PLANT HORMONES IN THE CONTROL OF ABSCISSION

Advances in Rice Seed Shattering

Seed shattering is an important trait that wild rice uses to adapt to the natural environment and maintain population reproduction, and weedy rice also uses it to compete with the rice crop. The loss of shattering is a key event in rice domestication. The degree of shattering is not only one of the main reasons for rice yield reduction but also affects its adaptability to modern mechanical harvesting methods. Therefore, it is important to cultivate rice varieties with a moderate shattering degree. In this paper, the research progress on rice seed shattering in recent years is reviewed, including the physiological basis, morphological and anatomical characteristics of rice seed shattering, inheritance and QTL/gene mapping of rice seed shattering, the molecular mechanism regulating rice seed shattering, the application of seed-shattering genes, and the relationship between seed-shattering genes and domestication.

Abscisic-Acid-Regulated Responses to Alleviate Cadmium Toxicity in Plants

High levels of cadmium (Cd) in soil can cause crop yield reduction or death. Cadmium accumulation in crops affects human and animal health as it passes through the food chain. Therefore, a strategy is needed to enhance the tolerance of crops to this heavy metal or reduce its accumulation in crops. Abscisic acid (ABA) plays an active role in plants' response to abiotic stress. The application of exogenous ABA can reduce Cd accumulation in shoots of some plants and enhance the tolerance of plants to Cd; therefore, ABA may have good application prospects. In this paper, we reviewed the synthesis and decomposition of ABA, ABA-mediated signal transduction, and ABA-mediated regulation of Cd-responsive genes in plants. We also introduced physiological mechanism underlying Cd tolerance because of ABA. Specifically, ABA affects metal ion uptake and transport by influencing transpiration and antioxidant systems, as well as by affecting the expression of metal transporter and metal chelator protein genes. This study may provide a reference for further research on the physiological mechanism of heavy metal tolerance in plants.

Seed Shattering: A Trait of Evolutionary Importance in Plants

Seed shattering refers to the natural shedding of seeds when they ripe, a phenomenon typically observed in wild and weedy plant species. The timing and extent of this phenomenon varies considerably among plant species. Seed shattering is primarily a genetically controlled trait; however, it is significantly influenced by environmental conditions, management practices and their interactions, especially in agro-ecosystems. This trait is undesirable in domesticated crops where consistent efforts have been made to minimize it through conventional and molecular breeding approaches. However, this evolutionary trait serves as an important fitness and survival mechanism for most weeds that utilize it to ensure efficient dispersal of their seeds, paving the way for persistent soil seedbank development and sustained future populations. Weeds have continuously evolved variations in seed shattering as an adaptation under changing management regimes. High seed retention is common in many cropping weeds where weed maturity coincides with crop harvest, facilitating seed dispersal through harvesting operations, though some weeds have notoriously high seed shattering before crop harvest. However, high seed retention in some of the most problematic agricultural weed species such as annual ryegrass (Lolium rigidum), wild radish (Raphanus raphanistrum), and weedy amaranths (Amaranthus spp.) provides an opportunity to implement innovative weed management approaches such as harvest weed seed control, which aims at capturing and destroying weed seeds retained at crop harvest. The integration of such management options with other practices is important to avoid the rapid evolution of high seed shattering in target weed species. Advances in genetics and molecular biology have shown promise for reducing seed shattering in important crops, which could be exploited for manipulating seed shattering in weed species. Future research should focus on developing a better understanding of various seed shattering mechanisms in plants in relation to changing climatic and management regimes.

Ethephon induced oxidative stress in the olive leaf abscission zone enables development of a selective abscission compound

BACKGROUND

Table olives (Olea europaea L.), despite their widespread production, are still harvested manually. The low efficiency of manual harvesting and the rising costs of labor have reduced the profitability of this crop. A selective abscission treatment, inducing abscission of fruits but not leaves, is crucial for the adoption of mechanical harvesting of table olives. In the present work we studied the anatomical and molecular differences between the three abscission zones (AZs) of olive fruits and leaves.

RESULTS

The fruit abscission zone 3 (FAZ3), located between the fruit and the pedicel, was found to be the active AZ in mature fruits and is sensitive to ethephon, whereas FAZ2, between the pedicel and the rachis, is the flower active AZ as well as functioning as the most ethephon induced fruit AZ. We found anatomical differences between the leaf AZ (LAZ) and the two FAZs. Unlike the FAZs, the LAZ is characterized by small cells with less pectin compared to neighboring cells. In an attempt to differentiate between the fruit and leaf AZs, we examined the effect of treating olive-bearing trees with ethephon, an ethylene-releasing compound, with or without antioxidants, on the detachment force (DF) of fruits and leaves 5 days after the treatment. Ethephon treatment enhanced pectinase activity and reduced DF in all the three olive AZs. A transcriptomic analysis of the three olive AZs after ethephon treatment revealed induction of several genes encoding for hormones (ethylene, auxin and ABA), as well as for several cell wall degrading enzymes. However, up-regulation of cellulase genes was found only in the LAZ. Many genes involved in oxidative stress were induced by the ethephon treatment in the LAZ alone. In addition, we found that reactive oxygen species (ROS) mediated abscission in response to ethephon only in leaves. Thus, adding antioxidants such as ascorbic acid or butyric acid to the ethephon inhibited leaf abscission but enhanced fruit abscission.

