Indole-3-Acetic Acid Controls Cambial Growth in Scots Pine by Positional Signaling1

Abstract

The vascular cambium produces secondary xylem and phloem in plants and is responsible for wood formation in forest trees. In this study we used a microscale mass-spectrometry technique coupled with cryosectioning to visualize the radial concentration gradient of endogenous indole-3-acetic acid (IAA) across the cambial meristem and the differentiating derivatives in Scots pine (Pinus sylvestris L.) trees that had different rates of cambial growth. This approach allowed us to investigate the relationship between growth rate and the concentration of endogenous IAA in the dividing cells. We also tested the hypothesis that IAA is a positional signal in xylem development ( Cambial growth involves the production of secondary xylem and phloem elements. The lateral meristem responsible for this growth, the vascular cambium, normally consists of 5 to 15 dividing cells in a radial direction (Larson, 1994), the so-called cambial zone. The rate of cambial growth is the major determinant for the production of wood in forest trees, and it is determined by both the radial number of dividing cells in the cambial zone and the rate of cell division for each of the cambial zone cells (Gregory and Wilson, 1968;Wilson and Howard, 1968; Gregory, 1971). Cambial growth is adjusted to the demands of water transport required by the leaf biomass and to provide the mechanical strength necessary to support the crown and to withstand wind forces (Zimmermann and Brown, 1971). It is also well established that stem-diameter growth is often found to be greatest within the young crown and to decrease gradually down the stem. As a consequence, the amount and pattern of growth along the stem are determined by the size and arrangement of the crown (Larson, 1963; Hall, 1965). Such coordination requires a signaling system that integrates apical with cambial growth.The plant hormone IAA appears to fulfill the requirements for such a signal. Developing buds and young shoots are major sources of IAA, which is transported in a basipetal polar fashion (Kaldeway, 1984; Little and Savidge, 1987). Experiments with exogenous IAA have clearly demonstrated that polarly transported IAA induces formation of primary vascular tissues (Sachs, 1981; Jacobs, 1984; Aloni, 1995) and maintains the structure and cell-division activity of the vascular cambium (Savidge, 1983). IAA also affects the rate of cambial growth, as measured by tracheary element production in both 1-year-old shoots and mature stems in a dose-dependent manner (Little and Savidge, 1987;Little and Sundberg, 1991). These findings strongly suggest a function for IAA as a signal that integrates apical growth with production of vascular tissues. Accordingly, variations in cambial growth patterns along the stem have been explained by the postulated existence of longitudinal concentration gradients of IAA (Larson, 1969; Aloni and Zimmermann, 1983).With the development of accurate techniques for measuring IAA in plant tissues, it has become possible to test these ideas. The physiological relevance of IAA...

Activity–dormancy transition in the cambial meristem involves stage-specific modulation of auxin response in hybrid aspen

“…Early analyses suggested that SDs could induce the cessation of cambial cell division by reducing cambial auxin levels (28,29). This hypothesis was subsequently rejected, however, when high-resolution MS analysis indicated that no reduction in cambial auxin levels occurred during the activitydormancy cycle (30). These results were surprising, because if cambial auxin levels remain unchanged during growth cessation, what would be the mechanism that inhibits cell division, a process normally promoted by auxin?…”

Lignin and Biomass: A Negative Correlation for Wood Formation and Lignin Content in Trees

“…All major classes of phytohormones may have important roles in xylogenesis (Aloni et al, 1990;Uggla et al, 1998;Andersson-Gunneras et al, 2006), and therefore could affect both growth and lignin. An integrated genetic, transcriptional, and metabolic approach could provide a systematic path to determine the mechanism of control.…”

Comparing needle and shoot phenology with xylem development on three conifer species in Italy

“…On the one hand, the influence of environmental factors, well documented in dendroecology through climate-growth relationships (Schweingruber, 1996), was demonstrated to be complicated by the asynchronous and long lasting production and maturation of xylem cells Fonti et al, 2007;Seo et al, 2008). On the other, the dynamics of wood formation is strongly determined by internal factors, such as cambial age, carbohydrate availability and hormone distribution Uggla et al, 1998;2001). In particular, since the 1960s biologists have been trying to explain (or interpret) the influence of auxin on cambial activity and latewood formation (Aloni 2001;Avery et al, 1937;Denne and Wilson, 1977;Larson, 1969).…”

“…Auxin participates in the reactivation of the vascular cambium, inducing the production of xylem and phloem (Larson, 1969) and regulating rate and duration of the developmental processes during xylogenesis (Tuominen et al, 1997;Uggla et al, 1998). Larson's hypothesis affirms that, with the auxin basipetal movement, periclinal divisions in the cambium should also begin at the base of the buds and spread downwards towards the stem (Denne, 1979;Lachaud et al, 1999;Larson, 1969).…”