Cytokinins in the xylem sap of desert-grown almond (Prunus dulcis†) trees: Daily courses and their possible interactions with abscisic acid and leaf conductance

Hormonal involvement in boosting the vegetative vigour underlying caffeine-related improvements in lentil production

Previous studies have shown that caffeine (1,3,7-trimethylxanthine) has some potential for its use as a biostimulant ingredient for boosting lentil production at suboptimal temperatures. However, some limitations to its use include its potential side effects as an emerging contaminant and the current lack of knowledge of its mechanism of action. Here, we aimed to study the mechanisms underlying improved lentil production upon caffeine application. Greenhouse-grown plants treated with caffeine (at 10-5 M, 10-4 M, and 10-3 M) were compared to an untreated, control treatment, and both reproductive and vegetative vigour were evaluated in parallel with endogenous foliar concentrations of phytohormones, including both stress and growth-related hormones. Results showed an enhanced lentil production at the highest caffeine concentration (10-3 M) which might be attributed, at least in part, to a greater vegetative vigour. The hormonal profiling revealed a dual effect. Firstly, there was a specific increase in jasmonoyl-isoleucine (JA-Ile) in the short term, which may provide a priming effect. Secondly, abscisic acid (ABA) content kept at low levels and the active cytokinin (CK) isopentenyl adenine (2-iP) increased and persisted at high levels throughout the reproductive stage. Cytokinin-mediated effects on growth, and more specifically the high CK/ABA ratios in leaves, appeared to mediate caffeine-related effects in boosting vegetative vigour. In conclusion, caffeine emerges as a compelling alkaloid for integration into biostimulant formulations due to its favorable effect in boosting lentil production through an improvement of vegetative vigour. These outcomes appear to be modulated by phytohormones, most notably jasmonates, priming plants for improved performance under suboptimal temperatures, and cytokinins, alongside ABA and its associated ratios, collectively enhancing plant growth and reproductive vigour in challenging conditions.

Diurnal variation of cytokinin, auxin and abscisic acid levels in tobacco leaves

As many processes are regulated by both light and plant hormones, evaluation of diurnal variations of their levels may contribute to the elucidation of the complex network of light and hormone signal transduction pathways. Diurnal variation of cytokinin, auxin, and abscisic acid levels was tested in tobacco leaves (Nicotiana tabacum L. cv. Wisconsin 38) grown under a 16/8 h photoperiod. The main peak of physiologically active cytokinins (cytokinin bases and ribosides) was found after 9 h of light, i.e. 1 h after the middle of the light period. This peak coincided with the major auxin peak and was closely followed by a minor peak of abscisic acid. Free abscisic acid started to increase at the light/dark transition and reached its maximum 3 h after dark initiation. The content of total cytokinins (mainly N-glucosides, followed by cis-zeatin derivatives and nucleotides) exhibited the main peak after 9 h of light and the minor peak after the transition to darkness. The main, midday peak of active cytokinins was preceded by a period of minimal metabolic conversion of tritiated trans-zeatin (less than 30%). The major cytokinin-degrading enzyme, cytokinin oxidase/dehydrogenase (EC 1.5.99.12), exhibited maximal activity after the dark/light transition and during the diminishing of the midday cytokinin peak. The former peak might be connected with the elimination of the long-distance cytokinin signal. These cytokinin oxidase/dehydrogenase peaks were accompanied by increased activity of beta-glucosidase (EC 3.2.1.21), which might be involved in the hydrolysis of cytokinin O-glucosides and/or in fine-tuning of active cytokinin levels at their midday peak. The achieved data indicate that cytokinin metabolism is tightly regulated by the circadian clock.

The effect of root cooling on hormone content, leaf conductance and root hydraulic conductivity of durum wheat seedlings (Triticum durum L.)

Root cooling of 7-day-old wheat seedlings decreased root hydraulic conductivity causing a gradual loss of relative water content during 45 min (RWC). Subsequently (in 60 min), RWC became partially restored due to a decrease in transpiration linked to lower stomatal conductivity. The decrease in stomatal conductivity cannot be attributed to ABA-induced stomatal closure, since no increase in ABA content in the leaves or in the concentration in xylem sap or delivery of ABA from roots was found. However, decreased stomatal conductance was associated with a sharp decline in the content of cytokinins in shoots that was registered shortly after the start of root cooling and linked to increases in the activity of cytokinin-oxidase. This decrease in shoot cytokinin content may have been responsible for closing stomata, since this hormone is known to maintain stomatal opening when applied to plants. In support of this, pre-treatment with synthetic cytokinin benzyladenine was found to increase transpiration of wheat seedlings with cooled roots and bring about visible loss of turgor and wilting.

