Beyond the sine law of plant gravitropism

Abstract

The great German plant physiologist Wilhelm Pfeffer rightly claimed that “no plant is entirely without the power of movement” (1). If this fact remains underappreciated, it is perhaps because plant movements typically unfold over minutes, hours, or days, and thus exceed the attention span of all but the most dedicated observer. Among the large array of plant movements, the tropisms—that is, those movements that are directed toward or away from an external stimulus such as gravity or light—are the most fascinating because they highlight beautifully the sentient nature of plants and the goal-directedness of their growth habit. The pervasiveness of plant tropisms is revealed when one stops to consider the unlikeliness that seeds lodged haphazardly within the crevices of a rugged terrain should sprout stems that reliably find their way up. Were it not for the ability of the young plants to sense light and gravity, forests would be impenetrable tangles of stems and branches growing in all directions. Their prevalence in the plant kingdom explains why tropisms have been an active area of research since the beginning of the 19th century. By the end of the 19th century, the field had reached such a level of development and popularity that eminent biologists, including Charles Darwin and Pfeffer, could devote entire books to the topic (1, 2). Given this long and illustrious tradition of research, one might expect today’s biologists to have extracted all useful information from standard observational research of plant tropisms. A paper published in PNAS should convince the readers that much can still be learned from careful, quantitative observation of biological processes. In a systematic study of shoot gravitropism in 11 taxa, Bastien et al. (3) at once establish the universal response to gravity as a process of initial stem curving followed by apical straightening and debunk the idea that current models of gravitropism offer a plausible explanation for the process.