Expression and function of the bHLH genes ALCATRAZ and SPATULA in selected Solanaceae species

Abstract

Summary The genetic mechanisms underlying fruit development have been identified in Arabidopsis and have been comparatively studied in tomato as a representative of fleshy fruits. However, comparative expression and functional analyses on the bHLH genes downstream the genetic network, ALCATRAZ (ALC) and SPATULA (SPT), which are involved in the formation of the dehiscence zone in Arabidopsis, have not been functionally studied in the Solanaceae. Here, we perform detailed expression and functional studies of ALC/SPT homologs in Nicotiana obtusifolia with capsules, and in Capsicum annuum and Solanum lycopersicum with berries. In Solanaceae, ALC and SPT genes are expressed in leaves, and all floral organs, especially in petal margins, stamens and carpels; however, their expression changes during fruit maturation according to the fruit type. Functional analyses show that downregulation of ALC/SPT genes does not have an effect on gynoecium patterning; however, they have acquired opposite roles in petal expansion and have been co‐opted in leaf pigmentation in Solanaceae. In addition, ALC/SPT genes repress lignification in time and space during fruit development in Solanaceae. Altogether, some roles of ALC and SPT genes are different between Brassicaceae and Solanaceae; while the paralogs have undergone some subfunctionalization in the former they are mostly redundant in the latter.

Expression of gynoecium patterning transcription factors in Aristolochia fimbriata (Aristolochiaceae) and their contribution to gynostemium development

“…The NGA genes are members of the B3 transcription factor family in angiosperms; the four Arabidopsis paralogs result from Brassicaceae-specific duplication events [12,13]. Functional data from Arabidopsis indicate that all four copies act redundantly to control style and stigma fusion and development [12,13,63,65]. The same functions have been identified for the NGA homologs in Eschscholzia californica and Nicotiana benthamiana.…”

“…Additionally, SPT specifies the differentiation of the valve margins and the dehiscence zone during carpel development and, together with INDEHISCENT (IND), it also regulates the auxin biosynthesis and distribution in the medial tissues during development [14,61,62]. Expression of SPT homologs in species of Solanaceae is broad in sepals, petals, stamens and carpels [63]. Functional analyses suggest that, together with ALC, SPT genes promote cell division and organ size as well as fruit maturation and ripening, likely by repressing lignification [63].…”

“…Expression of SPT homologs in species of Solanaceae is broad in sepals, petals, stamens and carpels [63]. Functional analyses suggest that, together with ALC, SPT genes promote cell division and organ size as well as fruit maturation and ripening, likely by repressing lignification [63]. Similar broad expression patterns in the perianth, the ovary and the lignified layer of the fruits (endocarp) has been found for SPT/ALC homologs in peach (Prunus persica, Rosaceae), and it has been suggested that they function in endocarp differentiation [64].…”

Comparative anatomy and genetic bases of fruit development in selected Rubiaceae (Gentianales)

“…Together, SPT and ALC genes control gynoecium patterning and the development of the separation layer in the Arabidopsis fruit (Alvarez and Smyth, 1999 ; Heisler et al, 2001 ; Groszmann et al, 2008 ; Rajani and Sundaresan, 2001 ). The two genes have opposite expression patterns in dry versus fleshy Solanaceae fruits but act redundantly to repress lignification in time and space independent of fruit type (Ortíz‐ Ramírez et al, 2018 , 2019 ). As expected from the mapping of the core eudicot duplication, most Rubiaceae species have gene representatives in the gene clades ALC and SPT .…”

“…The expression patterns detected for ALC / SPT homologs in C. corymbosa, G. hypocarpium , and P. angustifolia suggest that both ALC and SPT may play important roles during carpel development in C. corymbosa , and only SPT orthologs stay active during fruit development in C. corymbosa and P. angustifolia . However, a putative role in specifying when and how lignification patterns start, like those identified in Solanaceae (Ortíz‐Ramirez et al, 2018 , 2019 ) cannot be readily assigned to ALC / SPT genes in Rubiaceae. Similar to other gene lineages discussed so far, spatial expression resolution in additional developmental stages as well as more comparative studies in species with different fruit types are required to assess their contribution in Rubiaceae.…”