Root contraction, found among a variety of agave and yucca species, can help plants endure unfavorable seasons and is associated with greater root hydraulic conductivity near the soil surface. The goals of this study were to better understand the adaptive significance of contractile roots in a phylogenetic context and to further investigate the physiological processes that regulate root contraction. To better resolve evolutionary relationships within the genus Agave, we sequenced a number of genes for several species of agave with and without contractile roots. To better understand the physiology of contractile roots, we investigated the effects of the hormones gibberellin and ethylene on the process of root contraction. Wires inserted at the base of the plants just above the roots were measured weekly for eight weeks with gibberellin and sixteen weeks with ethylene, to determine the amount of root contraction that occurred.

Results/Conclusions

Acquired DNA sequences were combined with other published sequences as well as with morphological traits to generate a phylogenetic tree, allowing inferences to be made about the evolution of contractile roots in the agave family.  For plants under all hormone treatments, root contraction occurred over time; however, there was no significant difference in root contraction between plants treated with gibberellin and control plants (P = 0.524).  In  contrast, there were significant differences between plants treated with the ethylene precursor (and stimulus) 1-aminocyclopropane-1-carboxylic acid (ACC), those treated with aminoethoxyvinylglycine (AVG, an ethylene inhibitor), and control plants (ANOVA; P < 0.001).  Data collected from these two lines of research offer further understanding into the phenomenon of contractile roots including their phylogenetic distribution within the genus Agave as well as a better understanding of the role hormones may play in stimulating or inhibiting root contraction.