Friend or foe: Hybrid proline-rich proteins determine how plants respond to beneficial and pathogenic microbes.

Plant plastids generate signals, including some derived from lipids, that need to be mobilized to effect signaling. We used informatics to discover potential plastid membrane proteins involved in microbial responses in Arabidopsis (Arabidopsis thaliana). Among these are proteins co-regulated with the systemic immunity component AZELAIC ACID INDUCED 1, a hybrid proline-rich protein (HyPRP), and HyPRP superfamily members. HyPRPs have a transmembrane domain, a proline-rich region (PRR), and a lipid transfer protein domain. The precise subcellular location(s) and function(s) are unknown for most HyPRP family members. As predicted by informatics, a subset of HyPRPs has a pool of proteins that target plastid outer envelope membranes via a mechanism that requires the PRR. Additionally, two HyPRPs may be associated with thylakoid membranes. Most of the plastid- and nonplastid-localized family members also have pools that localize to the endoplasmic reticulum, plasma membrane, or plasmodesmata. HyPRPs with plastid pools regulate, positively or negatively, systemic immunity against the pathogen Pseudomonas syringae. HyPRPs also regulate the interaction with the plant growth-promoting rhizobacteria Pseudomonas simiae WCS417 in the roots to influence colonization, root system architecture, and/or biomass. Thus, HyPRPs have broad and distinct roles in immunity, development, and growth responses to microbes and reside at sites that may facilitate signal molecule transport.

Short-term response to waterlogging in Quercus petraea and Quercus robur: A study of the root hydraulic responses and the transcriptional pattern of aquaporins.

We characterized the short-term response to waterlogging in Quercus petraea (Matt.) Liebl. and Quercus robur L. as the initial response towards their known long-term differences in tolerance to waterlogging. One-month old seedlings were subjected to hypoxic stress and leaf gas exchange, shoot water potential (Ψs) and root hydraulic conductivity (Lpr) were measured. In parallel, the expression of nine aquaporins (AQPs) along the primary root was analysed by quantitative RT-PCR. Results showed a similar reduction in net assimilation (A) and stomatal conductance (gs) for the two species. Notably, the response of Lpr differed temporally between the two species. Q. robur seedlings exhibited a significant early decline of Lpr within the first 5 h that returned to control levels after 48 h, whereas Q. petraea seedlings showed a delayed response with a significant decrease of Lpr exhibited only after 48 h. Transcriptional profiling revealed that three genes (PIP1;3, TIP2;1 and TIP2;2) were differentially regulated under stress conditions in the two oak species. Taken together, these results suggested species-specific responses to short-term waterlogging in terms of root water transport.

Differential responses in sympatric tree species exposed to waterlogging.

Climate change is projected to have a significant ecological impact on natural ecosystems, most notably through direct and indirect modifications of local precipitation regimes. In addition, anthropic activities such as the removal of vegetation, soil proofing due to building, the absence of storm drains and crop over-irrigation will all increase the occurrence of flooding. As a result, forest species, and more specifically trees, will increasingly be exposed to soil waterlogging. It is now well established that such flooding events can lead to changes in forest distribution and composition. For such reasons, it is becoming increasingly important to study forest ecosystems and more particularly the adaptive potential of tree species to better understand the ecological plasticity of forest communities to environmental modifications.

Identification and expression of nine oak aquaporin genes in the primary root axis of two oak species, Quercus petraea and Quercus robur.

Aquaporins (AQPs) belong to the Major Intrinsic Protein family that conducts water and other small solutes across biological membranes. This study aimed to identify and characterize AQP genes in the primary root axis of two oak species, Quercus petraea and Quercus robur. Nine putative AQP genes were cloned, and their expression was profiled in different developmental root zones by real-time PCR. A detailed examination of the predicted amino acid sequences and subsequent phylogenetic analysis showed that the isolated AQPs could be divided into two subfamilies, which included six plasma membrane intrinsic proteins (PIPs) and three tonoplast intrinsic proteins (TIPs). We characterized the anatomical features of the roots and defined three developmental root zones: the immature, transition and mature zones. Expression analysis of the AQPs was performed according to these root developmental stages. Our results showed that the expression of PIP2;3 and TIP1 was significantly higher in Quercus petraea compared with Quercus robur in the three root zones. However, PIP2;1 and TIP2;1 were found to be differentially expressed in the mature zone of the two oak species. Of the nine AQP genes identified and analyzed, we highlighted four genes that might facilitate a deeper understanding of how these two closely related tree species adapted to different environments.

