Shoot (botany)

Last updated

In botany, a plant shoot consists of any plant stem together with its appendages like leaves, lateral buds, flowering stems, and flower buds. [1] [2] The new growth from seed germination that grows upward is a shoot where leaves will develop. In the spring, perennial plant shoots are the new growth that grows from the ground in herbaceous plants or the new stem or flower growth that grows on woody plants.

Contents

In everyday speech, shoots are often synonymous with stems. Stems, which are an integral component of shoots, provide an axis for buds, fruits, and leaves.

Young shoots are often eaten by animals because the fibers in the new growth have not yet completed secondary cell wall development, making the young shoots softer and easier to chew and digest. As shoots grow and age, the cells develop secondary cell walls that have a hard and tough structure. Some plants (e.g. bracken) produce toxins that make their shoots inedible or less palatable.

Shoot types of woody plants

Development of fruiting spurs on an apple tree. Left: A two-year-old shoot; Right: A three-year-old shoot with fruit spurs The pruning-book; a monograph of the pruning and training of plants as applied to American conditions (1903) (14581189429).jpg
Development of fruiting spurs on an apple tree. Left: A two-year-old shoot; Right: A three-year-old shoot with fruit spurs

Many woody plants have distinct short shoots and long shoots. In some angiosperms, the short shoots, also called spur shoots or fruit spurs, produce the majority of flowers and fruit. A similar pattern occurs in some conifers and in Ginkgo , although the "short shoots" of some genera such as Picea are so small that they can be mistaken for part of the leaf that they have produced. [3]

A related phenomenon is seasonal heterophylly, which involves visibly different leaves from spring growth and later lammas growth. [4] Whereas spring growth mostly comes from buds formed the previous season, and often includes flowers, lammas growth often involves long shoots.[ citation needed ]

Shoot Apical Meristem (SAM)

The shoot apical meristem (SAM) is the tiny growing tip at the very top of a plant’s shoot. It is made of small, constantly dividing cells that act like a “factory” for creating all new above-ground parts of the plant. It is very important to understand the organization of the SAM. The tunica-corpus model, first proposed by Schmidt in 1924, describes the organization of the shoot apical meristem into two structurally and functionally distinct regions based on the orientation of cell division. The tunica consists of the outer one or two cell layers (commonly referred to as L1 and L2), where cells predominantly divide anticlinally-perpendicular to the surface-thereby maintaining a continuous surface layer. Beneath the tunica lies the corpus, an inner mass of cells (L3 and deeper layers) that divide in multiple planes, including both anticlinal and periclinal orientations, contributing to the internal bulk and volume of the meristem [3] . Cells in the tunica divide mainly anticlinally to maintain a smooth outer surface and give rise to the epidermis of emerging leaves and stems. In contrast, the corpus contains cells that divide in multiple planes, contributing to the internal tissues and overall bulk of the growing shoot. Together, the tunica and corpus generate the continuous formation of new organs and tissues, allowing the shoot to elongate and produce leaves, branches, and reproductive structures.

Functions of the Shoot Apical Meristem

The shoot apical meristem performs two major functions: maintaining a self-renewing pool of stem cells at the center of the apex and generating new lateral organs from its flanks. These functions depend on the coordinated activity of its structural regions- the central zone, peripheral zone, and rib meristem-and on the layered organization described by the tunica- corpus model. Recent research has shown that these developmental roles are regulated by interacting hormone gradients and transcription factors within the meristem. According to Shani, Yanai and Ori (2006), high cytokinin activity in the central zone, promoted by KNOXI and WUSCHEL proteins, supports meristem maintenance and indeterminate growth. In contrast, elevated auxin and gibberellin levels at the periphery specify the P0 region, where lateral organs are initiated. These opposing hormonal domains create stable boundaries between the tunica and corpus regions and between the meristem center and emerging primordia, allowing the SAM to continuously balance self-renewal with organ production. [5]

Zones of the Shoot Apical Meristem

The shoot apical meristem is devided into zones. The central zone contains slowly dividing stem cells that maintain the meristem. Surrounding it, the peripheral zone has faster-dividing cells that give rise to leaf primordia and new lateral organs. Beneath these regions lies the rib zone, where cells divide and expand to contribute to the internal tissues of the stem. Together, these zones coordinate growth, organ initiation, and stem cell maintenance. The devision of cells could primarily be compared to that of human bones, if someone breaks a bone in their body, that bone starts deviding at the place of repture because of the compound Calcium. Just like when a human being grows tall and the bone cells starts multiplying, the cells in a plant shoots also start deviding to facilitate physical growth. Even the human bone has zones such as the middle and the top section which devide cells differently like in the case of plants.

