Developmental stages and episode-specific regulatory genes in andromonoecious melon flower development

BACKGROUND AND AIMS

Given the lack of specific studies on floral development in melon (Cucumis melo L.), we carried out an extensive study involving morphological and transcriptomic analyses to characterize floral development in this species.

METHODS

Using an andromonoecious line, we analysed the development of floral buds in male and hermaphrodite flowers with both light microscopy and scanning electron microscopy. Based on flower lengths, we established a correlation between the developmental stages and four main episodes of floral development and conducted an extensive RNA sequencing analysis of these episodes.

KEY RESULTS

We identified 12 stages of floral development, from the appearance of the floral meristems to anthesis. The main structural differences between male and hermaphrodite flowers appeared between stages 6 and 7; later stages of development leading to the formation of organs and structures in both types of flowers were also described. We analysed the gene expression patterns of the four episodes in flower development to find the genes that were specific to each given episode. Among others, we identified genes that defined the passage from one episode to the next according to the ABCDE model of floral development.

CONCLUSIONS

This work combines a detailed morphological analysis and a comprehensive transcriptomic study to enable characterization of the structural and molecular mechanisms that determine the floral development of an andromonoecious genotype in melon. Taken together, our results provide a first insight into gene regulation networks in melon floral development that are crucial for flowering and pollen formation, highlighting potential targets for genetic manipulation to improve crop yield of melon in the future.

Editing melon eIF4E associates with virus resistance and male sterility

The cap-binding protein eIF4E, through its interaction with eIF4G, constitutes the core of the eIF4F complex, which plays a key role in the circularization of mRNAs and their subsequent cap-dependent translation. In addition to its fundamental role in mRNA translation initiation, other functions have been described or suggested for eIF4E, including acting as a proviral factor and participating in sexual development. We used CRISPR/Cas9 genome editing to generate melon eif4e knockout mutant lines. Editing worked efficiently in melon, as we obtained transformed plants with a single-nucleotide deletion in homozygosis in the first eIF4E exon already in a T0 generation. Edited and non-transgenic plants of a segregating F2 generation were inoculated with Moroccan watermelon mosaic virus (MWMV); homozygous mutant plants showed virus resistance, while heterozygous and non-mutant plants were infected, in agreement with our previous results with plants silenced in eIF4E. Interestingly, all homozygous edited plants of the T0 and F2 generations showed a male sterility phenotype, while crossing with wild-type plants restored fertility, displaying a perfect correlation between the segregation of the male sterility phenotype and the segregation of the eif4e mutation. Morphological comparative analysis of melon male flowers along consecutive developmental stages showed postmeiotic abnormal development for both microsporocytes and tapetum, with clear differences in the timing of tapetum degradation in the mutant versus wild-type. An RNA-Seq analysis identified critical genes in pollen development that were down-regulated in flowers of eif4e/eif4e plants, and suggested that eIF4E-specific mRNA translation initiation is a limiting factor for male gametes formation in melon.

Virus-Infected Melon Plants Emit Volatiles that Induce Gene Deregulation in Neighboring Healthy Plants

It is well described that viral infections stimulate the emission of plant volatiles able to recruit viral vectors thereby promoting virus spread. In contrast, much less is known on the effects that emitted volatiles may have on the metabolism of healthy neighboring plants, which are potential targets for new infections through vector transmission. Watermelon mosaic virus (WMV) (genus Potyvirus, family Potyviridae) is an aphid-transmitted virus endemic in cucurbit crops worldwide. We have compared gene expression profiles of WMV-infected melon plants with those of healthy or healthy-but-cohabited-with-infected plants. Pathogenesis-related (PR) and small heat shock protein encoding genes were deregulated in cohabited plants, and PR deregulation depended on the distance to the infected plant. The signaling was short distance in the experimental conditions used, and cohabiting had a moderate effect on the plant susceptibility to WMV. Static headspace experiments showed that benzaldehyde and γ-butyrolactone were significantly over-emitted by WMV-infected plants. Altogether, our data suggest that perception of a volatile signal encoded by WMV-infected tissues triggers a response to prepare healthy tissues or/and healthy neighboring plants for the incoming infections.

Metabolic and cardiovascular adjustment to work in air and water at 18, 25, and 33 degrees C

By use of successive increments of discontinuous work with an arm-leg cycle ergometer the VO2, Q, SV, and HR were studied in six male subjects at rest and during exercise in air and in water at 18, 25, and 33 degrees C. The Q values obtained by CO2 rebreathing were reproducible. VO2 was linearly related to work with the plots for air and 33 degrees C water being similar. However, during work in 25 and 18 degrees C water, the VO2 averaged 9.0% (150 ml) and 25.3% (400 ml) higher, respectively, than values observed in 33 degrees C water, with the largest differences observed in leaner subjects. The plot of HR-VO2 was linear and almost identical during work in air and 33 degrees C water, but shifted significantly to the right in cooler water. VO2 averaged 250-700 ml higher in cold water compared to air and 33 degrees C water at a given mean heart rate. The Q vs. VO2 line was similar during work in air and in water with no effect of water or temperature. At similar levels of VO2, SV was significantly larger (P less than 0.05) in 25 and 18 degrees C water than in air or 33 degrees C water. Consequently, the reduction in heart rate during work in cold water was entirely compensated for by a proportionate increase in the SV of the heart. Q was therefore maintained at similar levels of energy expenditure in air and in 18, 25, and 30 degrees C water.

Specificity of swim training on maximum oxygen uptake

The present study was designed to evaluate the specificity-generality of the cardiorespiratory adaptation to swim training. Fifteen male, college age, recreational swimmers utilized interval swim training procedures 1 h/day, 3 days/wk for 10 wk. Maximum physiological measures (Vo2, Ve, HR, R, and work time) were determined prior to and following swim training during treadmill running and tethered swimming Vo2max tests. Identical measures were made on 15 control subjects who did not participate in any form of training. As a result of training, the experimental subjects significantly increased (all P is less than 0.01) their swimming Vo2max (380 ml/min) max Ve (14.9 l/min,btps) and max swim time (4.0 min), and significantly decreased (P is less than 0.05) their max HR (3.5 beats/min). However, there was no significant improvement in Vo2max when the same subjects were evaluated by the treadmill running test. Differences in Vo2max and associated measures during running and swimming tests remained essentially unchanged for control subjects. The results of the present study clearly demonstrate the specificity of the cardiorespiratory adaptation to swim training in male recreational swimmers.