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This study evaluated the growth characteristics of six guayule (Parthenium argentatum Gray) accessions irrigated with waters high in boron, selenium, and calcium, and sodium salts under greenhouse conditions. The synthetic irrigation waters used in this investigation mimic the water quality used frequently for irrigation in the westside of the San Joaquin Valley, California, U.S.A. Guayule is a natural rubber-producing industrial crop that is tolerant of marginal soils and drought, and it may be grown as an alternative crop with poor quality waters for this region. Among the six accessions investigated for 55 days, three of them showed increased natural rubber concentration when grown under saline irrigation conditions compared to a non-saline irrigation (control). Elemental analysis in the leaves revealed that guayule can also absorb and accumulate potentially toxic elements. Results from this study suggest that specific accessions of guayule can be grown under saline irrigation conditions. Guayule exhibited potential bioremediation benefits under agronomic conditions found in the westside of the San Joaquin Valley, CA.

The natural rubber producing plant guayule (Parthenium argentatum Gray) stores carbohydrates mainly in the form of fructans, synthesized and stored in the same tissues at the same time as the rubber polymer, and a potential source of carbon for rubber biosynthesis. The first committed step to fructan synthesis is catalyzed by sucrose:sucrose-1-fructosyltransferase (1-SST), which was downregulated to test whether reduction of carbohydrate synthesis would divert carbon instead to rubber biosynthesis. Guayule leaf strips were transformed by Agrobacterium-mediated technology, and plants with downregulated 1-SST were evaluated in the laboratory and greenhouse. The plant tissue fructan concentration was reduced significantly, and sucrose concentration increased, especially in root tissues of greenhouse-grown plants. However, increased natural rubber production did not result.

Summary Drought stress is the major limiting factor in agriculture. Wheat, which is the most widely grown crop in the world, is predominantly cultivated in drought‐prone rainfed environments. Since roots play a critical role in water uptake, root response to water limitations is an important component for enhancing wheat adaptation. In an effort to discover novel genetic sources for improving wheat adaptation, we characterized a wheat translocation line with a chromosomal segment from Agropyron elongatum, a wild relative of wheat, which unlike common wheat maintains root growth under limited‐water conditions. By exploring the root transcriptome data, we found that reduced transcript level of LATERAL ROOT DENSITY (LRD) gene under limited water in the Agropyron translocation line confers it the ability to maintain root growth. The Agropyron allele of LRD is down‐regulated in response to water limitation in contrast with the wheat LRD allele, which is up‐regulated by water deficit stress. Suppression of LRD expression in wheat RNAi plants confers the ability to maintain root growth under water limitation. We show that exogenous gibberellic acid (GA) promotes lateral root growth and present evidence for the role of GA in mediating the differential regulation of LRD between the common wheat and the Agropyron alleles under water stress. Suppression of LRD also had a positive pleiotropic effect on grain size and number under optimal growth conditions. Collectively, our findings suggest that LRD can be potentially useful for improving wheat response to water stress and altering yield components.

We report functional genomics studies of a CYP74 rubber particle protein from Parthenium argentatum, commonly called guayule. Previously identified as an allene oxide synthase (AOS), this CYP74 constitutes the most abundant protein found in guayule rubber particles. Transgenic guayule lines with AOS gene expression down-regulated by RNAi (AOSi) exhibited strong phenotypes that included agricultural traits conducive to enhancing rubber yield. AOSi lines had higher leaf and stem biomass, thicker stembark tissues, increased stem branching and improved net photosynthetic rate. Importantly, the rubber content was significantly increased in AOSi lines compared to the wild-type (WT), vector control and AOS overexpressing (AOSoe) lines, when grown in controlled environments both in tissue-culture media and in greenhouse/growth chambers. Rubber particles from AOSi plants consistently had less AOS particle-associated protein, and lower activity (for conversion of 13-HPOT to allene oxide). Yet plants with downregulated AOS showed higher rubber transferase enzyme activity. The increase in biomass in AOSi lines was associated with not only increases in the rate of photosynthesis and non-photochemical quenching (NPQ), in the cold, but also in the content of the phytohormone SA, along with a decrease in JA, GAs, and ABA. The increase in biosynthetic activity and rubber content could further result from the negative regulation of AOS expression by high levels of salicylic acid in AOSi lines and when introduced exogenously. It is apparent that AOS in guayule plays a pivotal role in rubber production and plant growth.

Guayule (Parthenium argentatum) is under development in the southwestern United States as a source of domestic natural rubber; increased rubber yield was found for greenhouse-grown guayule with downregulated allene oxide synthase (AOS). The objective of this study was to evaluate natural rubber production in guayule plants with varying levels of AOS gene expression grown in a field environment. Four plant genotypes: wildtype plants, vector controls, and transgenic plants with overexpressed and downregulated allene oxide synthase (total of 960 plants) were grown at the Bridgestone Guayule Research Farm in Eloy, Arizona between May 2016 to May 2018. Plant phenotypes were evaluated at 6-, 12-, 18-, and 24-months. Downregulated allene oxide synthase (AOSi) genotypes showed remarkable phenotypes including higher photosynthetic activity (net assimilation rate), total number of branches per plant, and plant stembark thickness compared with wildtype, vector control, and overexpressed AOS (AOSoe) lines. Moreover, the rubber particles were smaller and had higher activity (radiolabeled isoprenyl pyrophosphate (IPP) incorporation per g of rubber) in the AOSi genotypes than control and AOSoe lines. The downregulation of AOS also led to significant changes in phytohormone levels, which may have influenced the plants’ morphology and physiology. However, in contrast to laboratory and greenhouse studies, natural rubber concentration and yield were not higher in AOSi plants.

Root architecture traits are an important component for improving water stress adaptation. However, selection for aboveground traits under favorable environments in modern cultivars may have led to an inadvertent loss of genes and novel alleles beneficial for adapting to environments with limited water. In this study, we elucidate the physiological and molecular consequences of introgressing an alien chromosome segment (7DL) from a wild wheat relative species (Agropyron elongatum) into cultivated wheat (Triticum aestivum). The wheat translocation line had improved water stress adaptation and higher root and shoot biomass compared with the control genotypes, which showed significant drops in root and shoot biomass during stress. Enhanced access to water due to higher root biomass enabled the translocation line to maintain more favorable gas-exchange and carbon assimilation levels relative to the wild-type wheat genotypes during water stress. Transcriptome analysis identified candidate genes associated with root development. Two of these candidate genes mapped to the site of translocation on chromosome 7DL based on single-feature polymorphism analysis. A brassinosteroid signaling pathway was predicted to be involved in the novel root responses observed in the A. elongatum translocation line, based on the coexpression-based gene network generated by seeding the network with the candidate genes. We present an effective and highly integrated approach that combines root phenotyping, whole-plant physiology, and functional genomics to discover novel root traits and the underlying genes from a wild related species to improve drought adaptation in cultivated wheat.