红树植物淹水胁迫响应研究进展

潮汐淹水是红树植物面临的主要环境胁迫之一,也是导致目前红树林造林成活率低的一个关键因子。由于长期适应于水淹生境,红树植物发育出一套适应于潮间带生长的抗淹水机制。综述了与红树植物相关的抗淹水胁迫响应机制,包括了形态结构、生长、水分和光合作用、膜脂过氧化系统和根系脱氢酶系统、内源激素和胁迫多胺等5个方面。提出应用人工潮汐系统研究红树植物的淹水抗性机理是确定不同种类红树植物的耐淹水能力的有效手段。并指出生长的研究是淹水胁迫响应研究的基础,而与分子手段相结合的激素水平的研究将在红树植物抗性胁迫研究中得到重视。     

Global gene expression responses to waterlogging in roots of sesame (Sesamum indicum L.)

Waterlogging stress lowers yields in sesame (Sesamum indicum L.). A major component of waterlogging stress is the lack of oxygen available to submerged tissues. Although the morphology and physiology of plants grown under anaerobic conditions have been studied in detail, limited work has been done to elucidate adaptations at the molecular level. To gain comprehensive insight into how sesame responds to hypoxia at the genome level, we performed gene expression profiling at two time points during a 36-h period following hypoxic treatment using a whole-genome RNA-Seq analysis. We identified sets of significantly positively and negatively expressed genes (induced and repressed, respectively) in response to hypoxia with distinct temporal profiles. The genes that were affected were associated with glycolysis, nitrogen metabolism, starch and sucrose metabolism and plant hormone signal transduction and indicated the upregulation of particular pathways (glycolysis/glycogenesis) in the Kyoto Encyclopedia of Genes and Genomes. Moreover, significant changes in the expression of genes were found for pathways, including flavone and flavonol biosynthesis, steroid biosynthesis, photosynthesis, cysteine and methionine metabolism, glutathione metabolism, as well as phenylpropanoid biosynthesis, spliceosome, circadian rhythm. This study helps in elucidating the molecular mechanisms of waterlogging tolerance and provides a basis for the genetic engineering of sesame.     

Identification of genes expressed in maize root cortical cells during lysigenous aerenchyma formation using laser microdissection and microarray analyses

? To adapt to waterlogging in soil, some gramineous plants, such as maize (Zea mays), form lysigenous aerenchyma in the root cortex. Ethylene, which is accumulated during waterlogging, promotes aerenchyma formation. However, the molecular mechanism of aerenchyma formation is not understood. ? The aim of this study was to identify aerenchyma formation-associated genes expressed in maize roots as a basis for understanding the molecular mechanism of aerenchyma formation. Maize plants were grown under waterlogged conditions, with or without pretreatment with an ethylene perception inhibitor 1-methylcyclopropene (1-MCP), or under aerobic conditions. Cortical cells were isolated by laser microdissection and their mRNA levels were examined with a microarray. ? The microarray analysis revealed 575 genes in the cortical cells, whose expression was either up-regulated or down-regulated under waterlogged conditions and whose induction or repression was suppressed by pretreatment with 1-MCP. ? The differentially expressed genes included genes related to the generation or scavenging of reactive oxygen species, Ca(2+) signaling, and cell wall loosening and degradation. The results of this study should lead to a better understanding of the mechanism of root lysigenous aerenchyma formation.     

Physiology and biochemistry of waterlogging tolerance in plants

Waterlogging is a serious problem, which affects crop growth and yield in low lying rainfed areas. The main cause of damage under waterlogging is oxygen deprivation, which affect nutrient and water uptake, so the plants show wilting even when surrounded by excess of water. Lack of oxygen shift the energy metabolism from aerobic mode to anaerobic mode. Plants adapted to waterlogged conditions, have mechanisms to cope with this stress such as aerenchyma formation, increased availability of soluble sugars, greater activity of glycolytic pathway and fermentation enzymes and involvement of antioxidant defence mechanism to cope with the post hypoxia/anoxia oxidative stress. Gaseous plant hormone ethylene plays an important role in modifying plant response to oxygen deficiency. It has been reported to induce genes of enzymes associated with aerenchyma formation, glycolysis and fermentation pathway. Besides, nonsymbiotic-haemoglobins and nitric oxide have also been suggested as an alternative to fermentation for maintaining lower redox potential (low NADH/NAD ratio), and thereby playing an important role in anaerobic stress tolerance and signaling.     

Plant defense after flooding

Since the first study of hypoxic response in plants with cDNA microarray in 2002, the number of hypoxia-responsive genes has grown to more than 2000. However, to date, only small numbers of hypoxia-responsive genes are known to confer hypoxic resistance. Most investigations in this area have focused on identifying which genes are responsive and then characterized how these genes are induced during hypoxia, but the roles of numerous genes in hypoxic response are still unknown. In our recent study, we demonstrated that a group of genes are induced by submergence to trigger plant immunity, which is a response to protect plants against a higher probability of pathogen infection during or after flooding. This work offered a brand new perspective, i.e., that hypoxia-responsive genes can be induced for reasons other than conferring hypoxic resistance. Possible reasons why these responses were triggered are discussed herein.     

