Signaling in Microorganisms and Plants:- Signaling Systems of Plants Have Some of the Same Components Used by Microbes and Mammals
Like animals, vascular plants must have a means of communication between tissues to coordinate and direct growth and development; to adapt to conditions of O2, nutrients, light, and temperature; and to warn of the presence of noxious chemicals and damaging pathogens (Fig. 12–27). At least a billion years of evolution have passed since the plant and animal branches of the eukaryotes diverged, which is reflected in the differences in signaling mechanisms: some plant mechanisms are conserved—that is, are similar to those in animals (protein kinases, scaffold proteins, cyclic nucleotides, electrogenic ion pumps, and gated ion channels); some are similar to bacterial two-component systems; and some are unique to plants (light-sensing mechanisms, for example) (Table 12–7). The genome of the widely studied plant Arabidopsis thaliana, for example, encodes about 1,000 protein Ser/Thr kinases, including about 60 MAPKs and nearly 400 membrane-associated receptor kinases that phosphorylate Ser or Thr residues; a variety of protein phosphatases; scaffold proteins that bring other proteins together in signaling complexes; enzymes for the synthesis and degradation of cyclic nucleotides; and 100 or more ion channels, including about 20 gated by cyclic nucleotides. Inositol phospholipids are present, as are kinases that interconvert them by phosphorylation of inositol head groups.
FIGURE 12–27 Some stimuli that produce responses in plants.
However, some types of signaling proteins common in animal tissues are not present in plants, or are rep resented by only a few genes. Cyclic nucleotide dependent protein kinases (PKA and PKG) appear to be absent, for example. Heterotrimeric G proteins and protein Tyr kinase genes are much less prominent in the plant genome, and serpentine (G protein–coupled) receptors, the largest gene family in the human genome (>1,000 genes), are very sparsely represented in the plant genome. DNA-binding nuclear steroid receptors are certainly not prominent, and may be ab sent from plants. Although plants lack the most widely conserved light-sensing mechanism present in animals (rhodopsin, with retinal as pigment), they have a rich collection of other light-detecting mechanisms not found in animal tissues—phytochromes and cryptochromes, for example (Chapter 19). The kinds of compounds that elicit signals in plants are similar to certain signaling molecules in mammals (Fig. 12–28). Instead of prostaglandins, plants have jasmonate; instead of steroid hormones, brassinosteroids.
FIGURE 12–28 Structural similarities between plant and animal signals. The plant signals jasmonate, indole-3-acetate, and brassinolide resemble the mammalian signals prostaglandin E1, serotonin, and estradiol. About 100 different small peptides serve as plant signals, and both plants and animals use compounds derived from aromatic amino acids as signals.
