Catherine Lenne

Posture control and skeletal mechanical acclimation in terrestrial plants : Implications for mechanical modeling of plant architecture

Self-supporting plant stems are slender, erect structures that remain standing while growing in highly variable mechanical environments. Such ability is not merely related to an adapted mechanical design in terms of material-specific stiffness and stem tapering. As many terrestrial standing animals do, plant stems regulate posture through active and coordinated control of motor systems and acclimate their skeletal growth to prevailing loads. This analogy probably results from mechanical challenges on standing organisms in an aerial environment with low buoyancy and high turbulence. But the continuous growth of plants submits them to a greater challenge. In response to these challenges, land plants implemented mixed skeletal and motor functions in the same anatomical elements. There are two types of kinematic design: (1) plants with localized active movement (arthrophytes) and (2) plants with continuously distributed active movements (contortionists). The control of these active supporting systems involves gravi- and mechanoperception, but little is known about their coordination at the whole plant level. This more active view of the control of plant growth and form has been insufficiently considered in the modeling of plant architecture. Progress in our understanding of plant posture and mechanical acclimation will require new biomechanical models of plant architectural development.

Characterization and expression analysis under bending and other abiotic factors of PtaZFP2, a poplar gene encoding a Cys2/His2 zinc finger protein

In plants, mechanoperception and transduction of mechanical signals have been studied essentially in Arabidopsis thaliana L. and Lycopersicon esculentum L. plants, i.e., in nonwoody plants. Here, we have described the isolation of both the full-length cDNA and the regulatory region of PtaZFP2, encoding a member of Cys2/His2 zinc finger protein (ZFP) family in Populus tremula L. x Populus alba L. Time course analysis of expression demonstrated that PtaZFP2 mRNA accumulated as early as 5 min in response to a controlled stem bending and is restricted to the organ where the mechanical stimulus is applied. The real-time quantitative Reverse Transcriptase Polymerase Chain Reaction experiments showed that PtaZFP2 was also rapidly up-regulated in poplar stems in response to gravitropism suggesting that PtaZFP2 is induced by different mechanical signals. Abundance of PtaZFP2 transcripts also increased highly in response to wounding and to a weaker extent to salt treatment and cold, which is consistent with the numerous putative cis-elements found in its regulatory region. As in other species, these data suggest that Cys2/His2 ZFPs could function in poplar as key transcriptional regulators in the acclimation response to different environmental factors.

Phylogenetic Study of Plant Q-type C2H2 Zinc Finger Proteins and Expression Analysis of Poplar Genes in Response to Osmotic, Cold and Mechanical Stresses

Plant Q-type C2H2 zinc finger transcription factors play an important role in plant tolerance to various environmental stresses such as drought, cold, osmotic stress, wounding and mechanical loading. To carry out an improved analysis of the specific role of each member of this subfamily in response to mechanical loading in poplar, we identified 16 two-fingered Q-type C2H2-predicted proteins from the poplar Phytozome database and compared their phylogenetic relationships with 152 two-fingered Q-type C2H2 protein sequences belonging to more than 50 species isolated from the NR protein database of NCBI. Phylogenetic analyses of these Q-type C2H2 proteins sequences classified them into two groups G1 and G2, and conserved motif distributions of interest were established. These two groups differed essentially in their signatures at the C-terminus of their two QALGGH DNA-binding domains. Two additional conserved motifs, MALEAL and LVDCHY, were found only in sequences from Group G1 or from Group G2, respectively. Functional significance of these phylogenetic divergences was assessed by studying transcript accumulation of six poplar C2H2 Q-type genes in responses to abiotic stresses; but no group specificity was found in any organ. Further expression analyses focused on PtaZFP1 and PtaZFP2, the two genes strongly induced by mechanical loading in poplars. The results revealed that these two genes were regulated by several signalling molecules including hydrogen peroxide and the phytohormone jasmonate.

Jr-ZFP2, encoding a Cys2/His2-type transcription factor, is involved in the early stages of the mechano-perception pathway and specifically expressed in mechanically stimulated tissues in woody plants

Plants respond to environmental mechanical stimulation, such as wind, by modifying their growth and development. To study the molecular effects of stem bending on 3-week-old walnut trees, a cDNA-AFLP approach was developed. This study allowed the identification of a cDNA, known as Jr-ZFP2, encoding a Cys2/His2-type two-zinc-fingered transcription factor. Reverse transcriptase-polymerase chain reaction analysis confirmed that Jr-ZFP2 mRNA accumulation is rapidly and transiently induced after mechanical stimulation. After bending, Jr-ZFP2 transcript increase was restricted to the stem, the organ where the mechanical solicitation was applied. Furthermore, other abiotic factors, such as cold or salt, did not modify Jr-ZFP2 mRNA accumulation in walnut stems under our experimental conditions, whereas growth studies demonstrated that salt stress was actually perceived by the plants. These results suggest that the regulation of Jr-ZFP2 expression is more sensitive to mechanical stimulus. This gene will be a good marker for studying the early stages of mechanical perception in woody plants.

To respond or not to respond, the recurring question in plant mechanosensitivity

In nature, terrestrial plants experience many kinds of external mechanical stimulation and respond by triggering a network of signaling events to acclimate their growth and development. Some environmental cues, especially wind, recur on time scales varying from seconds to days. Plants thus have to adapt their sensitivity to such stimulations to avoid constitutive activation of stress responses. The study of plant mechanosensing has been attracting more interest in the last two decades, but plant responses to repetitive mechanical stimulation have yet to be described in detail. In this mini review, alongside classic experiments we survey recent descriptions of the kinetics of plant responses to recurrent stimulation. The ability of plants to modulate their responses to recurrent stimulation at the molecular, cellular, or organ scale is also relevant to other abiotic stimuli. It is possible that plants reduce their responsiveness to environmental signals as a function of their recurrence, recovering full sensitivity several days later. Finally, putative mechanisms underlying mechanosensing regulation are discussed.