AUTHOR=Conn Caitlin E. , Nelson David C. 

TITLE=Evidence that KARRIKIN-INSENSITIVE2 (KAI2) Receptors may Perceive an Unknown Signal that is not Karrikin or Strigolactone

JOURNAL=Frontiers in Plant Science

VOLUME=6

YEAR=2016

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2015.01219

DOI=10.3389/fpls.2015.01219

ISSN=1664-462X

ABSTRACT=<p>The α/β-hydrolases KAI2 and D14 are paralogous receptors for karrikins and strigolactones, two classes of plant growth regulators with butenolide moieties. KAI2 and D14 act in parallel signaling pathways that share a requirement for the F-box protein MAX2, but produce distinct growth responses by regulating different members of the SMAX1-LIKE/D53 family. <italic>kai2</italic> and <italic>max2</italic> mutants share seed germination, seedling growth, leaf shape, and petiole orientation phenotypes that are not found in <italic>d14</italic> or SL-deficient mutants. This implies that KAI2 recognizes an unknown, endogenous signal, herein termed KAI2 ligand (KL). Recent studies of ligand-specificity among <italic>KAI2</italic> paralogs in basal land plants and root parasitic plants suggest that karrikin and strigolactone perception may be evolutionary adaptations of KL receptors. Here we demonstrate that evolutionarily conserved <italic>KAI2c</italic> genes from two parasite species rescue multiple phenotypes of the <italic>Arabidopsis kai2</italic> mutant, unlike karrikin-, and strigolactone-specific <italic>KAI2</italic> paralogs. We hypothesize that KAI2c proteins recognize KL, which could be an undiscovered hormone.</p>