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Hyperspectral imaging is a promising tool for non-destructive phenotyping of plant physiological traits, which has been transferred from remote to proximal sensing applications, and from manual laboratory setups to automated plant phenotyping platforms. Due to the higher resolution in proximal sensing, illumination variation and plant geometry result in increased non-biological variation in plant spectra that may mask subtle biological differences. Here, a better understanding of spectral measurements for proximal sensing and their application to study drought, developmental and diurnal responses was acquired in a drought case study of maize grown in a greenhouse phenotyping platform with a hyperspectral imaging setup. The use of brightness classification to reduce the illumination-induced non-biological variation is demonstrated, and allowed the detection of diurnal, developmental and early drought-induced changes in maize reflectance and physiology. Diurnal changes in transpiration rate and vapor pressure deficit were significantly correlated with red and red-edge reflectance. Drought-induced changes in effective quantum yield and water potential were accurately predicted using partial least squares regression and the newly developed Water Potential Index 2, respectively. The prediction accuracy of hyperspectral indices and partial least squares regression were similar, as long as a strong relationship between the physiological trait and reflectance was present. This demonstrates that current hyperspectral processing approaches can be used in automated plant phenotyping platforms to monitor physiological traits with a high temporal resolution.

Jasmonate-mediated regulation of VOC emission has been extensively investigated in higher plants, however, only little is known about VOC production and its regulation in ferns. Here, we investigate whether the emission of VOCs from bracken fern Pteridium aquilinum is triggered by herbivory and if so - whether it is regulated by the octadecanoid signaling pathway. Interestingly, feeding of both generalist (Spodoptera littoralis) and specialist (Strongylogaster multifasciata) herbivores as well as application of singular and continuous mechanical wounding of fronds induced only very low levels of VOC emission. In contrast, treatment with jasmonic acid (JA) led to the emission of a blend of VOCs that was mainly comprised of terpenoids. Likewise, treatment with the JA precursor 12-oxo-phytodienoic acid (OPDA) and α-linolenic acid also induced VOC emission, albeit to a lower intesity than the JA treatment. Accumulation of endogenous JA was low in mechanically wounded fronds and these levels were unaffected by the application of oral secretions from both generalist or specialist herbivores. The emission of terpenoids upon JA treatment could be blocked with fosmidomycin and mevinolin, which are inhibitors of the MEP- and MVA pathways, respectively. These results indicate that similar to higher plants, terpenoid VOCs are produced via these pathways in bracken fern and that these pathways are JA-responsive. However, the very low amounts of terpenoids released after herbivory or mechanical damage are in stark contrast to what is known from higher plants. We speculate that S. multifasciata and S. littoralis feeding apparently did not induce the threshold levels of JA required for activating the MEP and MVA pathways and the subsequent volatile emission in bracken fern.

CORONATINE INSENSITIVE 1 (COI1) is a well-known key player in processes downstream of jasmonic acid (JA) biosynthesis: silencing COI1 in Nicotiana attenuata (ir-coi1) makes plants insensitive to JA, prevents the up-regulation of JA-mediated defenses and decreases the plants resistance to herbivores and pathogens. In agreement with previous studies, we observed that regulation of several JA biosynthesis genes elicited by Manduca sexta oral secretions (OS) is COI1 dependent. In response to wounding and application of OS ir-coi1 plants accumulate 75% less JA compared with wild-type plants (WT), resembling JA levels found in plants silenced in the key enzyme in JA biosynthesis LIPOXYGENASE 3 (as-lox). However, while OS-elicited as-lox plants also accumulated lower levels of the JA-conjugate JA–isoleucine (JA-Ile) than did WT plants, JA-Ile accumulation in ir-coi1 was higher, prolonged and peaked with a delay of 30 min. In vivo substrate feeding experiments of N. attenuata demonstrate that the increased and prolonged JA-Ile accumulation pattern in ir-coi1 is not the result of altered substrate availability, i.e. of JA and/or Ile, but is due to an ∼6-fold decrease in JA-Ile turnover. These results provide the first evidence for a second, novel regulatory feedback function of COI1 in enhancing JA-Ile turnover. Hence, in addition to its control over JA biosynthesis, COI1 might fine-tune the dynamics of the jasmonate response after induction by herbivore elicitors. [ABSTRACT FROM AUTHOR]

The fatty acid oxygenation up-regulated 2 (fou2) mutant in Arabidopsis thaliana creates a gain-of-function allele in a non-selective cation channel encoded by the Two Pore Channel 1 (TPC1) gene. This mutant genetically implicates cation fluxes in the control of the positive feedback loop whereby jasmonic acid (JA) stimulates its own synthesis. In this study we observed extensive transcriptome reprogramming in healthy fou2 leaves closely resembling that induced by treatment with methyl jasmonate, biotic stresses and the potassium starvation response. Proteomic analysis of fou2 leaves identified increased levels of seven biotic stress- and JA-inducible proteins. In agreement with these analyses, epistasis studies performed by crossing fou2 with aos indicated that elevated levels of JA in fou2 are the major determinant of the mutant phenotype. In addition, generation of fou2 aba1-5, fou2 etr1-1 and fou2 npr1-1 double mutants showed that the fou2 phenotype was only weakly affected by ABA levels and unaffected by mutations in NPR1 and ETR1. The results now suggest possible mechanisms whereby fou2 could induce JA synthesis/signaling early in the wound response. In contrast to fou2, transcriptome analysis of a loss-of-function allele of TPC1, tpc1-2, revealed no differential expression of JA biosynthesis genes in resting leaves. However, the analysis disclosed reduced mRNA levels of the pathogenesis-related genes PDF1.2a and THI2.1 in healthy and diseased tpc1-2 leaves. The results suggest that wild-type TPC1 contributes to their expression by mechanisms somewhat different from those affecting their expression in fou2. [ABSTRACT FROM AUTHOR]