Hormonal Regulation

Hormonal regulation of plant meristems involves an integrated network of signaling pathways that coordinate cell division, differentiation, and tissue patterning. Studies in Arabidopsis and other model species show that no single hormone acts in isolation; instead, developmental outcomes emerge from extensive cross-talk between auxin, cytokinin, gibberellins, ethylene, abscisic acid, and brassinosteroids.

Auxin plays a central role in pattern formation at both shoot and root apices. In the root apical meristem, auxin forms a stable concentration gradient maintained by coordinated activity of AUX/LAX influx carriers, PIN efflux proteins, and ABC transporters. This gradient specifies the position of the stem-cell niche and regulates the dosage-dependent activity of PLETHORA (PLT) transcription factors, which control stem-cell identity, mitotic activity, and differentiation.

Cytokinin acts largely antagonistically to auxin. In the root meristem, cytokinin promotes the transition of cells from division to differentiation, thereby influencing meristem size. Cytokinin signaling also contributes to vascular pattern formation and early embryonic establishment of the stem-cell region. Several studies show that auxin induces negative regulators of cytokinin signaling (such as ARR7 and ARR15), providing a molecular basis for auxin–cytokinin antagonism.

Ethylene interacts with auxin to regulate root elongation and stem-cell behavior. Ethylene stimulates auxin biosynthesis and enhances the expression of auxin transport components, leading to higher auxin accumulation in the elongation zone and reduced cell expansion. Ethylene also modulates quiescent-center division, linking hormonal signaling with maintenance of stem-cell quiescence.

Gibberellins (GAs) regulate root growth primarily by controlling cell expansion. GA signaling promotes degradation of DELLA proteins, which act as growth repressors. Auxin promotes GA-induced DELLA destabilization, whereas ethylene and abscisic acid stabilize DELLA proteins, illustrating a multi-directional regulatory interface among these pathways.

Brassinosteroids influence both meristematic cell division and elongation. Optimal brassinosteroid levels are required for proper auxin responsiveness, and brassinosteroid-deficient mutants show reduced meristem size and impaired auxin-induced transcriptional responses. Brassinosteroids also stimulate ethylene biosynthesis, adding an additional layer of interaction among hormonal pathways.

Abscisic acid (ABA) contributes to root meristem organization and integrates environmental stress signals with developmental responses. ABA affects sensitivity of meristematic cells, interacts with ethylene signaling components, and influences DELLA protein stability during stress conditions.

Collectively, hormonal regulation of meristems is achieved through a dynamic network in which hormones modulate each other’s biosynthesis, transport, and downstream gene expression. These interactions ensure that meristem activity is tightly coordinated with developmental stage and environmental inputs. [6]

See also

References

  1. ↑ Esau, K. (1953). Plant Anatomy. New York: John Wiley & Sons Inc. p. 411.
  2. ↑ Cutter, E.G. (1971). Plant Anatomy, experiment and interpretation, Part 2 Organs. London: Edward Arnold. p. 117. ISBN   0-7131-2302-8.
  3. ↑ Gifford, E.M.; Foster, A.S. (1989), "Morphology, and evolution of vascular plants", Taxon, 38 (4), New York: W. H. Freeman and Company: 613, Bibcode:1989Taxon..38Q.613S, doi:10.2307/1222641, JSTOR   1222641
  4. ↑ Eckenwalder, J.E. (1980), "Foliar Heteromorphism in Populus (Salicaceae), a Source of Confusion in the Taxonomy of Tertiary Leaf Remains", Systematic Botany, 5 (4): 366–383, Bibcode:1980SysBo...5..366E, doi:10.2307/2418518, JSTOR   2418518
  5. ↑ Shani, Eilon; Yanai, Osnat; Ori, Naomi (2006-10-01). "The role of hormones in shoot apical meristem function". Current Opinion in Plant Biology. Cell signalling and gene regulation / edited by Joseph Kieber and Takashi Araki. 9 (5): 484–489. doi:10.1016/j.pbi.2006.07.008. ISSN   1369-5266.
  6. ↑ Benková, Eva; Hejátko, Jan (2009-03-01). "Hormone interactions at the root apical meristem". Plant Molecular Biology. 69 (4): 383–396. doi:10.1007/s11103-008-9393-6. ISSN   1573-5028.

[1]

  1. ↑ Tooke, Fiona; Battey, Nick (2003). "Models of shoot apical meristem function". New Phytologist. 159 (1): 37–52. doi:10.1046/j.1469-8137.2003.00803.x. ISSN   1469-8137.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.