| Gravitropism, which guides plant organs' growth at a defined angle from the gravity vector, plays an important role in plant morphogenesis. Yet, only few proteins have thus far been implicated in gravity signal transduction. A proteomic approach was undertaken to identify root-tip Tris buffer-soluble proteins whose abundance, localization and/or modification are quickly altered in response to gravistimulation. One such protein is Adenosine Kinase (ADK), whose 2D protein spot intensity increases within 12 min of gravistimulation, then decreases to very low levels between 12 and 30 min of gravistimulation. The decrease in the 2D protein spot intensity appears to result from ADK relocalization to the nucleus. Reverse genetics showed that mutation in ADK1 results in reduced root sensitivity to gravistimulation and altered kinetics of root gravicurvature, suggesting its role in early gravi-perception and/or signal transduction. Interestingly, the PIN3 auxin efflux facilitator is misexpressed and does not relocalize to the new physical bottom of the columella cells upon gravistimulation in adk1-1, as it does in wild-type roots. Consequently, adk1-1 roots cannot develop a lateral auxin gradient across the cap, necessary for the curvature response. Surprisingly, mutant statocytes are as capable of responding to gravistimulation by cytoplasmic alkalinization as wild type, indicating that gravity-induced PIN3 relocalization and cytoplasmic alkalinization in root statocytes are outputs of distinct pathways.; Because ADK plays a key role in the regulation of the S-Adenosyl-L-Methionine cycle, a pathway that leads to polyamine synthesis and affects cytokinin homeostasis, I investigated a potential role for these regulatory compounds in root gravitropism. Firstly, I demonstrate that MGBG, an inhibitor of spermidine and spermine synthesis, inhibits wild-type root gravitropism, whereas addition of spermine in the growth medium partially rescues the root gravitropic defect of adk1-1. Furthermore, I show that a mutation in SPMS, a gene that encodes spermine synthases, also affects root gravitropism. Similarly, purine riboside, a cytokinin antagonist, also affects root gravitropism, whereas a mutation in the cytokinin receptors AHK2 affects both root gravitropism and root-growth sensitivity to NPA. These data suggest a role for cytokinin and the AHK2 cytokinin receptor in the regulation of polar auxin transport and root gravitropism. |