Contrasting growth and adaptive responses of two oak species to flooding stress: role of non-symbiotic haemoglobin.

Soil flooding is an environmental constraint that is increasingly important for forest ecosystems, affecting tree growth and regeneration. As a result, selection pressure will alter forest diversity and distribution by favouring tree species tolerant of soil oxygen deprivation. Sessile and pedunculate oaks are the most abundant oak species and they exhibit a strong differential tolerance to waterlogging. In order to gain some understanding of the mechanisms of tolerance of both species to hypoxia, we undertook the characterization of the physiological, morphological, cellular and molecular responses of both species to flooding stress. Our results indicate that pedunculate oak, the more tolerant species, succeeded in maintaining its growth, water status and photosynthetic activity at a higher level than sessile oak. Furthermore, pedunculate oak developed aerenchyma in its root cortex as well as adventitious roots. The later exhibited a strong accumulation of class1 non-symbiotic haemoglobin localized by in situ hybridization in the protoderm and in some cortical cells. In conclusion, the higher tolerance of pedunculate oak to flooding was associated with an enhanced capacity to maintain photosynthesis and water homeostasis, coupled with the development of adaptive features (aerenchyma, adventitious roots) and with a higher expression of non-symbiotic haemoglobin in the roots.

[Reactive oxygen species, stress and cell death in plants]. / Formes réactives de l'oxygène, stress et mort cellulaire chez les plantes.

Plants are constantly exposed to changes in environmental conditions. During periods of stress, the cellular redox homeostasis is altered as a result of reactive oxygen species accumulation. The change in redox is responsible for the symptoms commonly observed during periods of stress and reflects the phytotoxic nature of oxygen radical accumulation. However, oxygen radicals have recently been identified as key actors in the response to stress and their role as secondary messengers is now clearly established. The identification of their role in gene regulation has allowed one to identify them as key regulators in the induction and execution of programmed cell death typically observed during developmental processes as well as during stress responses. This review presents recent advances in the characterisation of the role of reactive oxygen species in plants.

A novel nonsymbiotic hemoglobin from oak: cellular and tissue specificity of gene expression.

This study presents the isolation and characterization of a novel nonsymbiotic Hb gene from sessile oak (Quercus petraea) seedlings, herein designated QpHb1. The cellular and tissue expression of QpHb1 was analysed by Northern blotting and in situ hybridization. The encoded protein was predicted to consist of 161 amino acid residues, and shares 71 and 51% amino acid sequence identity with the Arabidopsis class 1 and 2 nonsymbiotic Hb, respectively. Northern blot analysis revealed that QpHb1 was strongly expressed in roots. Spatial expression analysis of QpHb1 in the root apical region of sessile oak by in situ hybridization indicated that transcripts were mostly abundant in protoxylem cell initials, some cortical cells and the protoderm. In addition, when comparing the expression profile of QpHb1 in sessile and pedunculate oak (Quercus robur), two species with contrasted hypoxia tolerance, the transcript level of QpHb1 rose early in the most flood-tolerant species, pedunculate oak, during root submergence. The spatial-temporal expression of QpHb1 suggests that this gene could participate in perception and signalling during hypoxia.

A novel non-symbiotic hemoglobin from oak: Roles in root signalling and development?

The cellular and molecular adaptations of non-model woody species to environmental changes are still poorly understood. We have cloned and characterised a novel non-symbiotic hemoglobin from oak roots (QpHb1) which exhibits a specific cellular distribution in the root. The QpHb1 gene is strongly expressed in the protoderm and the protoxylem cells in two Quercus species (Q. petraea and Q. robur) with contrasting adaptive potential to drought and flooding. The constitutive expression of QpHb1 in both oak species in specific root tissues combined with the reported presence of nitric oxide in the same tissues and its potential for protein S-nitrosylation could support a role for non-symbiotic hemoglobins in signalling changes in the root environment and/or in controlling some aspects of root development.