1/7/2024 0 Comments Primordia bridge code![]() M-O (meristem-organ) boundaries separate leaf and flower primordia from the SAM, whereas O-O (organ-organ) boundaries develop between individual floral organs and create space between them. There are two types of boundaries in the developing shoot apices. Cells in the boundary have reduced rates of cell division, concave surfaces, elongated shapes, and exhibit low auxin concentration compared to the adjacent cells in meristems or primordia. Between the meristem and newly formed leaf or flower primordia, a boundary forms with specialized cells that separate meristematic activity from determinate organ growth. Each flower consists of four concentric whorls of organ types: the protective sepals, the showy petals, the male stamens, and the female carpels. Primordia are initiated from the peripheral zone of the SAM in a predictable pattern, and develop into leaves during the vegetative stage, and into flowers during the reproductive phase. Leaves and flowers originate from the shoot apical meristem (SAM), which contains pluripotent stem cells and resides at the tip of each stem. ![]() ![]() Thus, our findings shed light on the “bridge” role of boundaries between meristem and organ primordia during flower development in Arabidopsis. We showed that boundary-expressing HAN communicates with the meristem through PNH, regulates floral organ development via JAG and BOP2, and maintains boundary morphology through CYTOKININ OXIDASE 3 ( CKX3)-mediated cytokinin homeostasis. Here we combined genetic, molecular and biochemical tools to explore interactions between the boundary gene HANABA TARANU ( HAN) and two meristem regulators BREVIPEDICELLUS (BP) and PINHEAD ( PNH), and three primordia-specific genes PETAL LOSS (PTL), JAGGED ( JAG) and BLADE-ON-PETIOLE (BOP) during flower development. But whether and how boundaries communicate with meristem and organ primordia during flower development remains largely unknown. Previous studies showed that boundary genes interact with meristem regulators and primordia genes during embryogenesis or leaf development. Boundary genes are specifically expressed in boundaries and function in boundary formation and maintenance. Between the meristem and the newly formed organ primordia, a boundary with specialized cells is formed to separate them. The shoot apical meristem is the stem cell pool in plants that gives rise to all above-ground organs including leaves, flowers and fruits. Our data suggest that boundary-expressing HAN communicates with the meristem through the PNH, regulates floral organ development via JAG and BOP2, and maintains boundary morphology through CKX3 during flower development in Arabidopsis. Further, HAN directly binds to the promoter and intron of CYTOKININ OXIDASE 3 ( CKX3) to modulate cytokinin homeostasis in the boundary. HAN physically interacts with JAG, and directly stimulates the expression of JAG and BOP2 to regulate floral organ development. HAN and PNH transcriptionally promote each other, and biochemically interact to regulate meristem organization. ![]() We demonstrated the key role of HAN in determining petal number, as part of a set of complex genetic interactions. We combined genetic, molecular and biochemical tools to explore interactions between the boundary gene HANABA TARANU ( HAN) and two meristem regulators BREVIPEDICELLUS (BP) and PINHEAD ( PNH), and three primordia-specific genes PETAL LOSS (PTL), JAGGED ( JAG) and BLADE-ON-PETIOLE (BOP) during flower development. Despite interactions that have been found between boundary regulators with genes controlling meristem maintenance or primordial development, most boundary studies were performed during embryogenesis or vegetative growth, hence little is known about whether and how boundaries communicate with meristem and organ primordia during the reproductive stage. Between the meristem and the newly formed organ primordia, a boundary with specialized cells is formed that separates meristematic activity from determinate organ growth. Shoot organ primordia are initiated from the shoot apical meristem and develop into leaves during the vegetative stage, and into flowers during the reproductive phase.
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