Bulbil initiation: a comprehensive review on resources, development, and utilisation, with emphasis on molecular mechanisms, advanced technologies, and future prospects

Abstract

Bulbil is an important asexual reproductive structure of bulbil plants. It mainly grows in leaf axils, leaf forks, tubers and the upper and near ground ends of flower stems of plants. They play a significant role in the reproduction of numerous herbaceous plant species by serving as agents of plant propagation, energy reserves, and survival mechanisms in adverse environmental conditions. Despite extensive research on bulbil-plants regarding their resources, development mechanisms, and utilisation, a comprehensive review of bulbil is lacking, hindering progress in exploiting bulbil resources. This paper provides a systematic overview of bulbil research, including bulbil-plant resources, identification of development stages and maturity of bulbils, cellular and molecular mechanisms of bulbil development, factors influencing bulbil development, gene research related to bulbil development, multi-bulbil phenomenon and its significance, medicinal value of bulbils, breeding value of bulbils, and the application of plant tissue culture technology in bulbil production. The application value of the Temporary Immersion Bioreactor System (TIBS) and Terahertz (THz) in bulbil breeding is also discussed, offering a comprehensive blueprint for further bulbil resource development. Additionally, additive, seven areas that require attention are proposed: (1) Utilization of modern network technologies, such as plant recognition apps or websites, to collect and identify bulbous plant resources efficiently and extensively; (2) Further research on cell and tissue structures that influence bulb cell development; (3) Investigation of the network regulatory relationship between genes, proteins, metabolites, and epigenetics in bulbil development; (4) Exploration of the potential utilization value of multiple sprouts, including medicinal, ecological, and horticultural applications; (5) Innovation and optimization of the plant tissue culture system for bulbils; (6) Comprehensive application research of TIBS for large-scale expansion of bulbil production; (7) To find out the common share genetics between bulbils and flowers.

Endogenous Hormone Levels and Transcriptomic Analysis Reveal the Mechanisms of Bulbil Initiation in Pinellia ternata

“…Different plants exhibit varying developmental pathways for their bulbils. For example, bulbils morphologically progress from a ‘torpedo shape’ to an ‘embryo shape’ and finally to a ‘spherical shape’ during their development in the Lilium lancifolium [ 7 ]. We found that a P. ternata bulbil emerges from a white punctate structure, which subsequently undergoes both longitudinal elongation and lateral expansion, ultimately developing into an oval spherical configuration.…”

“…Phytohormones, also known as plant hormones or growth regulators, regulate various processes in plant growth, development, and environmental adaptation at extremely low concentrations [ 7 ]. These processes work independently yet cooperatively to collectively regulate nutritional growth, reproductive growth, seed germination, seed maturation dormancy, embryo development, and adaptation to biotic and abiotic stresses [ 36 , 37 , 38 ].…”

“…Bulbils fulfil a dual role as both agents of reproduction and sources of nourishment; they typically emerge in specific parts of plants, such as leaf axils, stems, or inflorescences, primarily serving as a means of asexual reproduction [ 7 ]. Numerous medicinal species, e.g., Dioscorea opposita and Lilium sulphureum , produce bulbils on their leaves and petioles [ 8 ]; conversely, plants such as Agave tequilana and Polygonum viviparum develop bulbils on their inflorescences [ 9 , 10 ], and bulbils can also grow on the leaves of plants such as Pinellia cordata , Amorphophallus bulbifer , and Ornithogalum thyrsoide [ 11 ].…”

“…Therefore, in P. ternata , bulbils grow on the petioles with diverse phenotypes. The bulbils are generally spherical or oval, though other irregular shapes also occur, depending on the specific characteristics and physiological state of the plant [ 7 ]. When bulbils grow at both the top and base of the petiole, they are termed double-bulbil type (DB); when bulbils only grow at the base of the petiole and the top of the petiole does not grow bulbils, they are termed single-bulbil type (SB).…”

“…During the bulbil formation of A. tequilana , AtqKNOX1 was significantly upregulated in the initial stage, while the expression of AtqKNOX2 increased during later stages [ 29 ]. In Dioscorea alata , several TFs from Aux/IAA, ERF (Ethylene-Responsive Factor), MYB (MYB-related) and bHLH (basic helix-loop-helix) families have been shown to play key roles in triggering bulbil formation [ 7 ].…”