Evolution of the Receptor-Like Kinase gene family in land plants
Melissa D. Lehti-Shiu and Shin-Han Shiu
Department of Plant Biology, Michigan State University, East Lansing, MI 48824, USA.
The Receptor-Like Kinase (RLK)/Pelle family is one of the largest plant gene families.
Members of this family are presumed to be the major players in the perception and
transduction of environmental and developmental signals in plants. To uncover? the
evolutionary history of this gene family, we conducted a phylogenetic analysis of the
kinase superfamily in A. thaliana, rice, poplar, and moss (Physcomitrella patens) and the
unicellular algae, Chlamydomonas reinhardtii. We found that moss has a substantial
number of RLK/Pelle genes (both receptor and non-receptor types); however, no
RLK/Pelle genes were found in Chlamydomonas. Intrestingl, expansion of this family
has occurred independently in all land plant lineages, indicating that RLKs were a major
source of innovation in general among land plants. In addition to large family size,
another important feature of RLKs is the diversity in its domain organization, suggesting
that natural selection favor the creation of new types of RLKs through domain fusion
events. Out of 25 types of RLKs with distinct domain configurations, 22 and 17 can be
found in poplar and moss, respectively, indicating that most of the receptor
configurations were established early in land plant evolution. These highly conserved
RLK types include receptor kinases with extracellular domains capable of interacting
with fungal and bacterial cell wall components. The conservation of domains involved in
sensing biotic stresses suggests that RLKs have played important roles in stress response
throughout the evolution of land plants. Previously we showed that, based on very
limited functional information, RLKs involved in defense responses were more likely to
be expanded and located in tandem clusters than those RLKs with roles in development.
To determine whether this is a general feature of RLKs involved in the perception of
biotic and abiotic stresses, we compared the expression patterns of RLKs after treatment
with different stress conditions based on the AtGenExpress stress array data. We found
that RLKs that have undergone recent expansion are more likely to be induced under the
stress conditions examined. Furthermore, this relationship can be attributed mostly to the
expansion of tandem duplicated RLKs. We also observed that different RLK/Pelle
subfamilies differ significantly in the breadth of responsiveness to stress. Together these
results point to a mechanism by which RLK duplication could have lead to improved
stress responsiveness in plants.