Hop (Humulus lupulus L.) response mechanisms in drought stress: Proteomic analysis with physiology

Climate-induced decline in the quality and quantity of European hops calls for immediate adaptation measures

A recent rise in the global brewery sector has increased the demand for high-quality, late summer hops. The effects of ongoing and predicted climate change on the yield and aroma of hops, however, remain largely unknown. Here, we combine meteorological measurements and model projections to assess the climate sensitivity of the yield, alpha content and cone development of European hops between 1970 and 2050 CE, when temperature increases by 1.4 °C and precipitation decreases by 24 mm. Accounting for almost 90% of all hop-growing regions, our results from Germany, the Czech Republic and Slovenia show that hop ripening started approximately 20 days earlier, production declined by almost 0.2 t/ha/year, and the alpha content decreased by circa 0.6% when comparing data before and after 1994 CE. A predicted decline in hop yield and alpha content of 4-18% and 20-31% by 2050 CE, respectively, calls for immediate adaptation measures to stabilize an ever-growing global sector.

Resilience of hop (Humulus lupulus L.) to salinity, heat and drought stresses: A mini-review

Over recent years, the cultivation of hops (Humulus lupulus L.) has spread widely in the Mediterranean, also affecting the southern regions of Spain and Italy with a typical semi-arid climate. Several and recent studies have investigated the responses of this species to the main abiotic stresses, which is an aspect of absolute relevance to the knowledge of the adaptive capacity of hops to the growing conditions of a new cultivation environment. Moreover, given the fact that hops' phytochemical composition is determined primarily by genetic and environmental factors, and that the species is perennial, the lack of knowledge on the effects of abiotic stress could be reflected in subsequent years, which means multi-year economic risks. This review work therefore aims to showcase, based on an in-depth investigation of the available literature, the response of hop to the main abiotic stresses, and the effect of these on productive and qualitative crop performances. The data presented will be useful to the understanding of constraints and to the identification of useful coping strategies to the cultivation of hops in semi-arid Mediterranean environments.

Plant Growth Regulators Improve Grain Production and Water Use Efficiency of Foeniculum vulgare Mill. under Water Stress

The development of methods increasing plant water use efficiency (WUE) would enhance the ability to grow wild aromatic and medicinally important species. The aim of this research was to determine the effect of plant growth regulators (PGRs) applied by spraying on stress resistance and WUE of fennel subjected to water stress. Plants in the generative stage were more drought tolerant than those in the vegetative stage. Water stress at vegetative stage decreased plant biomass and grain yield by 60% and 61%, respectively. Severe water stress in vegetative stage reduced grain production by 56%, and grains had 43% lower mass than those from non-stressed plants. Application of PGRs at both stages of growth increased grain yield and biomass, but the magnitude of increase depended on the type and application time of PGRs. Plants grown in well-watered conditions and sprayed with methyl jasmonate during the vegetative stage had the highest grain production (2.7 g plant⁻¹), whereas under moderate water stress, plants yielded the best (2.1 g plant⁻¹) when sprayed with epibrassinolide. The maximum WUE for grain (0.91 g L⁻¹) and essential oil production (20 mg L⁻¹) was noted in plants exposed to moderate stress and treated with methyl jasmonate during the vegetative stage.

Gene expression for secondary metabolite biosynthesis in hop (Humulus lupulus L.) leaf lupulin glands exposed to heat and low-water stress

Hops are valued for their secondary metabolites, including bitter acids, flavonoids, oils, and polyphenols, that impart flavor in beer. Previous studies have shown that hop yield and bitter acid content decline with increased temperatures and low-water stress. We looked at physiological traits and differential gene expression in leaf, stem, and root tissue from hop (Humulus lupulus) cv. USDA Cascade in plants exposed to high temperature stress, low-water stress, and a compound treatment of both high temperature and low-water stress for six weeks. The stress conditions imposed in these experiments caused substantial changes to the transcriptome, with significant reductions in the expression of numerous genes involved in secondary metabolite biosynthesis. Of the genes involved in bitter acid production, the critical gene valerophenone synthase (VPS) experienced significant reductions in expression levels across stress treatments, suggesting stress-induced lability in this gene and/or its regulatory elements may be at least partially responsible for previously reported declines in bitter acid content. We also identified a number of transcripts with homology to genes shown to affect abiotic stress tolerance in other plants that may be useful as markers for breeding improved abiotic stress tolerance in hop. Lastly, we provide the first transcriptome from hop root tissue.

UPLC-QTof-MSE Metabolomics Reveals Changes in Leaf Primary and Secondary Metabolism of Hop (Humulus lupulus L.) Plants under Drought Stress

The hop (Humulus lupulus L.) is an important specialty crop used in beer production. Untargeted UPLC-QTof-MS^E metabolomics was used to determine metabolite changes in the leaves of hop plants under varying degrees of drought stress. Principal component analysis revealed that drought treatments produced qualitatively distinct changes in the overall chemical composition of three out of four genotypes tested (i.e., Cascade, Sultana, and a wild var. neomexicanus accession but not Aurora), although differences among treatments were smaller than differences among genotypes. A total of 14 compounds consistently increased or decreased in response to drought stress, and this effect was generally progressive as the severity of drought increased. A total of 10 of these marker compounds were tentatively identified as follows: five glycerolipids, glutaric acid, pheophorbide A, abscisic acid, roseoside, and dihydromyricetin. Some of the observed metabolite changes likely occur across all plants under drought conditions, while others may be specific to hops or to the type of drought treatments performed.

The disadvantages of being a hybrid during drought: A combined analysis of plant morphology, physiology and leaf proteome in maize

A comparative analysis of various parameters that characterize plant morphology, growth, water status, photosynthesis, cell damage, and antioxidative and osmoprotective systems together with an iTRAQ analysis of the leaf proteome was performed in two inbred lines of maize (Zea mays L.) differing in drought susceptibility and their reciprocal F1 hybrids. The aim of this study was to dissect the parent-hybrid relationships to better understand the mechanisms of the heterotic effect and its potential association with the stress response. The results clearly showed that the four examined genotypes have completely different strategies for coping with limited water availability and that the inherent properties of the F1 hybrids, i.e. positive heterosis in morphological parameters (or, more generally, a larger plant body) becomes a distinct disadvantage when the water supply is limited. However, although a greater loss of photosynthetic efficiency was an inherent disadvantage, the precise causes and consequences of the original predisposition towards faster growth and biomass accumulation differed even between reciprocal hybrids. Both maternal and paternal parents could be imitated by their progeny in some aspects of the drought response (e.g., the absence of general protein down-regulation, changes in the levels of some carbon fixation or other photosynthetic proteins). Nevertheless, other features (e.g., dehydrin or light-harvesting protein contents, reduced chloroplast proteosynthesis) were quite unique to a particular hybrid. Our study also confirmed that the strategy for leaving stomata open even when the water supply is limited (coupled to a smaller body size and some other physiological properties), observed in one of our inbred lines, is associated with drought-resistance not only during mild drought (as we showed previously) but also during more severe drought conditions.