Physiological and biochemical changes in plants under waterlogging

Waterlogging usually results from overuse and/or poor management of irrigation water and is a serious constraint with damaging effects. The rapidly depleting oxygen from submerged root zone is sensed and plant adjusts expressing anaerobic proteins. Plant cells shift their metabolism towards low energy yielding anaerobic fermentation pathways in the absence of oxygen. Structural modifications are also induced as aerenchyma formation and adventitious rootings, etc. Studies at molecular and biochemical levels to facilitate early perception and subsequent responses have also been worked out to produce resistant transgenic plants. This review explores the sequential changes of plant responses at different levels regarding their defense strategies and efforts made to enhance them, tailoring crucial regulators so that they can withstand waterlogging stress.     

Nitric oxide is essential for the development of aerenchyma in wheat roots under hypoxic stress

In response to flooding/waterlogging, plants develop various anatomical changes including the formation of lysigenous aerenchyma for the delivery of oxygen to roots. Under hypoxia, plants produce high levels of nitric oxide (NO) but the role of this molecule in plant‐adaptive response to hypoxia is not known. Here, we investigated whether ethylene‐induced aerenchyma requires hypoxia‐induced NO. Under hypoxic conditions, wheat roots produced NO apparently via nitrate reductase and scavenging of NO led to a marked reduction in aerenchyma formation. Interestingly, we found that hypoxically induced NO is important for induction of the ethylene biosynthetic genes encoding ACC synthase and ACC oxidase. Hypoxia‐induced NO accelerated production of reactive oxygen species, lipid peroxidation, and protein tyrosine nitration. Other events related to cell death such as increased conductivity, increased cellulase activity, DNA fragmentation, and cytoplasmic streaming occurred under hypoxia, and opposing effects were observed by scavenging NO. The NO scavenger cPTIO (2‐(4‐carboxyphenyl)‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide potassium salt) and ethylene biosynthetic inhibitor CoCl₂ both led to reduced induction of genes involved in signal transduction such as phospholipase C, G protein alpha subunit, calcium‐dependent protein kinase family genes CDPK, CDPK2, CDPK 4, Ca‐CAMK, inositol 1,4,5‐trisphosphate 5‐phosphatase 1, and protein kinase suggesting that hypoxically induced NO is essential for the development of aerenchyma.     

Roots of willow (Salix viminalis L.) show marked tolerance to oxygen shortage in flooded soils and in solution culture

Responses to soil flooding and oxygen shortage were studied in field, glasshouse and controlled environment conditions. Established stools ofSalix viminalis L., were compared at five field sites in close proximity but with contrasting water table levels and flooding intensities during the preceding winter. There was no marked effect of site on shoot extension rate, time to half maximum length or final length attained. When rooted cuttings were waterlogged for 4 weeks in a glasshouse, soil redox potentials quickly decreased to below zero. Shoot extension was slowed after a delay of 20 d, while, in the upper 100 mm of soil, formation and outgrowth of unbranched adventitious roots with enhanced aerenchyma development was promoted after 7 d. At depths of 100–200 mm and 200–300 mm, extension by existing root axes was halted by soil flooding, while adventitious roots from above failed to penetrate these deeper zones. After 4 weeks waterlogging, all arrested root tips recommenced elongation when the soil was drained; their extension rates exceeding those of roots that were well-drained throughout. Growth in fresh mass was also stimulated. The additional aerenchyma found in adventitious roots in the upper 100 mm of soil may have been ethylene regulated since gas space development was inhibited by silver nitrate, an ethylene action inhibitor. The effectiveness of aerenchyma was tested by blocking the entry of atmospheric oxygen into plants with lanolin applied to lenticels of woody shoots of plants grown in solution culture. Root extension was halved, while shoot growth remained unaffected. H Lambers Section editor     

How plants sense low oxygen

The recent identification of the oxygen-sensing mechanism in plants is a breakthrough in plant physiology. The presence of a conserved N-terminal motif on some ethylene responsive factors (ERFs), targets the protein for post-translational modifications finally leading to degradation under normoxia and thus providing a mechanism for sensing the presence of oxygen. The stabilization of the N-terminus under low oxygen activates these ERFs, which regulate low oxygen core genes that enable plants to tolerate abiotic stress such as flooding. Additional mechanisms that signal low-oxygen probably also exist, and the production of reactive oxygen species (ROS) has been observed under low oxygen, suggesting that ROS might be part of the network involved in plant acclimation. Here, we review the most recent findings related to oxygen sensing.     

