Arabinogalactan proteins (AGPs) are a family of hydroxyproline-rich glycoproteins (HRGP) ubiquitous in the plant kingdom. They are probably one of the most heterogeneous and complex families of macromolecules, making them able to perform different and multiple functions. Located at the plasma membrane-cell wall interface, AGPs are involved in several processes, from plant growth and development to reproduction. An additional function of AGPs in response to biotic and abiotic stress has been suggested by several studies. The purpose of this review is to summarize critically and analytically the available knowledge on the effects of abiotic stress (low and high temperatures, drought, flooding, anoxia and metal deficiency/toxicity) and biotic stress (bacteria, fungi, nematodes and viruses) on AGPs. A deeper understanding of the role of AGPs during these conditions can be an important tool for understanding AGP biology and for the possible development of efficient breeding strategies.

Drought stress is one of the most severe conditions for plants, especially in the face of the emerging problem of global warming. This issue is important when considering economically relevant crops, including the tomato. For these plants, a promising solution is the valorization of local agrobiodiversity as a source of genetic variability. In this paper we investigated how six Italian tomato varieties react to a prolonged period of water depletion. We used a multidisciplinary approach, from genetics to plant physiology and cytology, to provide a detailed overview of the response of plants to stress. The varieties analyzed, each characterized by a specific genetic profile, showed a genotype-specific response with the variety ‘Fragola’ being the most resistant and the variety ‘Pisanello’ the most susceptible. For all the parameters evaluated, ‘Fragola’ performed in a manner comparable to that of control plants. On the contrary, ‘Pisanello’ appeared to be more affected and showed an increase in the number of stomata and a drastic increase in antioxidants, a symptom of acute oxidative stress. Our work suggests the existence of a valuable reservoir of genetic biodiversity with more drought-tolerant tomato genotypes opening the way to further exploitation and use of local germplasm in breeding programs.

Land plants are constantly subjected to multiple unfavorable or even adverse environmental conditions. Among them, abiotic stresses (such as salt, drought, heat, cold, heavy metals, ozone, UV radiation, and nutrient deficiencies) have detrimental effects on plant growth and productivity and are increasingly important considering the direct or indirect effects of climate change. Plants respond in many ways to abiotic stresses, from gene expression to physiology, from plant architecture to primary, and secondary metabolism. These complex changes allow plants to tolerate and/or adapt to adverse conditions. The complexity of plant response can be further influenced by the duration and intensity of stress, the plant genotype, the combination of different stresses, the exposed tissue and cell type, and the developmental stage at which plants perceive the stress. It is therefore important to understand more about how plants perceive stress conditions and how they respond and adapt (both in natural and anthropogenic environments). These concepts were the basis of the Special Issue that International Journal of Molecular Sciences expressly addressed to the relationship between environmental stresses and plants and that resulted in the publication of 5 reviews and 38 original research articles. The large participation of several authors and the good number of contributions testifies to the considerable interest that the topic currently receives in the plant science community, especially in the light of the foreseeable climate changes. Here, we briefly summarize the contributions included in the Special Issue, both original articles categorized by stress type and reviews that discuss more comprehensive responses to various stresses.

In clementine, failure of fertilization can result in parthenocarpic fruit development, which has several advantages, such as seedless fruit, longer shelf-life, and greater consumer appeal. Recently, S-RNases have been identified in Citrus grandis , thus revealing that the self-incompatibility (SI) reaction relies on the S-RNase gametophytic mechanism. The fundamental role of environmental factors, mostly temperature, in determining the numbers of pollen tubes reaching the ovary is also well established in Citrus . In the present work, temperature-dependent pollen–pistil interactions in C. clementina were analyzed, focusing on several morphological aspects, as well as on polyamine (PA) content and the activity and distribution of transglutaminase (TGase), both reported to be involved in the SI response in pear and in pummelo. Results clearly indicate that temperature contributed to a different activation of the SI response, which occurs at optimal temperature of 25°C but was by-passed at 15°C. TGase activity was stimulated during the SI response, and it localized differently in the compatible and incompatible interaction: in compatible pollinated styles, TGase localized inside the style canal, while it was detected all around it in incompatible crosses. TGase localization and activity were congruent with the levels of soluble and insoluble conjugated PAs and with morphological evidences, which highlighted cell wall modification occurring as a result of SI.

