Isoprene and ß-caryophyllene confer plant resistance via different plant internal signalling pathways.

Isoprene and other terpenoids are important biogenic volatile organic compounds in terms of atmospheric chemistry. Isoprene can aid plant performance under abiotic stresses, but the fundamental biological reasons for the high emissions are not completely understood. Here, we provide evidence of a previously unrecognized ecological function for isoprene and for the sesquiterpene, ß-caryophyllene. We show that isoprene and ß-caryophyllene act as core components of plant signalling networks, inducing resistance against microbial pathogens in neighbouring plants. We challenged Arabidopsis thaliana with Pseudomonas syringae, after exposure to pure volatile terpenoids or to volatile emissions of transformed poplar or Arabidopsis plants. The data suggest that isoprene induces a defence response in receiver plants that is similar to that elicited by monoterpenes and depended on salicylic acid (SA) signalling. In contrast, the sesquiterpene, ß-caryophyllene, induced resistance via jasmonic acid (JA)-signalling. The experiments in an open environment show that natural biological emissions are enough to induce resistance in neighbouring Arabidopsis. Our results show that both isoprene and ß-caryophyllene function as allelochemical components in complex plant signalling networks. Knowledge of this system may be used to boost plant immunity against microbial pathogens in various crop management schemes.

Roles of AGCVIII Kinases in the Hypocotyl Phototropism of Arabidopsis Seedlings.

Regulation of protein function by phosphorylation and dephosphorylation is an important mechanism in many cellular events. The phototropin blue-light photoreceptors, plant-specific AGCVIII kinases, are essential for phototropic responses. Members of the D6 PROTEIN KINASE (D6PK) family, representing a subfamily of the AGCVIII kinases, also contribute to phototropic responses, suggesting that possibly further AGCVIII kinases may potentially control phototropism. The present study investigates the functional roles of Arabidopsis (Arabidopsis thaliana) AGCVIII kinases in hypocotyl phototropism. We demonstrate that D6PK family kinases are not only required for the second but also for the first positive phototropism. In addition, we find that a previously uncharacterized AGCVIII protein, AGC1-12, is involved in the first positive phototropism and gravitropism. AGC1-12 phosphorylates serine residues in the cytoplasmic loop of PIN-FORMED 1 (PIN1) and shares phosphosite preferences with D6PK. Our work strongly suggests that the D6PK family and AGC1-12 are critical components for both hypocotyl phototropism and gravitropism, and that these kinases control tropic responses mainly through regulation of PIN-mediated auxin transport by protein phosphorylation.

Dynamic PIN-FORMED auxin efflux carrier phosphorylation at the plasma membrane controls auxin efflux-dependent growth.

The directional distribution of the phytohormone auxin is essential for plant development. Directional auxin transport is mediated by the polarly distributed PIN-FORMED (PIN) auxin efflux carriers. We have previously shown that efficient PIN1-mediated auxin efflux requires activation through phosphorylation at the four serines S1-S4 in Arabidopsis thaliana The Brefeldin A (BFA)-sensitive D6 PROTEIN KINASE (D6PK) and the BFA-insensitive PINOID (PID) phosphorylate and activate PIN1 through phosphorylation at all four phosphosites. PID, but not D6PK, can also induce PIN1 polarity shifts, seemingly through phosphorylation at S1-S3. The differential effects of D6PK and PID on PIN1 polarity had so far been attributed to their differential phosphosite preference for the four PIN1 phosphosites. We have mapped PIN1 phosphorylation at S1-S4 in situ using phosphosite-specific antibodies. We detected phosphorylation at PIN1 phosphosites at the basal (rootward) as well as the apical (shootward) plasma membrane in different root cell types, in embryos, and shoot apical meristems. Thereby, PIN1 phosphorylation at all phosphosites generally followed the predominant PIN1 distribution but was not restricted to specific polar sides of the cells. PIN1 phosphorylation at the basal and apical plasma membrane was differentially sensitive to BFA treatments, suggesting the involvement of different protein kinases or trafficking mechanisms in PIN1 phosphorylation control. We conclude that phosphosite preferences are not sufficient to explain the differential effects of D6PK and PID on PIN1 polarity, and suggest that a more complex model is needed to explain the effects of PID.