CONCLUSION

Our findings suggest that treating olive-bearing trees with a combination of ethephon and antioxidants reduces the detachment force (DF) of fruit without weakening that of the leaves. Hence, this selective abscission treatment may be used in turn to promote mechanized harvest of olives.

The influence of abscisic acid on the ethylene biosynthesis pathway in the functioning of the flower abscission zone in Lupinus luteus

Flower abscission is a highly regulated developmental process activated in response to exogenous (e.g. changing environmental conditions) and endogenous stimuli (e.g. phytohormones). Ethylene (ET) and abscisic acid (ABA) are very effective stimulators of flower abortion in Lupinus luteus, which is a widely cultivated species in Poland, Australia and Mediterranean countries. In this paper, we show that artificial activation of abscission by flower removal caused an accumulation of ABA in the abscission zone (AZ). Moreover, the blocking of that phytohormone's biosynthesis by NDGA (nordihydroguaiaretic acid) decreased the number of abscised flowers. However, the application of NBD - an inhibitor of ET action - reversed the stimulatory effect of ABA on flower abscission, indicating that ABA itself is not sufficient to turn on the organ separation. Our analysis revealed that exogenous ABA significantly accelerated the transcriptional activity of the ET biosynthesis genes ACC synthase (LlACS) and oxidase (LlACO), and moreover, strongly increased the level of 1-aminocyclopropane-1-carboxylic acid (ACC) - ET precursor, which was specifically localized within AZ cells. We cannot exclude the possibility that ABA mediates flower abscission processes by enhancing the ET biosynthesis rate. The findings of our study will contribute to the overall basic knowledge on the phytohormone-regulated generative organs abscission in L. luteus.

Cross-talk between environmental stresses and plant metabolism during reproductive organ abscission

In plants, flowering is a crucial process for reproductive success and continuity of the species through time. Fruit production requires the perfect development of reproductive structures. Abscission, a natural process, can occur to facilitate shedding of no longer needed, infected, or damaged organs. If stress occurs during flower development, abscission can intervene at flower level, leading to reduced yield. Flower abscission is a highly regulated developmental process simultaneously influenced and activated in response to exogenous (changing environmental conditions, interactions with microorganisms) and endogenous (physiological modifications) stimuli. During climate change, plant communities will be more susceptible to environmental stresses, leading to increased flower and fruit abscission, and consequently a decrease in fruit yield. Understanding the impacts of stress on the reproductive phase is therefore critical for managing future agricultural productivity. Here, current knowledge on flower/fruit abscission is summarized by focusing specifically on effects of environmental stresses leading to this process in woody plants. Many of these stresses impair hormonal balance and/or carbohydrate metabolism, but the exact mechanisms are far from completely known. Hormones are the abscission effectors and the auxin/ethylene balance is of particular importance. The carbohydrate pathway is the result of complex regulatory processes involving the balance between photosynthesis and mobilization of reserves. Hormones and carbohydrates together participate in complex signal transduction systems, especially in response to stress. The available data are discussed in relation to reproductive organ development and the process of abscission.

Quo vadis plant hormone analysis?

Plant hormones act as chemical messengers in the regulation of myriads of physiological processes that occur in plants. To date, nine groups of plant hormones have been identified and more will probably be discovered. Furthermore, members of each group may participate in the regulation of physiological responses in planta both alone and in concert with members of either the same group or other groups. The ideal way to study biochemical processes involving these signalling molecules is 'hormone profiling', i.e. quantification of not only the hormones themselves, but also their biosynthetic precursors and metabolites in plant tissues. However, this is highly challenging since trace amounts of all of these substances are present in highly complex plant matrices. Here, we review advances, current trends and future perspectives in the analysis of all currently known plant hormones and the associated problems of extracting them from plant tissues and separating them from the numerous potentially interfering compounds.

Leaf mines: their effect on leaf longevity

The effects of a number of factors, notably leaf mining insects, on the longevity of beech and holm oak leaves have been studied. The regular monitoring of individually labelled leaves was complemented by analysis of leaf fall data. Both methods confirm that these mining insects have only a slight impact on their host trees. The presence of first generation Phyllonorycter maestingella mines on beech leaves and winter generation P. messaniella mines on holm oak leaves accelerates leaf loss. Beech leaves mined by second generation P. maestingella and Rhynchaenus fagi did not show this accelerated loss. Their patterns of leaf fall can be explained by within-tree variation in both mine distribution and the timing of leaf fall. It is argued that this premature leaf fall is a damage response, and is not an attempt by the tree to regulate miner numbers.

