Although plants and animals are both eukaryotes, they have evolved separately for more than a billion years. Their last common ancestor is thought to have been a unicellular eukaryote that had mitochondria but no chloroplasts; the plant lineage acquired chloroplasts after plants and animals diverged. The earliest fossils of multicellular animals and plants date from almost 600 million years ago. Thus, it seems that plants and animals evolved multicellularity independently, each starting from a different unicellular eukaryote, some time between 1.6 and 0.6 billion years ago (Figure 15–68).
If multicellularity evolved independently in plants and animals, the molecules and mechanisms used for cell communication will have evolved separately and would be expected to be different. There should be some degree of resemblance, however, because the genes in both plants and animals diverged from those contained by their last common unicellular ancestor. Thus, whereas both plants and animals use nitric oxide, cyclic GMP, Ca2+, and Rho family GTPases for signaling, there are no homologs of the nuclear receptor family, Ras, JAK, STAT, TGFβ,
Figure 15–68 The proposed divergence of plant and animal lineages from a common unicellular eukaryotic ancestor.
The plant lineage acquired chloroplasts after the two lineages diverged. Both lineages independently gave rise to multicellular organisms—plants and animals. (Paintings courtesy of John Innes Foundation.) unicellular ancestor of animals unicellular ancestor of plants CHLOROPLASTS chloroplast
Notch, Wnt, or Hedgehog encoded by the completely sequenced genome of Arabidopsis thaliana, the small flowering plant. Similarly, plants do not seem to use cyclic AMP for intracellular signaling. Nevertheless, the general strategies underlying signaling are frequently very similar in plants and animals. Both, for example, use enzyme-coupled cell-surface receptors, as we now discuss.
Receptor Serine/Threonine Kinases Are the Largest Class  of Cell-Surface Receptors in Plants
Most cell-surface receptors in plants are enzyme-coupled. However, whereas the largest class of enzyme-coupled receptors in animals is the receptor tyrosine kinase (RTK) class, this type of receptor is extremely rare in plants. Instead, plants rely largely on a great diversity of transmembrane receptor serine/threonine kinases, which have a typical serine/threonine kinase cytoplasmic domain and an extracellular ligand-binding domain. The most abundant types of these receptors have a tandem array of extracellular leucine-rich repeat structures and are therefore called leucine-rich repeat (LRR) receptor kinases.
There are about 175 LRR receptor kinases encoded by the Arabidopsis genome. These include a protein called Bri1, which forms part of a cell-surface steroid hormone receptor. Plants synthesize a class of steroids that are called brassinosteroids because they were originally identified in the mustard family Brassicaceae, which includes Arabidopsis. These signal molecules regulate the growth and differentiation of plants throughout their life cycle. Binding of a brassinosteroid to a Bri1 cell-surface receptor kinase initiates an intracellular signaling cascade that uses a GSK3 protein kinase and a protein phosphatase to regulate the phosphorylation and degradation of specific transcription regulatory proteins in the nucleus, and thereby specific gene transcription. Mutant plants that are deficient in the Bri1 receptor kinase are insensitive to brassinosteroids and are therefore dwarfs.