Control of Longitudinal Growth by Phytochrome

Abstract

The study of growth has been an important part of plant physiology especially since the term “growth” was narrowed to the meaning of “cellular lengthening” and auxin was generally regarded as being the growth-regulating factor. However, the molecular basis of growth regulation is still unsolved: the action mechanism of auxin is still under debate, as is the significance of extensin [142] and the existence of growth-limiting proteins [38]. There are a number of plant physiologists who feel that the importance of auxin has been greatly overestimated in the past and that the intense studies of auxin action in excised plant parts have possibly led to an unsatisfactory concept as far as the significance of auxin for the growth of an intact plant is concerned (cf. [76]). Indeed, there are very few instances in which the application of auxin to intact plants led to any increase in elongation at all. While the application of auxin to entire plants does produce effects on the orientation of leaves and sometimes curvatures of stems, an induction of significant alterations of the overall growth rate of stems and roots has not been observed. These facts have led some to the belief that auxin does not really control the rate of elongation under conditions of symmetrical straight growth in intact plants. Instead, some plant physiologists prefer to believe that auxin in the intact plant is mainly concerned with tropisms (cf. [76]).