Phenotypic plasticity of aposporous embryo sac development in Hieracium praealtum

Abstract

Apomixis in Hieracium praealtum follows a developmental pathway of apospory, where an unreduced embryo sac develops from a somatic ovule cell without meiosis. The avoidance of meiosis together with fertilization-independent seed formation leads to clonal progeny genetically identical to the maternal plant. We have previously described the initial developmental steps of aposporous embryo sac formation in H. praealtum and here, we cytologically observed more than 500 ovules with a focus on the later stages of embryo sac maturation. Aposporous embryo sac maturation is a stochastic process in H. praealtum with single or multiple embryo sacs formed, in addition to off-types and embryo sac abortion. The frequency of twin embryo sacs growing at the same rate is a rare event and, in most ovules, the additional embryo sac undergoes developmental arrest suggesting dominance or growth promotion of a single embryo sac. Observed deviations from the Polygonum-type embryo sac in H. praealtum indicate developmental plasticity during embryo sac maturation. Nevertheless, fertilizationindependent seed formation successfully occurs.

Efficient CRISPR/Cas9-Mediated Knockout of an Endogenous PHYTOENE DESATURASE Gene in T1 Progeny of Apomictic Hieracium Enables New Strategies for Apomixis Gene Identification

“…Polyembryony occurred in 2% and 7% of the seed from the T0 HPDS lines 1 and 2, respectively ( Table 2 ). These frequencies align with previous observations of polyembryony in D36, measured at embryo sac maturation [ 42 ] and germination [ 43 ]. These results suggest that most T1 seeds formed in the transgenic plants arose via apomixis.…”

Controlling Apomixis: Shared Features and Distinct Characteristics of Gene Regulation

“…In aposporous Hieracium subgenus Pilosella, for example, the specification of the sexual MMC is preceding the formation of the aposporous initial cell, while the apomictic reproductive lineage subsequently suppresses further development of the sexual lineage [22]. Nevertheless, certain developmental flexibility and the formation of two gametophytes in the same ovule occurs in Hieracium praeltum and also in apomictic Boechera, providing the opportunity to reinforce the facultative nature of apomixis [33,34]. Also the occurrence of both, diplospory and apospory, demonstrates a striking flexibility of developmental concepts in Boechera [33].…”

“…Also heat shock proteins are stress responsive proteins tightly associated to ribosome function, as they typically assist folding of newly derived polypeptide sequences to proteins as chaperones. Evidence for roles of heat shock proteins in apomixis regulation comes from different types of apomixis, including apospory in Hieracium prealtum [34], apomixis in Paspalum notatum [92], somatic embryogenesis in Citrus [76], and apogamy in the fern Drypteris affinis [174]. In addition, it is interesting to note that AP2/ERF transcription factors are important players in the integration of hormonal pathways and stress responses to control developmental decisions [154].…”