As the primary root emerges from a germinating seed, it shows strong positive gravitropism leading to rapid downward curvature (Figure 8.8a). This enables the root tip quickly to penetrate the soil, giving anchorage and access to water, the latter being a vital factor in successful establishment. Root gravitropism has been investigated for over a century, but its mechanism is still not fully understood. However, we do know that gravity is detected in the root cap, and that normally both root cap and root tip need to be present for straight growth and curvature to occur. Because the elongation zone is situated behind the tip, information about the root’s position must be transferred from the sensing site in the cap to the elongation zone.
Shoots sense gravity differently. Both the shoot tip and the growing zone behind it can detect and respond to gravity (Figure 8.8b), so that even decapitated shoots retain an ability to curve upwards when displaced from the vertical. The shoot tip, unlike the root tip, is therefore not essential for gravitropism.
Detecting the direction of gravity is the essential ﬁrst step in gravitropism. Plant organs achieve this by sensing the move-ment and position of starch grains contained within amyloplasts of specialised cells called statocytes (Figure 8.9a).
In roots, statocytes are located in the root cap (Figure 8.9b) which also serves to protect the root meristem from abrasion by soil particles as it grows through the soil. Root cap involvement was ﬁrst demonstrated in maize, when a needle was used to prise off the root cap. This procedure did not inhibit growth, but ability to sense and respond to gravity were completely lost until a new cap grew over the root tip about one day later. Subsequently, a gravity-insensitive mutant of maize was found that does not secrete the mucilage which normally covers and protects the root cap and tip. Mucilage artiﬁcially applied to mutant roots immediately restored the gravity response indicating that the root cap transmits infor-mation through the mucilage. This information is probably in the form of a small diffusible molecule, moving either in the mucilage or through the root apoplasm. Researchers have not yet been able to identify this chemical.
In dicotyledonous shoots, statocytes form a cylindrical tube one cell thick, which surrounds the vascular tissue (Figure 8.9c). This cylinder is known as the ‘starch sheath’, because numerous starch grains show up very clearly in stem sections stained with starch-speciﬁc iodine solution. These statocytes are distributed along the length of the shoot and so can sense gravity in the absence of the apex. In grasses and cereals, stem statocytes are restricted to the stem node and leaf sheath pulvinus. Consequently, only the nodes and pulvini respond to gravity (Figure 8.10).
The involvement of statocyte starch grains in gravity perception was proved by keeping barley plants in the dark for 5d, which resulted in disappearance of starch grains as the starch was consumed in respiration. These starchless plants completely lost their gravity response, but feeding with sucrose resulted in starch grains reforming and restoration of gravity sensing. Additional evidence comes from a maize mutant known as amylomaize, which has abnormally small starch grains and very slow gravitropic response.
Proof that the controlling force is gravity, and not, for example, lines of magnetic ﬁeld, comes from experiments in which a centrifugal force was substituted for gravity. If a germinating bean seed was placed at the axis of a horizontal centrifuge rotating at one revolution per second, to give an acceleration of 4 x 10–3g, this effectively counteracted gravity. The starch grains in the root cap developed in the centre of the cell and were unable to generate a displacement message. Consequently, the root remained straight. At two revolutions per second, equivalent to 2 x 10–2g, the starch grains were forced against the outside wall of the statocytes. As a result, the root commenced to curve, bringing the tip parallel with the centrifugal force, that is, growing radially outwards. Now the centrifugal force acted along the length of the root and the starch grains were displaced onto the normally lower sides of the statocyte cells in the root cap, leading to straight growth. Experiments on plants under ‘micro-gravity’ conditions in space orbit have conﬁrmed much of what was previously deduced from experiments on earth (Halstead and Dutcher 1987).
How do amyloplasts enable gravity sensing? Because of their high density and relatively large mass, they normally occupy the lowest part of the statocyte. When a root is dis-placed from the vertical, statocyte orientation is changed and the starch grains roll or slide ‘downhill’ through the cytoplasm to reach the new low point. Statocytes, possibly through stretch or displacement receptors in the plasma membrane, are able to recognise that starch grains have moved to new positions. An asymmetric message is then transmitted from the root cap to the growing region and a correction curvature is initiated until the cap returns to vertical. Similar events occur in shoots.
Many organs naturally grow at an angle to gravity. This is a type of gravitropism termed plagiotropism and occurs in lateral shoots and roots, and also in some prostrate primary shoots, for example runners of strawberry and subterranean rhizomes of some grasses and sedges (Figure 7.18). The lateral growth angle is variable but is at least partly under genetic control, giving every plant a recognisable architecture. In shoots, the angle is also influenced by the vertical primary stem and by environmental factors. For example, exposure to bright sunlight tends to increase the angle to the vertical, while shade reduces it. Couch grass illustrates the requirement for exposure to direct sunlight. When their runners grow into shade, the plagiotropic tendency disappears and stems grow vertically in search of higher light intensity. The primary shoot apex also influences direction of growth of lateral shoots, which often changes to vertical if the primary shoot tip is removed. This response is probably linked to apical dominance.