Phenotypes of Floral Nectaries in Developmental Mutants of Legumes and What They May Tell about Genetic Control of Nectary Formation

Abstract

The vast majority of angiosperms attracts animal pollinators with the nectar secreted through specialized floral nectaries (FNs). Although there is evidence that principal patterns of regulation of FN development are conserved in large angiosperm clades, these structures are very diverse considering their morphology and position within a flower. Most data on genetic control of FN formation were obtained in surveys of a model plant species, Arabidopsis thaliana (Brassicaceae). There are almost no data on genetic factors affecting FN development in Leguminosae, the plant family of a high agricultural value and possessing outstandingly diverse flowers. In this work, the morphology of FNs was examined in a set of leguminous species, both wild-type and developmental mutants, by the means of a scanning electron microscopy. Unlike Brassicaceae, FNs in legumes are localized between stamens and a carpel instead of being associated with a certain floral organ. FNs were found stable in most cases of mutants when perianth and/or androecium morphology was affected. However, regulation of FN development by BLADE-ON-PETIOLE-like genes seems to be a shared feature between legumes (at least Pisum) and Arabidopsis. In some legumes, the adaxial developmental program (most probably CYCLOIDEA-mediated) suppresses the FN development. The obtained results neither confirm the role of orthologues of UNUSUAL FLORAL ORGANS and LEAFY in FN development in legumes nor reject it, as two studied pea mutants were homozygous at the weakest alleles of the corresponding loci and possessed FNs similar to those of wild-type.

Floral nectaries in Leguminosae: structure, diversity, and possible evolution

“…The best, and probably only, known example of naturally occurring dorsalised leguminous 'peloria' is Cadia with its polysymmetric flowers clearly derived from monosymmetric ones through ectopic expression of a CYC-like gene (Citerne et al, 2006). However, flowers of this plant are nectariferous (Cronk & Ojeda, 2008) which agrees with probable absence of inhibition of FN development in the adaxial domain of a related genus Calpurnia having a papilionate corolla (Sinjushin, 2022). It means that, if FN is polysymmetric, a broader range of symmetry changes is available in a given lineage.…”

“…Their contrast pigmentation is also believed to attract visitors (Cronk & Ojeda, 2008). Probably the same preventing function is fulfilled by the pocket-like ridge on the hypanthium in resupinate flowers of Clitoria ternatea L. (Sinjushin, 2022) and the overall shape of the FN in Centrosema spp. (Aponte & Jáuregui, 2019).…”

“…In pea, the mutation cochleata1 (coch1) of a BOP1/2 orthologue is known to affect floral development (Couzigou et al, 2012). In flowers of a coch1 missense mutant with a mild expression of anomalies, nectariferous stomata were found on a receptacle whereas no such stomata were detected in flowers of a nonsense mutant with a more severe phenotype (Sinjushin, 2022). This may suggest that the BOP1/2-dependent control of FN development is shared between Arabidopsis and legumes.…”

“…However, a nectar disc with stomata is found in 'double' flowers of garden Wisteria floribunda f. violaceoplena (C.K.Schneid.) Rehder & E.H.Wilson, although its functionality was not tested (Sinjushin, 2022). Floral phenotypes with extra petals are known in several legume species but it has not been confirmed whether the causative mutations are in AG-like genes and if these 'double' flowers produce nectar.…”

“…One of these mutations in pea is keeled wings (k) (Wang et al, 2008). In flowers of k mutants, the nectar-secreting area expands to lateral parts of the receptacle whereas FN has the abaxial position in wild-type flowers (Sinjushin, 2022). This may point at the CYC-mediated suppression of FN development in leguminous flowers but this interaction requires a deeper examination.…”