plant cells have meristematic stem cells that are always in plant, can make any type of cell (totipotent)

meristems: region of unspecialized plant tissue/cells, found chiefly at growing tips of roots and shoots and in the cambium, consisting of actively dividing cells forming new tissue

where does plant growth occur

• plants cell wall limits ability to divide + expand

• growth therefore only occurs where there are immature cells e.g. the meristem

types of meristem

• Apical meristems

  • at the tips or apex of roots and shoots 

  • results in longer roots and shoots

• Lateral bud meristems

  • in buds and could give rise to side shoots

• Lateral meristems

  • found in a cylinder near the outside of roots and shoots

  • cause them to widen

• Intercalary meristems

  • located between nodes

  • makes the shoot longer

apical dominance

dicotyledonous plants have apical meristems (undifferentiated cells)

• apical bud of plant is primary growing point located at apex (tip) of the stem

• it is the dominant bud, since it can cause all the lateral buds below them to remain dormant

• terminal buds have special tissue called apical meristem. these cells can divide indefinitely and produces all the differentiated tissue

• growth of this bud exhibits apical dominance - it inhibits growth of lateral buds

auxin role in apical dominance

• auxins are also known to play a role in apical dominance - high auxin conc stimulates growth at apical shoot + inhibits growth of lateral shoots. plants then grow taller and compete more efficiently for sunlight

• this is known as APICAL DOMINANCE

• if terminal or apical tip of plant is removed, then lateral buds elongate and grow

• (also regardless of genotype more gibberellin means more lateral growth)

apical dominance: terminal bud releases auxin inhibiting the growth of lateral buds

pruning (apical bud decapitation): removing the terminal bud via selective pruning allows dormant lateral buds to develop, producing bushier plants

controlling apical dominance

• imposed by young leaves of apical bud

• these synthesise indoleacetic acid (IAA)

• IAA inhibits growth of buds below

• if apical bud removed, one or more lateral buds take over

• if apical stump is treated with IAA, growth of lateral buds is suppressed

why?

• auxin in the apical bud is transported down the shoot through the cells and accumulates at the nodes where its presence inhibits growth of the nearby lateral buds. This prevents the lateral bud competing with the apical tip for light and nutrients.

PROOF

• If apical bud removed – the lateral buds grow, and the plant grows sideways.

• If the bud is removed and synthetic hormone IAA applied to the cut, the lateral buds don’t grow.

auxins

plant hormones used to control growth, acts as growth promoter

• causes increased transport of hydrogen ions from cytoplasm to cell wall

• this activates enzymes expansins which breaks crosslinks between cellulose fibres

• increasing cell wall flexibility

• cells expand as they absorb water

• auxin also alters gene expression to promote cell growth

how they are produced

auxins produced at apex + then diffuse or actively transported to the cells in zone of elongation → they elongate, shoot grows

gibberellin

• cause stem elongation, concentration affects internodes

• discovered as they are produced by fungus from genus Gibberella

• infected rice seedlings grew extremely tall and thin

• dwarf plants produce little/no gibberellin