Populus × canescens root suberization in reaction to osmotic and salt stress is limited to the developing younger root tip region

Root endodermal suberization induced by nitrate stress regulate apoplastic pathway rather than nitrate uptake in tobacco (Nicotiana tabacum L.)

Nitrogen levels and distribution in the rhizosphere strongly regulate the root architecture. Nitrate is an essential nutrient and an important signaling molecule for plant growth and development. Hydroponic experiments were conducted to investigate the differences in endodermal suberization in tobacco (Nicotiana tabacum L.) roots at three nitrate levels. Nitrogen accumulation was detected in the roots, shoots, and xylem sap. Nitrate influx on the root surface was also measured using the non-invasive self-referencing microsensor technique (SRMT). RNA-Seq analysis was performed to identify the genes related to endodermal suberization, nitrate transport, and endogenous abscisic acid (ABA) biosynthesis. The results showed that root length, root-shoot ratio, nitrate influx on the root surface, and NiA and NRT2.4 genes were regulated to maintain the nitrogen nutrient supply in tobacco under low nitrate conditions. Low nitrate levels enhanced root endodermal suberization and hence reduced the apoplastic transport pathway, and genes from the KCS, FAR, PAS2, and CYP86 families were upregulated. The results of exogenous fluridone, an ABA biosynthesis inhibitor, indicated that suberization of the tobacco root endodermis had no relevance to radial nitrate transport and accumulation. However, ABA enhances suberization, relating to ABA biosynthesis genes in the CCD family and degradation gene ABA8ox1.

Suberin deficiency and its effect on the transport physiology of young poplar roots

The precise functions of suberized apoplastic barriers in root water and nutrient transport physiology have not fully been elucidated. While lots of research has been performed with mutants of Arabidopsis, little to no data are available for mutants of agricultural crop or tree species. By employing a combined set of physiological, histochemical, analytical, and transport physiological methods as well as RNA-sequencing, this study investigated the implications of remarkable CRISPR/Cas9-induced suberization defects in young roots of the economically important gray poplar. While barely affecting overall plant development, contrary to literature-based expectations significant root suberin reductions of up to 80-95% in four independent mutants were shown to not evidently affect the root hydraulic conductivity during non-stress conditions. In addition, subliminal iron deficiency symptoms and increased translocation of a photosynthesis inhibitor as well as NaCl highlight the involvement of suberin in nutrient transport physiology. The multifaceted nature of the root hydraulic conductivity does not allow drawing simplified conclusions such as that the suberin amount must always be correlated with the water transport properties of roots. However, the decreased masking of plasma membrane surface area could facilitate the uptake but also leakage of beneficial and harmful solutes.

Changes in wax composition but not amount enhance cuticular transpiration

This study focuses on the role of the qualitative leaf wax composition in modulating the cuticular water loss using a Populus × canescens cer6 mutant line, which accumulates C34-C46 wax ester dimers and is reduced in wax monomers >C24. The two literature-based hypotheses to be tested were the importance of the amount of wax esters and the weighted mean carbon chain length in restricting cuticular water loss. The main results were acquired by chemical analysis of cuticular wax and gravimetric cuticular transpiration measurements. Besides additional physiological measurements, the leaf surface properties were characterised by scanning electron microscopy and spectrophotometric light reflectance quantification. Mutation of the CER6 gene resulted in striking changes in qualitative wax composition but not quantitative wax amount. Based on the strong accumulation of dimeric wax esters, the relative proportion of esters increased to >90%, and the weighted mean carbon chain length increased by >6 carbon atoms. These qualitative alterations were found to increase the cuticular transpiration of leaves by twofold. Our results do not support the hypotheses that enhanced amounts of wax esters or increased weighted mean carbon chain lengths of waxes lead to reduced cuticular transpiration.

Ectomycorrhizal symbiosis prepares its host locally and systemically for abiotic cue signaling

Tree growth and survival are dependent on their ability to perceive signals, integrate them, and trigger timely and fitted molecular and growth responses. While ectomycorrhizal symbiosis is a predominant tree-microbe interaction in forest ecosystems, little is known about how and to what extent it helps trees cope with environmental changes. We hypothesized that the presence of Laccaria bicolor influences abiotic cue perception by Populus trichocarpa and the ensuing signaling cascade. We submitted ectomycorrhizal or non-ectomycorrhizal P. trichocarpa cuttings to short-term cessation of watering or ozone fumigation to focus on signaling networks before the onset of any physiological damage. Poplar gene expression, metabolite levels, and hormone levels were measured in several organs (roots, leaves, mycorrhizas) and integrated into networks. We discriminated the signal responses modified or maintained by ectomycorrhization. Ectomycorrhizas buffered hormonal changes in response to short-term environmental variations systemically prepared the root system for further fungal colonization and alleviated part of the root abscisic acid (ABA) signaling. The presence of ectomycorrhizas in the roots also modified the leaf multi-omics landscape and ozone responses, most likely through rewiring of the molecular drivers of photosynthesis and the calcium signaling pathway. In conclusion, P. trichocarpa-L. bicolor symbiosis results in a systemic remodeling of the host's signaling networks in response to abiotic changes. In addition, ectomycorrhizal, hormonal, metabolic, and transcriptomic blueprints are maintained in response to abiotic cues, suggesting that ectomycorrhizas are less responsive than non-mycorrhizal roots to abiotic challenges.

