Radial concentration gradients of morphogenetic substances are likely to be involved in positional signalling for the developmental programming during cambial growth and wood differentiation. In fact, the multiple roles of auxin in the regulation of vascular development and wood formation might be better understood within the conceptional framework of positional signalling. However, this view has generally been neglected due to the lack of knowledge about its distribution patterns over the vascular cambium.
With the use of a novel microscale mass-spectrometry method for accurate measurements of pg levels of IAA in extracts from 1 mg of plant tissue, the radial distribution of endogenous IAA over the cambial region of Pinus sylvestris (L.) has been visualised. Thirty µm consecutive tangential cryosections were obtained from the cambial region, including tissues between the mature phloem and xylem, and IAA was analysed in each section. In an active cambium, steep concentration gradients were found over the cambial meristem and its developing derivatives, with the highest concentration in the cell division zone levelling down to trace amounts in the mature tissues on both sides. It was also found that the IAA concentration in the dormant cambium (obtained in mid-January) was similar to that in the active cambium. Based on the occurrence of a radial IAA gradient together with additional data on endogenous IAA dynamics in the vascular region, a model of IAA regulation of seasonal and spatial variations in wood formation will be presented. The model suggests a role for polarly transported IAA in defining the developmental state of the cells during cambial growth and wood differentiation.
Key words: radial concentration gradients, Pinus sylvestris, wood development.
Correspondence: Björn Sundberg, Dept. of Forest Genetics and Plant Physiology, The Swedish Univ. of Agricultural Sciences, S-901 83 Umeå, Sweden
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E-mail: bjorn.sundberg@genfys.slu.se