Ethylene-promoted tomato flower abscission and the possible involvement of an inhibitor

The abscission zone in tomato (Lycopersicon esculentum (L.) Mill. flower pedicels is morphologically distinguishable prior to separation and is delineated by an indentation of the epidermis. Exposure of excised pedicels with the flower attached to ethylene results in abscission within 12 h and this can be accelerated by flower removal. Abscission of excised pedicels with the flower removed takes place in the absence of exogenous ethylene but this is delayed by pretreatment with aminoethoxyvinyl glycine, an inhibitor of ethylene biosynthesis. The data presented support the hypothesis that flower tissue is the source of an abscission inhibitor.

Abscission in Phaseolus vulgaris the positional differentiation and ethylene-induced expansion growth of specialised cells

Cell separation in abscission zones of explants of Phaseolus vulgaris L. is preceded by the enlargement of a single row of cells. Unlike the other cells surrounding them, the cells of this row enlarge in response to ethylene and not to auxin.Enlargement follows an increase in the activity of carboxymethyl 1: 4-glucanase in the abscission zone; also a response to ethylene and not to auxin.Eventual cell separation occurs between the enlarging cells and the non-enlarging tissue distal to them.It is proposed that the formation of cells at the abscission zone, with growth properties dissimilar to those of the rest of the pulvinus and petiole, is an example of precise positional differentiation. The presence and enlargement of such cells may be a general prerequisite for the abscission of organs or tissues distal to them.

Cavitation studies on whole Ricinus plants by acoustic detection

Acoustic detection has been used to investigate the incidence of cavitation in whole potted Ricinus plants subjected to water stress by withholding water. Cavitation proceeded rather slowly and was detectable before and during wilting. Techniques which restricted water uptake more drastically such as root cooling or overlapping cuts induced more rapid "click" production and wilting; a response already described for excised leaves. When water stress was removed by rewatering, or rewarming a cooled root system, cavitation soon ceased. This response was more sluggish of over-delayed.Cavitation in aging leaves on well watered plants has also been examined. Despite the onset of senescence over many days there was no evidence that dry patches, which often develop extensively, are a consequence of water shortage induced by xylem blockage. Leaves, falling naturally by abscission in still air, were often remarkably turgid with water potentials similar to those of healthy attached leaves. Only after losing water was cavitation apparent, as usual for excised mature leaves. Sometimes more persistent leaves did cavitate in situ, just before abscission, showing that in normal leaves xylem blockage can occasionally precede leaf fall by several hours.

Peroxidases in tobacco abscission zone tissue. I. Fine-structural localization in cell walls during ethylene-induced abscission

The fine-structural localization of peroxidases during ethylene-induced abscission of flower pedicels of Nicotiana tabacum L. cv. ‘Little Turkish’ has been investigated. Peroxidase activity has been localized in both the cell walls and intercellular spaces of ethylene-treated flower pedicels which were fixed in glutaraldehyde, incubated in diaminobenzidine (DAB) medium with postfixation in 2 % osmium tetroxide. Peroxidase staining is present in the cell walls and intercellular spaces of control tissue but is not as intense as in ethylene-treated tissue. Increased peroxidase staining is evident in the intercellular spaces and cell walls after 2 h of exposure to ethylene and increases in intensity between 2 and 5 h. At 5 h, ethylene-induced abscission occurs. Fine-structural investigations revealed prominent staining in the middle-lamellar and peripheral areas of the cell walls in ethylene-treated tissue. The peroxidase staining appears to be due to peroxidase as prior incubation with potassium cyanide gives a marked reduction in the staining reaction. Incubation of the ethylene-exposed tissue in aminotriazole, a specific inhibitor of catalase, does not reduce peroxidase staining, except in the microbodies, which reportedly contain catalase.

Fine structure of abscission zones : IV. Effect of ethylene on the ultrastructure of abscission cells of tobacco flower pedicels

The effect of ethylene on abscission of flower pedicels of tobacco plants has been investigated. For the first 2 h of exposure to C2H4, the pedicels bend rather than break in response to applied force, but after 2.5 h exposure they break at the abscission zone under an applied force of 40 g. The break strength of the abscission zone decreases exponentially with time to 5 g at 5 h after beginning of the C2H4 treatment. An examination of the tissue at the fine structural level 2 h after exposure to C2H4 reveals the accumulation of rough endoplasmic reticulum (RER) in the abscission cells. Rough ER becomes increasingly abundant by 3-5 h exposure of the tissue to C2H4. There is approximately a 30 fold increase in RER by 5 h of exposure, as compared to untreated tissue.Loss in the integrity of the membranes of microbodies occurs by 5 h exposure of the tissue to C2H4. As cell wall degradation proceeds, fibrous material, vesicular structures, and electron dense bodies-the latter often appearing striated-develop in the disintegrating wall. Little change is seen in the structure of nuclei, mitochondria, chloroplasts and in the crystalloid cores of microbodies during the first 5 h of exposure of the tissue to C2H4. However, disorganization of cytoplasmic components does occur in cells where cell wall breakdown is at an advanced stage.