The exodermis: A forgotten but promising apoplastic barrier

The endodermis and exodermis are widely recognized as two important barriers in plant roots that play a role in regulating the movement of water and ions. While the endodermis is present in nearly all plant roots, the exodermis, characterized by Casparian strips and suberin lamellae is absent in certain plant species. The exodermis can be classified into three types: uniform, dimorphic, and inducible exodermis. Apart from its role in water and ion transport, the exodermis acts as a protective barrier against harmful substances present in the external environment. Furthermore, the exodermis is a complex barrier influenced by various environmental factors, and its resistance to water and ions varies depending on the type of exodermis and the maturity of the root. Therefore, investigations concerning the exodermis necessitate a plant-specific approach. However, our current understanding of the exodermal physiological functions and molecular mechanisms governing its development is limited due to the absence of an exodermis in the model plant Arabidopsis. Due to that, unfortunately, the exodermis has been largely overlooked until now. In this review, we aim to summarize the current fundamental knowledge regarding the exodermis in common research used crop species and propose suggestions for future research endeavors.

Apoplastic barriers of Populus × canescens roots in reaction to different cultivation conditions and abiotic stress treatments

Populus is an important tree genus frequently cultivated for economical purposes. However, the high sensitivity of poplars towards water deficit, drought, and salt accumulation significantly affects plant productivity and limits biomass yield. Various cultivation and abiotic stress conditions have been described to significantly induce the formation of apoplastic barriers (Casparian bands and suberin lamellae) in roots of different monocotyledonous crop species. Thus, this study aimed to investigate to which degree the roots of the dicotyledonous gray poplar (Populus × canescens) react to a set of selected cultivation conditions (hydroponics, aeroponics, or soil) and abiotic stress treatments (abscisic acid, oxygen deficiency) because a differing stress response could potentially help in explaining the observed higher stress susceptibility. The apoplastic barriers of poplar roots cultivated in different environments were analyzed by means of histochemistry and gas chromatography and compared to the available literature on monocotyledonous crop species. Overall, dicotyledonous poplar roots showed only a remarkably low induction or enhancement of apoplastic barriers in response to the different cultivation conditions and abiotic stress treatments. The genetic optimization (e.g., overexpression of biosynthesis key genes) of the apoplastic barrier development in poplar roots might result in more stress-tolerant cultivars in the future.

Cutinized and suberized barriers in leaves and roots: Similarities and differences

Anatomical, histochemical, chemical, and biosynthetic similarities and differences of cutinized and suberized plant cell walls are presented and reviewed in brief. Based on this, the functional properties of cutinized and suberized plant cell walls acting as transport barriers are compared and discussed in more detail. This is of general importance because fundamental misconceptions about relationships in plant-environment water relations are commonly encountered in the scientific literature. It will be shown here, that cuticles represent highly efficient apoplastic transport barriers significantly reducing the diffusion of water and dissolved compounds. The transport barrier of cuticles is mainly established by the deposition of cuticular waxes. Upon wax extraction, with the cutin polymer remaining, cuticular permeability for water and dissolved non-ionized and lipophilic solutes are increasing by 2-3 orders of magnitude, whereas polar and charged substances (e.g., nutrient ions) are only weakly affected (2- to 3-fold increases in permeability). Suberized apoplastic barriers without the deposition of wax are at least as permeable as the cutin polymer matrix without waxes and hardly offer any resistance to the free movement of water. Only upon the deposition of significant amounts of wax, as it is the case with suberized periderms exposed to the atmosphere, an efficient transport barrier for water can be established by suberized cell walls. Comparing the driving forces (gradients between water potentials inside leaves and roots and the surrounding environment) for water loss acting on leaves and roots, it is shown that leaves must have a genetically pre-defined highly efficient transpiration barrier fairly independent from rapidly changing environmental influences. Roots, in most conditions facing a soil environment with relative humidities very close to 100%, are orders of magnitude more permeable to water than leaf cuticles. Upon desiccation, the permanent wilting point of plants is defined as -1.5 MPa, which still corresponds to nearly 99% relative humidity in soil. Thus, the main reason for plant water stress leading to dehydration is the inability of root tissues to decrease their internal water potential to values more negative than -1.5 MPa and not the lack of a transport barrier for water in roots and leaves. Taken together, the commonly mentioned concepts that a drought-induced increase of cuticular wax or root suberin considerably strengthens the apoplastic leaf or root transport barriers and thus aids in water conservation appears highly questionable.