Bernard Genty

Perturbation of indole-3-butyric acid homeostasis by the UDP-glucosyltransferase UGT74E2 modulates Arabidopsis architecture and water stress tolerance.

The hydrogen peroxide–responsive UDP-glucosyltransferase UGT74E2 from Arabidopsis thaliana is shown to be involved in modulation of plant architecture and water stress response through its activity toward the auxin indole-3-butyric acid (IBA). Evidence is provided that, during water stress, IBA and IBA-glucose levels increase, and auxins help maintain the photosynthetic capacity under stress. Reactive oxygen species and redox signaling undergo synergistic and antagonistic interactions with phytohormones to regulate protective responses of plants against biotic and abiotic stresses. However, molecular insight into the nature of this crosstalk remains scarce. We demonstrate that the hydrogen peroxide–responsive UDP-glucosyltransferase UGT74E2 of Arabidopsis thaliana is involved in the modulation of plant architecture and water stress response through its activity toward the auxin indole-3-butyric acid (IBA). Biochemical characterization of recombinant UGT74E2 demonstrated that it strongly favors IBA as a substrate. Assessment of indole-3-acetic acid (IAA), IBA, and their conjugates in transgenic plants ectopically expressing UGT74E2 indicated that the catalytic specificity was maintained in planta. In these transgenic plants, not only were IBA-Glc concentrations increased, but also free IBA levels were elevated and the conjugated IAA pattern was modified. This perturbed IBA and IAA homeostasis was associated with architectural changes, including increased shoot branching and altered rosette shape, and resulted in significantly improved survival during drought and salt stress treatments. Hence, our results reveal that IBA and IBA-Glc are important regulators of morphological and physiological stress adaptation mechanisms and provide molecular evidence for the interplay between hydrogen peroxide and auxin homeostasis through the action of an IBA UGT.

Constitutive activation of a plasma membrane H+‐ATPase prevents abscisic acid‐mediated stomatal closure

Light activates proton (H+)‐ATPases in guard cells, to drive hyperpolarization of the plasma membrane to initiate stomatal opening, allowing diffusion of ambient CO2 to photosynthetic tissues. Light to darkness transition, high CO2 levels and the stress hormone abscisic acid (ABA) promote stomatal closing. The overall H+‐ATPase activity is diminished by ABA treatments, but the significance of this phenomenon in relationship to stomatal closure is still debated. We report two dominant mutations in the OPEN STOMATA2 (OST2) locus of Arabidopsis that completely abolish stomatal response to ABA, but importantly, to a much lesser extent the responses to CO2 and darkness. The OST2 gene encodes the major plasma membrane H+‐ATPase AHA1, and both mutations cause constitutive activity of this pump, leading to necrotic lesions. H+‐ATPases have been traditionally assumed to be general endpoints of all signaling pathways affecting membrane polarization and transport. Our results provide evidence that AHA1 is a distinct component of an ABA‐directed signaling pathway, and that dynamic downregulation of this pump during drought is an essential step in membrane depolarization to initiate stomatal closure.

Bacteriophytochrome controls photosystem synthesis in anoxygenic bacteria

Plants use a set of light sensors to control their growth and development in response to changes in ambient light. In particular, phytochromes exert their regulatory activity by switching between a biologically inactive red-light-absorbing form (Pr) and an active far-red-light absorbing form (Pfr). Recently, biochemical and genetic studies have demonstrated the occurrence of phytochrome-like proteins in photosynthetic and non-photosynthetic bacteria—but little is known about their functions. Here we report the discovery of a bacteriophytochrome located downstream from the photosynthesis gene cluster in a Bradyrhizobium strain symbiont of Aeschynomene. The synthesis of the complete photosynthetic apparatus is totally under the control of this bacteriophytochrome. A similar behaviour is observed for the closely related species Rhodopseudomonas palustris, but not for the more distant anoxygenic photosynthetic bacteria of the genus Rhodobacter, Rubrivivax or Rhodospirillum. Unlike other (bacterio)phytochromes, the carboxy-terminal domain of this bacteriophytochrome contains no histidine kinase features. This suggests a light signalling pathway involving direct protein–protein interaction with no phosphorelay cascade. This specific mechanism of regulation may represent an important ecological adaptation to optimize the plant–bacteria interaction.

Bacteriophytochrome and regulation of the synthesis of the photosynthetic apparatus in Rhodopseudomonas palustris: pitfalls of using laboratory strains

The synthesis of the photosynthetic apparatus of different strains of Rhodopseudomonas palustris has been studied as a function of the oxygen concentration and far-red light. For strain CEA001, only a small amount of photosynthetic apparatus is synthesized in the dark for oxygen concentration higher than 8% whereas synthesis is strongly enhanced by far-red light illumination. This enhancement is due to the action of a bacteriophytochrome (ORF2127/ORF2128), which antagonizes the repressor PpsR. On the contrary, a large fraction of photosystem is synthesized in the dark and far-red illumination induces no enhancement in strain CGA009. This difference in phenotype of strain CGA009 is explained by a single point-mutation R428C in the helix-turn-helix DNA binding motif of PpsR, rendering it inactive. In addition, a frame-shift mutation had occurred in the gene encoding bacteriophytochrome (ORF2127/ORF2128), conducting to a truncated inactive sensor. We propose that these mutations occurred in culture. Bacteria have developed a sophisticated regulatory process to synthesize their photosynthetic apparatus when light is available. This process is a critical advantage for the bacteria under natural conditions since they optimize their development depending on the available energy resources. On the contrary, under laboratory growth conditions where there is no substrate limitation, there is no crucial need for such a regulation and deleterious mutations affecting this process are of no importance.

Estimating Mesophyll Conductance from Measurements of C18OO Photosynthetic Discrimination and Carbonic Anhydrase Activity

A new model based on the relationship between C18OO photosynthetic discrimination and carbonic anhydrase activity estimates mesophyll conductance in C3 and C4 plants. Carbonic anhydrase (CA) activity in leaves catalyzes the 18O exchange between CO2 and water during photosynthesis. This feature has been used to estimate the mesophyll conductance to CO2 (gm) from measurements of online C18OO photosynthetic discrimination (∆18O). Based on CA assays on leaf extracts, it has been argued that CO2 in mesophyll cells should be in isotopic equilibrium with water in most C3 species as well as many C4 dicot species. However, this seems incompatible with ∆18O data that would indicate a much lower degree of equilibration, especially in C4 plants under high light intensity. This apparent contradiction is resolved here using a new model of C3 and C4 photosynthetic discrimination that includes competition between CO2 hydration and carboxylation and the contribution of respiratory fluxes. This new modeling framework is used to revisit previously published data sets on C3 and C4 species, including CA-deficient plants. We conclude that (1) newly ∆18O-derived gm values are usually close but significantly higher (typically 20% and up to 50%) than those derived assuming full equilibration and (2) despite the uncertainty associated with the respiration rate in light, or the water isotope gradient between mesophyll and bundle sheath cells, robust estimates of ∆18O-derived gm can be achieved in both C3 and C4 plants.