Arabinogalactan-proteins are a family of highly glycosylated hydroxyproline-rich glycoproteins widely distributed in the plant kingdom and mainly located at the cell surface. Because of their great heterogeneity, abundance and ubiquitous localization, arabinogalactan-proteins are thought to play important and different roles in plant growth and development. Many evidences also indicate a role of arabinogalactan-proteins during reproduction as well as in response to stress conditions. In the present work, we investigated the distribution of arabinogalactanproteins recognised by JIM8 antibody in pistils of Solanum lycopersicum cv Micro-Tom heat-stressed for 3 h at 42 C at different developmental stages (5 and 10 days before anthesis). Our results indicate that high temperature strongly affects the distribution and content of arabinogalactan-proteins in stigma and ovule, particularly in samples heat-stressed 5 days before anthesis. In stigmas, cells exhibited an altered pattern of JIM8-labelled AGPs, dispersed and less abundant. In ovules, the embryo sacsurrounding cells showed a clear reduction in the content of JIM8-labelled arabinogalactan proteins. These evidences suggest that heat stress affects both content and distribution of AGPs. Considering the role of AGPs in plant reproduction, from the acquisition of stigmatic receptivity to pollen guidance into the ovule, we can suppose that high temperature affects all these processes through the alteration of AGPs

Resequencing in resistant cultivar 'Nure' and structural comparison with the same region of susceptible 'Morex' was performed in order to gain a better insight into barley Frost-resistance-H2 locus. Accurate annotation showed copy number variation (CNV) in the proximal part of the locus. In 'Nure', two exact copies of the HvCBF4-HvCBF2A region and one of the HvCBF4-HvCBF2B segment were observed, while in 'Morex' the corresponding region harboured a single HvCBF4-HvCBF2A (22 kb) segment. Abundance and diversity of repetitive element classes, gene function gain/losses, regulatory motifs and SNPs in gene sequences were identified. An expression study of key HvCBFs with/without CNV on selected genotypes contrasting for frost resistance and estimated HvCBF4-HvCBF2B copy number (2-10 copies) was also performed. Under light stimulus at warm temperature (23°C), CNV of HvCBF2A and HvCBF4 correlated with their expression levels and reported frost resistance of genotypes; moreover, expression levels of HvCBF2A and HvCBF14 were strongly correlated (r=0.908, p<0.01). On the other hand, frost resistance correlated to HvCBF14 expression (r=0.871, p<0.01) only after cold induction (6°C) in the dark. A complex interplay of HvCBFs expression levels under different light/temperature stimuli is discussed in light of CNV and presence/number of regulatory elements that integrate different signal transduction pathways.

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Callose is a cell wall polysaccharide involved in several fundamental biological processes, ranging from plant development to response to abiotic and biotic stresses. To understand how callose deposition is regulated, it is important to know how its synthesizing enzyme, i.e., callose synthase, is regulated and if it interacts with vesicular-cytoskeletal system of plant cells. Actin filaments are thought to determine the long-range distribution of callose synthase through transport vesicles. Unlike other enzymes (such as cellulose synthase) that synthesize cell wall polysaccharides, the spatial and biochemical relationships between callose synthase and microtubules are poorly understood. Some experimental evidence already support the association between callose synthase and tubulin, however, despite its importance in maintaining plant integrity, knowledge about regulation of callose biosynthesis is still limited. Here we investigated the association between callose synthase and cytoskeleton by biochemical and ultrastructural analyses in a model system, pollen tube, where callose is an essential cell wall component. Native 2-D electrophoresis and isolation of the callose synthase complex confirmed that callose synthase is associated with tubulin and can interface with cortical microtubules. In contrast, actin and sucrose synthase (which supplies UDP-glucose to callose synthase) are not permanently associated with callose synthase. Immunogold labeling showed strong colocalization of the enzyme and microtubules; this association is occasionally mediated by vesicles. The association between callose synthase and vesicles was also demonstrated by co-distribution between the enzyme and Rab11b; in addition, the not homogeneous distribution of callose synthase in cell membranes is also shown by analysis of membrane microdomains.