TARGET OF RAPAMYCIN is essential for asexual vegetative reproduction in Kalanchoë

Abstract

The unique mechanism by which leaf margin cells regain potency and then form a plantlet in Kalanchoë spp. remains elusive but involves organogenesis and embryogenesis in response to age, day length, nutrient availability and drought stress. In light of this, we investigated whether TARGET OF RAPAMYCIN (TOR), a conserved protein kinase in eukaryotes that controls cell growth and metabolism in response to nutrient and energy availability, may regulate plantlet formation. KdTOR was expressed in the leaf margin at the site of plantlet initiation, in the early plantlet cotyledons, and in the root tip of the developed plantlet. Both chemical and genetic inhibition of TOR Kinase activity in Kalanchoë daigremontiana leaves disrupted plantlet formation. Furthermore, downregulation of KdTOR in transgenic plants led to wide-ranging transcriptional changes, including decreased K. daigremontiana SHOOTMERISTEMLESS and K. daigremontiana LEAFYCOTYLEDON1 expression, whereas auxin treatments induced KdTOR expression in the plantlet roots. These results suggest that the KdTOR pathway controls plantlet development in cooperation with auxin, organogenesis, and embryogenesis pathways. The ancient and highly conserved TOR Kinase therefore controls diverse and unique developmental pathways, such as asexual reproduction within the land plant lineage.

Comparative Transcriptome Analysis of Two Kalanchoë Species during Plantlet Formation

“…Additionally, few plants developed a preference for asexual reproduction under certain stress conditions [3]. Moreover, K. daigremontiana plantlet formation is triggered only under long-day conditions [14,26]. With evidence of light-dependent selective preference for asexual or sexual reproduction [3], light or light-associated stress response might be regulating plantlet initiation of K. daigremontiana.…”

“…The developing leaves might be responding to radiation to obtain sufficient light for growth, but also for protection from radiation damage [66,67]. At the same time, the plants might be detecting whether there is sufficient light exposure for plantlet formation as plantlet formation occurs only under long-day conditions [14,26].…”

“…In some cases, bud dormancy does not affect root initiation as adventitious-like roots emerge when leaves are still attached to the plant [10]. On the other hand, plantlet formation in the K. daigremontiana constitutiveplantlet-forming species resembles zygotic embryogenesis, but skips dormancy and the seedling stage [13,14]. In contrast with inducible-plantlet-forming species, constitutiveplantlet-forming species form pedestal structures in the notches, from which plantlet primordia emerge.…”

“…In contrast with inducible-plantlet-forming species, constitutiveplantlet-forming species form pedestal structures in the notches, from which plantlet primordia emerge. Once the plantlets are fully formed, excision sites are established at the base of the plantlet through a programmed cell death pathway, allowing them to fall on the ground and start growing independently [7,14].…”

Induction of Somatic Embryogenesis in Plants: Different Players and Focus on WUSCHEL and WUS-RELATED HOMEOBOX (WOX) Transcription Factors

“…In particular, somatic embryogenesis in vivo is a remarkable phenomenon, but, unfortunately, it has not been studied in detail. It might be interesting to learn, in more depth, the role of WOX genes in epiphylly, since only a few genes have been investigated at present [26,[294][295][296][297]. In this regard, the recent study by Jácome-Blásquez et al [298] appeared to increase knowledge regarding the genes involved in the ectopic differentiation of plantlets in vivo.…”