Plant Organ Primordia

Abstract

The shoot apical meristem (SAM) in plants is composed of totipotent cells that generate the major plant organs including leaves and flowers. Within the SAM synthesis and movement of auxin creates a hormonal flux. In production of leaf primordia, the hormonal flux alters a series small ribonucleic acids (RNAs) that impact the expression of homeotic genes. This results in the generation of a WUSCHEL/CLAVATA feedback loop that generate niches that result in the placement of leaf primordia. This placement of leaf primordia generates the overall phyllotaxis and nodal architecture that defines the major plant body. In perennial plants, seasonal cues alter primordia development resulting in shifts between true leaves and bud scales creating the overwintering meristem. How the transition between bud scales and leaf primordia occurs in unclear.

Key Concepts

  • The initiation of leaf primordia at nodes is regulated by an auxin gradient within the SAM.
  • A series of homeotic genes are associated with both SAM formation and establishment of leaf primordia and leaf organ symmetry including Knotted1, WUSCHEL‐RELATED Homeobox 1 and CLAVAT‐type (CLV) homoeotic genes.
  • Small RNAs play a critical role in the developmental processes associated with the SAM development.
  • Auxin flux is directed by PIN1 transporter polarity controlled by MONOPTEROS.
  • Responses to seasonal variation alter leaf primordia development.

Keywords: auxin; shoot apical meristem; SAM; organ primordia; plant architecture; WUS/CLV; pathway; MONOPTEROS; PIN‐FORMED; perennial buds; Arabidopsis thaliana

Figure 1. Variable primordia pattern formations found in shoot apices. These patterns are genetically controlled and specific to species.
Figure 2. Factors that control auxin efflux within the SAM and leaf primordia. Developmental zones are shown in blue. Orange indicates the CLV‐WUS feedback loop. Red arrows indicate auxin efflux, which is controlled by PIN1 polarity (pink). Controlling the PIN1 polarity in P0 is MP shown in green. ARR, Arabidopsis response regulator; CLV1‐3, CLAVATA1‐3; CK, cytokinin; CZ, central zone; GA, gibberellin; MP, MONOPTEROS; OC, organizing center; P0‐P1, leaf primordial; PIN1, PIN‐FORMED1; PZ, peripheral zone; RZ, rib zone; WUS, WUSCHEL. Adapted from Wang and Li and Bhatia et al. .
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Further Reading

Drea S (2010) Petals. In: eLS. John Wiley & Sons Ltd: Chichester. DOI: 10.1002/9780470015902.a0002065.pub2.

Egea‐Cortines M and Weiss J (2013) Control of plant organ size. In: eLS. John Wiley & Sons Ltd: Chichester. DOI: 10.1002/9780470015902.a0003363.pub2.

Lin Sang Y, Juan Cheng Z and Sheng Zhang X (2018) iPSCs: a comparison between animals and plants. Trends in Plant Science 23 (8): 660–666. DOI: 10.1016/j.tplants.2018.05.008.

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Campbell, Michael, and Adams, Rachael(Mar 2020) Plant Organ Primordia. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002055.pub3]