Abstract
Although plant organ shapes are defined by spatio-temporal variations of directional tissue expansion, this is a little characterized aspect of organ growth regulation. Although it is well known that the plant hormone gibberellin increases the leaf length/with ratio, its effects on cell expansion in the growing leaf are largely unknown. To understand how variations in rate and anisotropy of growth establish the typical monocotelydonous leaf shape, we studied the leaf growth zone of maize (Zea mays) with a kinematic analysis of cell expansion in the three directions of growth: proximo-distal, medio-lateral, and dorso-ventral. To determine the effect of gibberellin, we compared a gibberellin-deficient dwarf3 mutant and the overproducing UBI::GA20OX-1 line with their wild types. We found that, as expected, longitudinal growth was dominant throughout the growth zone. The highest degree of anisotropy occurred in the division zone, where relative growth rates in width and thickness were almost zero. Growth anisotropy was smaller in the elongation zone, due to higher lateral and dorso-ventral growth rates. Growth in all directions stopped at the same position. Gibberellin increased the size of the growth zone and the degree of growth anisotropy by stimulating longitudinal growth rates. Inversely, the duration of expansion was negatively affected, so that mature cell length was unaffected, while width and height of cells were reduced. Our study provides a detailed insight in the dynamics of growth anisotropy in the maize leaf and demonstrates that gibberellin specifically stimulates longitudinal growth rates throughout the growth zone.
Highlights
The characteristic elongated and flattened morphology of a monocotyledonous leaf suggests that, during its development, there must be large differences in growth along its three axes: proximodistal, medio-lateral, and dorso-ventral, respectively (Figure 1)
To understand how cell growth is coordinated in the three main axes of growth (Figure 1), we quantified the overall dimensions of the growing 4th leaf of maize seedlings, at 3 days after its emergence from the whorl of older leaves
We previously showed that for longitudinal cell expansion gibberellin increases the length of the growth zone (Nelissen et al, 2012), leading to the question if it has the same effect on growth in width and height
Summary
The characteristic elongated and flattened morphology of a monocotyledonous leaf suggests that, during its development, there must be large differences in growth along its three axes: proximodistal (longitudinal), medio-lateral (width), and dorso-ventral (thickness), respectively (Figure 1). The orientation of these axes is established by oriented cell division patterns during leaf primordium formation (Hudson, 1999). Giberellin Regulates Leaf Growth Anisotropy B studied (Benhajsalah and Tardieu, 1995; Gázquez and Beemster, 2017) It is clear from the shape of the leaf and from previous investigations (Muller et al, 2007) that longitudinal growth is the dominant process in monocotyledonous leaf development.
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