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 eu-karyotes 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, elec-trogenic ion pumps, and gated ion channels); some are similar to bacterial two-component systems; and some are unique to plants (light-sensing mechanisms, for ex-

FIGURE 12-26 The two-component signaling mechanism in bacterial chemotaxis. When an attractant ligand (A) binds to the receptor domain of the membrane-bound receptor, a protein His kinase in the cytosolic domain (component 1) is activated and autophosphorylates on a His residue. This phosphoryl group is then transferred to an Asp residue on component 2 (in some cases a separate protein; in others, another domain of the receptor protein). After phosphorylation on Asp, component 2 moves to the base of the flagellum, where it determines the direction of rotation of the flagellar motor.

Attractant

Receptor His kinase (component 1)

Receptor His kinase (component 1)

Attractant

Acetly Phosphate

Phosphorylated form of component 2 reverses direction of motor

Plasma membrane

Response regulator (component 2)

Rotary motor (controls flagellum)

Plasma membrane

Response regulator (component 2)

Phosphorylated form of component 2 reverses direction of motor

Rotary motor (controls flagellum)

Microorganisms FIGURE 12-27 Some stimuli that produce responses in plants.

ample) (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 phospho-rylation of inositol head groups.

However, some types of signaling proteins common in animal tissues are not present in plants, or are represented 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 absent 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 jas-monate; instead of steroid hormones, brassinosteroids.

TABLE 12-7 Signaling Components Present in Mammals, Plants, or Bacteria

Signaling protein Mammals Plants Bacteria

Electrogenic ion pumps + + +

Two-component His kinases + + +

Guanylyl cyclase + +

Receptor protein kinases (Ser/Thr) + +

Ca2+ as second messenger + +

Calmodulin, CaM-binding protein + +

Cyclic nucleotide-gated channels + +

IP3-gated Ca2+ channels + +

Phosphatidylinositol kinases + +

Serpentine receptors + +/-

Pl-specific phospholipase C + ?

Tyrosine kinase receptors + ?

Nuclear steroid receptors +

Protein kinase A +

Protein kinase G +

Plants

Plants

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