Abscisic acid (ABA) flows from Hordeum vulgare to the hemiparasite Rhinanthus minor and the influence of infection on host and parasite abscisic acid relations

Using the facultative root hemiparasite Rhinanthus minor and Hordeum vulgare as a host, the flows, depositions, and metabolism of abscisic acid (ABA) within the host, within the parasite, and between host and parasite have been studied. When the plants were supplied with 5 mM NO(3)(-), there were weak or no effects of parasitism on ABA flows, biosynthesis, and ABA degradation in barley. However, ABA deposition was significantly affected in the leaf laminae (3-fold) and in the leaf sheath (2.4-fold), but not in roots. Dramatic changes in ABA flows, metabolism, and deposition on a per plant basis, however, have been observed in Rhinanthus. Biosynthesis in the roots was 12-fold higher after attachment, resulting in 14-fold higher ABA flows in the xylem. A large portion of this ABA was metabolized, a small portion was deposited. Phloem flows of ABA were increased 13-fold after attachment. The concentrations of ABA in tissues and transport fluids were higher in attached Rhinanthus by an order of magnitude than in host tissues and xylem sap. The same tendency was also found in a comparison between single Rhinanthus and unparasitized barley. As compared with 5 mM NO(3)(-), lower NO(3)(-) or 1 mM NH(4)(+) supply doubled the ABA concentrations in barley leaf laminae, while having only small or non-significant effects in the other organs. The possible function of ABA for the parasite is discussed.

Influence of a drying cycle on post-drought xylem sap abscisic acid and stomatal responses in young temperate deciduous angiosperms

•   Post-drought patterns of water relations and gas exchange were studied in relation to xylem sap abscisic acid (ABA) concentration during recovery for young plants of five woody species. The potential role of xylem sap [ABA] in these responses was the object of study. •   Potted plants were allowed to deplete soil water and then were rewatered. At peak drought and during recovery, predawn and midday leaf water potential (Ψ_l ), stomatal conductance (g_s ), and xylem sap [ABA] were measured. •   Water potentials recovered rapidly after rewatering but stomatal re-opening was delayed. Xylem sap [ABA] was elevated early in recovery and might have affected stomatal opening, but after 1 d at high soil water content [ABA] in recovering plants was equal to or lower than in control plants. Stomata appeared to be more sensitive to xylem sap [ABA] in recovering than droughted plants. •   Xylem sap [ABA] may play some role in delayed recovery of stomatal opening after drought, but may not completely explain the responses.

A possible stress physiological role of abscisic acid conjugates in root-to-shoot signalling

Abscisic acid (ABA) conjugates, predominantly their glucose esters, have recently been shown to occur in the xylem sap of different plants. Under stress conditions, their concentration can rise substantially to levels that are higher than the concentration of free ABA. External ABA conjugates cannot penetrate apoplastic barriers in the root. They have to be hydrolysed by apoplastic enzymes in the root cortex. Liberated free ABA can then be redistributed to the root symplast and dragged directly across the endodermis to the stele. Endogenous ABA conjugates are formed in the cytosol of root cells, transported symplastically to the xylem parenchyma cells and released to the xylem vessels. The mechanism of release is unknown; it may include the action of ABC-transporters. Because of its extremely hydrophilic properties, ABA-GE is translocated in the xylem of the stem without any loss to the surrounding parenchyma. After arrival in the leaf apoplast, transporters for ABA-GE in the plasmalemma have to be postulated to redistribute the conjugates to the mesophyll cells. Additionally, apoplastic esterases can cleave the conjugate and release free ABA to the target cells and tissues. The activity of these esterases is increased when barley plants are subjected to salt stress.

ABA-based chemical signalling: the co-ordination of responses to stress in plants

There is now strong evidence that the plant hormone abscisic acid (ABA) plays an important role in the regulation of stomatal behaviour and gas exchange of droughted plants. This regulation involves both long-distance transport and modulation of ABA concentration at the guard cells, as well as differential responses of the guard cells to a given dose of the hormone. We will describe how a plant can use the ABA signalling mechanism and other chemical signals to adjust the amount of water that it loses through its stomata in response to changes in both the rhizospheric and the aerial environment. The following components of the signalling process can play an important part in regulation: (a) ABA sequestration in the root; (b) ABA synthesis versus catabolism in the root; (c) the efficiency of ABA transfer across the root and into the xylem; (d) the exchange of ABA between the xylem lumen and the xylem parenchyma in the shoot; (e) the amount of ABA in the leaf symplastic reservoir and the efficiency of ABA sequestration and release from this compartment as regulated by factors such as root and leaf-sourced changes in pH; (f) cleavage of ABA from ABA conjugates in the leaf apoplast; (g) transfer of ABA from the leaf into the phloem; (h) the sensitivity of the guard cells to the [ABA] that finally reaches them; and lastly (i) the possible interaction between nitrate stress and the ABA signal.

