Unipolar cell divisions in the yeast S. cerevisiae lead to filamentous growth: Regulation by starvation and RAS

It has previously been postulated that when plants are stressed by certain changes in patterns of weather they become a better source of food for invertebrate herbivores because this stress causes an increase in the amount of nitrogen available in their tissues for young herbivores feeding on them. And this may cause outbreaks of such phytophagous invertebrates. Evidence is now presented that a similar physiological mechanism appears to operate when a wide variety of apparently unrelated environmental factors impinge on plants or parts of plants in such a way as to perturb their metabolism. A broken branch, lightning strike, fire, nutrient deficiencies or an otherwise adverse site; all may have this effect. With the advent of modern man the available agencies increase and diversify to include pesticides, irradiation and air pollutants. One common metabolic response by plants to all such agents impinging on them seems to be equivalent to that found in senescing plant tissues — the breakdown and mobilization of nitrogen in soluble form away from the senescing/stressed tissues. Young herbivores which chance to feed on such stressed/senescing tissues have a greater and more readily available supply of nitrogen in their food than they would have had if feeding on unstressed plants. As a result many more of them survive, and there is an increase in abundance of their kind. Such increases may be quite localised and short-lived or more widespread and persistent, depending on the extent and duration of the stress experienced by the plants. And in the face of this improved nutrition and survival of the very young, predators and parasites seem to have only a minor influence on subsequent changes in abundance of their herbivorous prey. Another effect of increased mobilization of nitrogen in stressed plants is an increase in the quantity of the seed that they set. This has led to the conclusion that increased abundance of some species of birds at such times is due to a greater supply of seeds as winter food for recent fledglings. But it may be that the increased abundance is due to the synchronous increase in phytophagous insects providing a richer source of protein food for laying hens and growing nestlings.

https://www.jstor.org/stable/info/4217356

The abundance of invertebrate herbivores in relation to the availability of nitrogen in stressed food plants.

This review synthesizes the available knowledge on drought-disease interactions in forest trees with a focus on (1) evidence and patterns of drought-disease interactions, (2) current understanding of processes and mechanisms, and (3) three well documented cases studies. The first part is based on the analysis of a database of slightly more than one hundred studies, obtained by keyword searches combining drought, diseases or pathogens, and forest trees. A large majority of published studies referred to a positive association between drought and disease, i.e. disease favoured by drought or drought and disease acting synergistically on tree health status, with a predominance of canker/dieback diseases, caused by pathogens like Botryosphaeria, Sphaeropsis, Cytospora and Biscognauxia (Hypoxylon). The type of disease-related variables (incidence vs. severity) and the intensity and timing of water stress were shown to be significant factors affecting the drought- infection interaction. Interactions with other abiotic stresses and species-specific and genetic effects, related to host or pathogen, have also been reported. Direct effects of drought on pathogens are generally negative, although most fungal pathogens exhibit an important plasticity and can grow at water potentials well below the minimum for growth of their host plants. Studies on indirect effects of drought on pathogens through other community interactions are still relatively scarce. Positive drought-infection effects can mostly be explained by indirect effects of drought on host physiology. The predisposition and the multiple stress hypotheses are presented, as well as recent developments in the study of the molecular basis of abiotic and biotic stress, and their interactions. Sphaeropsis sapinea on pines, Biscognauxia mediterranea on oaks and root pathogens in declines associated with drought provide illustrative examples, treated as case studies, of pathogens of current significance associated with drought. The conclusion highlights some knowledge gaps, e.g. the role of latent parasites and the shift to a pathogenic stage, or the genetics of some fungal groups. The need for prevention of pathogen dispersal, especially crucial in the case of latent pathogens, is emphasized.

