Control Systems in the Living World - Building the Most Complex Structure on Earth - page 45

Auxin

Cytoplasmic
perception

Nuclear
perception

Extracellular
perception

Tr anscriptional
responses

Nontranscriptional
responses

Inactivation Synthesis Import Export

Auxin distribution dynamics in cells and tissues

Patterns of cell elongation division and differentiation

Figure 1.26 Feedback between the auxin action machinery occurs at multiple levels ( Leyser,

2010 ).

Bud formation is a good example to illustrate the hormonal mechanism of organ
formation in plants. In a simplified form, this mechanism illustrates the coordinated
interactions of two hormones, auxine and cytokinine, in determining the develop-
ment of the leaf bud ( Figure 1.27 ).
One of the main functions of cytokinins is stimulating cell division and leaf
expansion. Cytokinins bind to the histidine protein kinase receptor CRE1 and con-
trol the number of early cell divisions via an intracellular signaling pathway stim-
ulating nuclear transcription activators (ARR1, ARR2, and ARR10) or repressors
(ARR4, ARR5, ARR6, and ARR7) ( Hwang and Sheen, 2001; Sakai et al., 2001;
Schmuelling, 2002 ). The effects of cytokinins in some respects oppose those of
auxin (they inhibit stem elongation and stimulate a plant's bushiness), and the bal-
ance of these hormones may control the size of the meristem ( Beemster and Baskin,
2000 ).
Initiation of bud formation requires the export of auxin from the bud. The antago-
nistic action of auxin and cytokinin in bud formation is explained by the produc-
tion of cytokinin, which increases the flow of auxin to the stem, which inhibits bud
growth. The local synthesis of cytokinin is responsible for bud formation, although
cytokinin is produced primarily in roots. Recently discovered hormones called str-
igolactones are also involved in bud formation; by dampening auxin transport, they
inhibit bud growth ( Crawford et al., 2010 ).
Gibberellins are another group of hormones involved in the germination and elon-
gation of the stem, as well as floral timing and development ( Schwechheimer, 2008 ).
They are synthesized under the influence of external cues such as the lengthening of
the day and the drop in temperature.