Xylem-transported chemical signals and the regulation of plant growth and physiology

There is now a substantial body of evidence that shoot growth and physiology of plants rooted in drying soil may be regulated by chemical signals moving from the root to the shoot in the xylem stream. Although some evidence suggests that soil drying can reduce the supply of promoters of leaf growth and stomatal opening, there is now compelling evidence for an enhanced flux of inhibitors in the xylem stream of draughted plants. Some of this inhibitory activity is still to be identified but at least in some plants the bulk of activity can be explained by the enhanced concentration of the plant hormone abscisic acid (ABA). A series of field experiments has now shown that ABA, moving as a signal from the roots to the leaves in the transpiration stream, can provide a measure of the access that the plant has to water in the soil in the rooting zone. We show here how this signal may be a variation in the concentration of ABA arriving at the sites of action in the leaf. The response to such a signal apparently varies as a function of the physiological state of the leaf. The basis of such variation in the sensitivity of response is also discussed. One other interpretation of the field data is that leaves respond to the amount of ABA arriving in the leaf, rather than the concentration. We show some evidence for this contention.

Exploitation of the xylem stream by parasitic organisms

A taxonomically diverse group of angiosperms and certain homopteran insects derive water, inorganic and organic solutes from angiosperm xylem sap. Parasitic angiosperms are connected to their host(s) by a specialized organ (the haustorium) and form close cellular contacts with host xylem tissue, while insects tap xylem vessels by means of stylets. Adaptations to phytophagy are discussed with respect to gaining access to xylem tissue and the nutrition of sap feeders. Parasitic angiosperm-host interactions are examined in relation to recent advances in our understanding of root-to-shoot communication via the xylem (the influence of host-sourced signals on the parasite) and the functional significance of high rates of transpiration in parasitic angiosperms.

The pH gradients in the root system and the abscisic acid concentration in xylem and apoplastic saps

Abscisic acid (ABA) is a stress signal that is transported from the root system to leaves, and induces stomatal closure before water relations of the leaves are affected by soil drying. Xylem vessels are in direct contact with the leaf apoplasm, the only leaf compartment that is directly connected with the primary site of ABA action, the outer surface of the guard cell plasma membrane (Hartung 1983). ABA distributes among the leaf compartments according to the anion trap concept and the Henderson-Hasselbalch equation, with the free acid as the permeating and the anion as the nearly non-permeating molecular species. Applying this concept, a flattening of the intracellular pH gradients increases the apoplastic ABA concentration. Indeed, stress increases the apoplastic pH (Hartung et al. 1988) and decreases slightly the cytosolic pH . The validity of this concept has been shown repeatedly and was confirmed by a mathematical leaf model (Slovik et al. 1992). It is appropriate to ask whether these mechanisms also contribute to ABA compartmentation and redistribution in the root system. Therefore, we have incorporated compartmental pH values of unstressed and stressed root cells, the permeability coefficients of root membranes for ABA and anatomical data into a mathematical model, similar to that of Slovik et al. (1992). The simulation shows that ABA redistribution in roots caused by changing pH gradients can account for up to a 2 to 3-fold accumulation of ABA in the xylem sap of stressed plants. The model also predicts that the pH gradient across the cortical plasma membrane has the most distinct effects on redistribution of ABA into the xylem sap of stressed plants and, additionally, that the ABA concentration in the rhizospheric aqueous solution can play an im portant role in root-to-shoot signalling.

Stress-related compounds in xylem fluid of blight-diseased citrus containingFusarium solaninaphthazarin toxins and their effects on the host

Naphthazarin toxins of Fusarium solani were detected and quantified by competitive enzyme-linked immunosorbent assay (ELISA) in xylem fluid of scaffold roots from blight-diseased trees. These toxins alter plant metabolic activity; this study examined their effects on xylem health by measuring physiological components in xylem fluid. Protein concentration in fluid was positively correlated with increases in toxin concentration. In fluid containing about 100 μg∙L−1toxin, total amino acids reached levels 2.5 to 3.0 times greater than those in fluid containing no detectable toxin; asparagine, glutamic acid, proline, glycine, and arginine were the most abundant. Levels of phenylalanine ammonia-lyase, polyphenol oxidase, chlorogenic acid oxidase, and superoxide dismutase activity did not increase in xylem fluid containing toxin, which may be a reason why vascular discoloration did not occur. Xylem fluid containing about 20 μg∙L−1toxin was associated with a 9-fold increase in total phenolics and a 15-fold increase in peroxidase. Peroxidases were predominantly anionic and may function in defense. Some of these peroxidases may function as lignases, releasing phenolic and other constituents from cells and cell walls. These toxins are known to enhance membrane permeability, which may be the main reason for the accumulation of these stress metabolites in xylem fluid. These data explain the disruption of hydraulic conductivity in blight tree roots and the eventual physiological breakdown of roots on diseased trees.Key words: phytotoxins, isomarticin, ELISA, fungi, roots.