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Interactive effects of drought and pathogens in forest trees

Predisposition, Stress, and Plant Disease

Defence mechanisms against fungi in bark, xylem, roots, and foliage of woody angiosperms and gymnosperms are addressed in this newest addition to the "Springer Series in Wood Science". The topic is approached from an anatomical and physiological angle, with emphasis on the relationships between plant structure and function and their role in defensive reactions and wound repair. Contributions on the biochemistry of defensive reactions, in-depth treatises on specific diseases such as Dutch elm disease, and Fusiform rust of pines, as well as chapters on wound reactions in bark tissues, microenvironmental aspects of xylem decay, the role of mycorrhizal fungi in defence, physiological responses of trees to fungal symbionts of insects, and the application of tissue culture and immunocytochemistry to the study of defense mechanisms in trees are included.

Defense Mechanisms Of Woody Plants Against Fungi

The development of Hypoxylon canker of Populus tremuloides: role of interacting environmental factors

A technique was devised for obtaining extensive canker development on wounded greenhouse-grown aspens (Populus tremuloides Mich.) inoculated with mycelium of Hypoxylon pruinatum (Klot.) Cke. Cankers developed rapidly only on water-stressed plants, and under such conditions often averaged 75 mm in length within 10 days. Alcohol treatment of stems before inoculation did not affect the course of canker development. Cankers developed most rapidly on plants watered every third or every fifth day, and developed least in plants watered daily. Stem wounds exposing the wood were essential for canker development. Wounds required an 8-day period before inoculation to heal sufficiently to inhibit canker development. Wound sites midway on the stem were most susceptible to canker development, while sites near the growing point were the least susceptible. Time for symptom appearance was prolonged in inoculated plants of larger diameters.

Factors affecting canker development on Populus tremuloides artificially inoculated with Hypoxylon pruinatum

Proline and γ-amino-n-butyric acid were present in considerable amounts in the sap of resistant species of the Ulmaceae such as Ulmus pumila, Hemiptelea davidii, and Celtis occidentalis, but occurred only in trace amounts in the sap of susceptible U. americana, U. japonica, and U. carpinifolia. The xylem sap of U. japonica, C. occidentalis, and H. davidii contained large amounts of alanine. Total concentrations of amino acids and ammonia were higher in resistant species than in susceptible species. Fructose, glucose, and sucrose were present in the sap of all the six species. Sucrose was the main sugar in the sap of U. japonica, U. pumila, and C. occidentalis; in U. americana and U. carpinifolia sucrose and fructose were present in about equal amounts; in H. davidii fructose was the major sugar.

Nitrogenous and carbohydrate compounds in the xylem sap of Ulmaceae species varying in resistance to Dutch elm disease

Thirty-two ninhydrin-positive nitrogenous compounds were present in the xylem sap of 6- to 9-year- old American elms. Twenty-six of these compounds were identified. Total concentration of nitrogenous materials was several times higher in diseased than in healthy sap after inoculation during the susceptible period in the spring, but was lower after inoculations during the late resistant period. Percentage compositions of γ-amino-n-butyric acid, proline, and alanine in diseased sap increased after both spring and mid-summer inoculations. Proline, which was present in trace amounts in healthy sap, constituted 14 to 38% of the total in diseased sap. Percentage concentrations of amide nitrogen in diseased sap were reduced 50% or more in all inoculated trees. Percentages of ammonia, aspartic acid, glutamic acid, and several other amino acids did not change.

Nitrogenous compounds in the xylem sap of American elms with Dutch elm disease

/pdf/the-development-of-hypoxylon-canker-of-populus-tremuloides-16lghj2pgp.pdf

E.B. Smalley

Papers

The development of Hypoxylon canker of Populus tremuloides: role of interacting environmental factors

Factors affecting canker development on Populus tremuloides artificially inoculated with Hypoxylon pruinatum

Nitrogenous and carbohydrate compounds in the xylem sap of Ulmaceae species varying in resistance to Dutch elm disease

Nitrogenous compounds in the xylem sap of American elms with Dutch elm disease

The development of Hypoxylon canker of Populus tremuloides: role of ascospores, conidia, and toxins