Perspectives for a Framework to Understand Aril Initiation and Development

Abstract

A differentiated structure called “aril” has been described in seeds of several plant species during the course of evolution and might be considered as a supernumerary integument. Besides its ecological function in seed dispersal, the structure also represents a relevant character for systematic classification and exhibits important properties that impart agronomic value in certain species. Little is known about the molecular pathways underlying this morphological innovation because it is absent in currently used model species. A remarkable feature of the seeds of Passiflora species is the presence of a conspicuous aril. This genus is known for the ornamental, medicinal, and food values of its species. In view of the molecular resources and tools available for some Passiflora species, we highlight the potential of these species as models for developmental studies of the aril.

A genome sequence resource for the genus Passiflora, the genome of the wild diploid species Passiflora organensis

“…The MADS‐box family of transcription factors is involved in many important developmental processes, and they are well‐known for their role in phase transition and establishing the identity of floral organs [reviewed by Kramer (2019)]. We chose this gene family because some of its members have already been studied in detail by our group and might have important biological and evolutionary roles in Passiflora , especially with regard to reproductive development (Cutri & Dornelas, 2012; Scorza et al., 2017; Silveira et al., 2016). We initially found 104 putative MADS‐box family members in P. organensis .…”

The effect of gut passage by waterbirds on the seed coat and pericarp of diaspores lacking “external flesh”: Evidence for widespread adaptation to endozoochory in angiosperms

“…However, unitegmic ovules, and thus structurally more minimalistic seed coats, have evolved in core-eudicots [48, 54, 56]. More complex seed coats with a fleshy sarcotesta or a huge variety of arils have evolved numerous times in both gymnosperm and angiosperms, and these are typically assumed to be supplementary adaptations for endozoochory or synzoochory (e.g., elaiosomes and myrmecochory; [4, 48, 54, 57, 58].…”

Seed coat thickness in the evolution of angiosperms

“…By contrast, some angiosperms evolved a third cell sheath in addition to oi and ii. Annonaceae ovules develop a so-called third integument in between oi and ii [ 12 ], whereas Myristicaceae and other groups initiate a third envelop, referred to as aril (not to be confused with arillodes), later on in development in a region proximal to the oi that has been interpreted as the funiculus [ 17 , 66 , 77 ]. Interestingly, arils can also be found in gymnosperms, such as Taxus baccata [ 54 ].…”