheeosfededrycofraut iatnidn twhheipchertichaerpseaerde cfuoasetdantodgtehtheepretroicfaorrpmatrheefcuasreydoptosgise.tAheflr otoreftoirsma ttyhpeecoafrydoisppseisr.saAl uflnoirte, twishearetybpyethoef cdaisrpyoerpssails uisncito,vwerheedrebbyyththeelecmarmyoapasnisdispacloevae. rIendabsypitkheelelte,mamfloaraent dorpflaolereat.sIanrea fsupritkheelrete,naclfoloseredt boyr gfllourmetessa(ree.gf.u, rTtrhiteircuemnctluorsgeidubmysgsplu.mdiecsoc(ceo.igd.e,sT).rNitiocutem, intucragridyuompssis,pfl. odriceotcacnoiddessp).ikNeloette,,thine ecamryboryposiiss, eflnocrleotseadndbysptwikoe,lteht,reteheanedmfboruyroprisotenctcilvoeseladyebrys, trwesop,etchtirveeelya.nEdmf,oeumrbprryoot;eGctli,vgelulmayee;rAs,wr,easwpenct(iavoenlyg. aEpmp,eenmdabgryeoa;tGthl,eglleummme;aA). w, awn (a long appendage at the lemma)
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