Waterlogging‐induced adventitious root formation in cucumber is regulated by ethylene and auxin through reactive oxygen species signalling

Development of adventitious roots (ARs) at the base of the shoot is an important adaptation of plants to waterlogging stress; however, its physiological mechanisms remain unclear. Here, we investigated the regulation of AR formation under waterlogged conditions by hormones and reactive oxygen species (ROS) in Cucumis sativus L., an agriculturally and economically important crop in China. We found that ethylene, auxin, and ROS accumulated in the waterlogged cucumber plants. On the other hand, application of the ethylene receptor inhibitor 1‐methylcyclopropene (1‐MCP), the auxin transport inhibitor 1‐naphthylphthalamic acid (NPA), or the NADPH oxidase inhibitor diphenyleneiodonium (DPI) decreased the number of ARs induced by waterlogging. Auxin enhanced the expression of ethylene biosynthesis genes, which led to ethylene entrapment in waterlogged plants. Both ethylene and auxin induced the generation of ROS. Auxin‐induced AR formation was inhibited by 1‐MCP, although ethylene‐induced AR formation was not inhibited by NPA. Both ethylene‐ and auxin‐induced AR formation were counteracted by DPI. These results indicate that auxin‐induced AR formation is dependent on ethylene, whereas ethylene‐induced AR formation is independent of auxin. They also show that ROS signals mediate both ethylene‐ and auxin‐induced AR formation in cucumber plants.     

Antioxidative responses and morpho-anatomical adaptations to waterlogging in Sesbania virgata

Sesbania virgata (Leguminosae) is tolerant of long periods of soil inundation. However, its morphological adaptations to anoxia and its response to possible damage from oxidative stress are still unknown. Here, we provide new information that helps to explain the ability of S. virgata plants to grow in flooded environments. Plants containing six expanded leaves were placed in masonry tanks and were subjected to the following conditions: control (well watered), soil waterlogging (water to the setup level of 1 cm above the soil surface—roots and parts of the stems flooded), and complete submergence (whole plant flooded). Plants exposed to flooding (soil waterlogging and complete submergence) significantly increased their production of hydrogen peroxide (H₂O₂), indicating the extent of oxidative injury posed by stress conditions. We demonstrate that plants exposed to flooding develop an efficient scavenger of ROS (generated during stress) in the roots through the coordinated action of nonenzymatic ascorbic acid (Asc) and dehydroascorbate (DHA) as well as the enzymatic antioxidants superoxide dismutase (SOD), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) that are present in the tissues. Moreover, we observed the development of morpho-anatomical structures such as adventitious roots, lenticels, and cracks in the stem of plants under soil waterlogging. The secondary root of plants under soil waterlogging showed a thinner cortex and larger number of elements of small diameter vessels. Numerous aerenchymas were observed in the newly formed in the adventitious roots. We conclude that these antioxidative responses and morpho-anatomical adaptations in the roots are part of a suite of adaptations that allow S. virgata plants to survive long periods of flooding, notably under waterlogged conditions.     

Physiological and anatomical adaptations induced by flooding in Cotula coronopifolia

Cotula coronopifolia is a wild annual Asteraceae that grows in periodically-flooded prone environments and seems highly tolerant to periodic flooding. Seedlings of about 15 cm were collected directly from the edge of Soliman sabkha (N-E Tunisia, semi-arid stage) and grown under greenhouse conditions. Two treatments were considered: drainage and flooding. After 56 days of treatment, flooded plants showed a pronounced growth increase. This performance was essentially associated with significant increment in biomass production of both shoots and roots (about 220% of the control). The appropriate response to flooding was also characterized by the ability of the species to maintain its water status under such conditions. Neither water content nor water potential showed a significant variation as compared to those of non-flooded plants. However, transpiration rate decreased slightly but significantly in flooded plants (from 0.86 to 0.64 mmol H2O m-2 s-1). Na+ and K+ concentrations were practically maintained under waterlogging conditions, except a significant increase of Na+ content in roots of flooded plants (157% of the control). These responses were concomitant with maintenance of photosynthetic rate. However, the contents of chlorophylls a and b increased to 167% and 295%, respectively. It seems that the enhancement in these photosynthetic pigments together with a significant improvement in water use efficiency (from 4.66 to 6.07 mmol CO2 mol-1 H2O) allowed to the species to compensate the decrease in photosynthetic rate. At the anatomical level, this species responded to flooding by a significant development of its root aerenchyma (+63%) and an increase in the lignification of its stem xylem tissues (+37%). Based on the presented data, the plant fitness under flooding conditions was a result of dynamic readjustment of several morphological, physiological, and anatomical adaptive traits. Flood requirement together with salt tolerance are responsible for the predominance of C. coronopifolia in a large area in its natural biotope where most plants cannot tolerate interactive effects of flooding and salinity.