Insect Herbivory of Leaves Affects the Auxin Flux Along Root Apices in Arabidopsis thaliana

Plants reduce their growth rate when confronted by insect attack, and in turn resources are redirected from growth to enhance resistance against herbivory. In this study, one possible signaling cascade was investigated for establishing the underlying mechanism for the slow growth rate that has been observed following insect herbivory. Our results showed that free jasmonate (JA) and auxin levels were elevated in both leaves and roots after Plutella xylostella L. attack which was accompanied by the transcriptional increase of the auxin biosynthetic YUCCA3 and YUCCA8 genes. Further examination of endogenous auxin flux using physiological micro-sensor profiling showed that near the surface of the root transition zone, the net auxin flux decreased after insect attack. Conversely, insect herbivory caused an increase in the net H + flux along the root surface with the most pronounced response occurring in the transition zone. Transcript levels of auxin transporter genes PIN1, PIN2, PIN3, PIN7 , and AUX1 were also reduced after insect attack. Together, the auxin and H + flux results indicate that the reduced growth after insect attack was likely associated with a decrease of auxin flux and proton secretion along the root tip.