Development of Hypocotyls and Apical Hooks in Dicotyledons and Their Regulatory Mechanisms for Seedling Emergence

doi:10.13560/j.cnki.biotech.bull.1985.2023-1084

Abstract

Abstract:

Seedling emergence is a crucial stage in plant production as it greatly impacts or even determines the final yield. For dicotyledonous plants, the growth of the hypocotyl and the development of the apical hook play a significant role in seedling emergence. Hypocotyl growth and apical hook development are a continuously dynamic process and are important guarantees for seedling emergence. The elongation of the hypocotyls provides the necessary force for seedlings to penetrate the soil, while the apical hook protects the delicate apical meristems and cotyledons from the damages during the soil topping process. Focusing on the development of the hypocotyl and apical hook, this paper summarized the molecular mechanisms regulating hypocotyl elongation and thickening by arranging themselves through the cytoskeleton in dicotyledonous plants, as well as the growth mechanisms of cellular differentiation involved in apical hook formation, maintenance, and unfolding. This paper examined the synergistic effects and mechanisms of major environmental factors such as light and temperature, as well as plant endogenous hormones, on hypocotyl and apical hook development. Furthermore, the paper explored agricultural practices, including optimizing environmental factors through sowing methods and seedling thinning to enhance hypocotyl growth and apical hook development, which in turn improves seedling emergence rate and stand establishment rate. Lastly, the paper provided suggestions for further research on the molecular mechanisms regulating hypocotyl and apical hook development, along with insights into the adaptation mechanism of apical hook and hypocotyl development to adversity, as well as for achieving effective control of seedling emergence and stand establishment in dicotyledonous plants. This paper serves as a valuable reference for a deeper understanding of the seedling emergence mechanism in dicotyledonous plants and innovative plant seeding and seedling preservation techniques.

Key words: hypocotyl, apical hook, dicotyledons, regulation mechanism, seedling emergence

HUA Zi-qing, ZHOU Jing-yuan, DONG He-zhong. Development of Hypocotyls and Apical Hooks in Dicotyledons and Their Regulatory Mechanisms for Seedling Emergence[J]. Biotechnology Bulletin, 2024, 40(4): 23-32.

Figures/Tables 2

Fig. 1 Process of seed germination and seedling emergence in dicotyledonous plants（cotton as an example） During the early stage of seedling emergence, cells on both sides of the apical hypocotyl grow differentially to form the apical hook. Under the mechanical pressure of the soil, the differential growth of cells on the inner and outer sides of the apical hook is intensified to entere the maintenance stage, while the hypocotyl thickened laterally. With the upward growth of the seedling, the mechanical pressure of the soil decreases, the hypocotyl elongates rapidly, the growth rate of the inner cells of the apical hook increases, and finally the apical hook and the cotyledons get unfolded to complete the successful emergence

Fig. 2 Factors influencing the development of apical hook and hypocotyl and their mechanisms in regulating seedling emergence and stand establishment Environmental signals, such as light and temperature, interact with phytohormones like ethylene and gibberellin, to jointly regulate the expressions of genes related to the development of hypocotyls and apical hooks, and to promote seedling emergence and stand establishment. Normal arrows indicate positive regulatory effects, while T-arrows indicate negative regulatory effects. PHYs/CRYs: Phytochromes/Cryptochromes; SA: salicylic acid; JA: jasmonic acid; GA: gibberellic acid; ABA: abscisic acid; BR: brassinosteroids; ET: ethylene; SL: strigolactone; NPR1: salicylic acid receptor protein; MYC2: jasmonic acid signaling pathway transcription factors; DELLAs: DELLA family proteins; BZR1: brassinosteroids receptor protein; EBF1/2: ethylene insensitive 3 binding F-box protein 1/2; CBF: AP2/EREBP transcription factor family members

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