Novel Bisexual Flower Control Gene Regulates Sex Differentiation in Melon (Cucumis melo L.)

The sex-control system involves several mechanisms in melon. The present study identified a novel bisexual flower control gene from the hermaphroditic melon germplasm, different from the previously recognized one. Genetic analysis showed that a single recessive gene in the newly identified locus b controlled the bisexual flower phenotype in melons. We generated 1431 F₂ segregating individuals for genetic mapping of locus b, which was delimited to a 47.94 kb region. Six candidate genes were identified in the delimited interval, and candidate No. 4 encoding melon CPR5 protein was selected as the suitable one for locus b and was denoted CmCPR5. CPR5 reportedly interacted with ethylene receptor ETR1 to regulate ethylene signal transduction. Moreover, the ethephon assays showed that the parental lines (unisexual line and bisexual line) had contrasting expression patterns of CmCPR5. The BiFC and LCI assays also confirmed that CmCPR5 interacted with CmETR1 in 0426 but not in Y101. However, crossover tests showed that CmETR1 functioned normally in both parental lines, suggesting CPR5 malfunction in Y101. This study proposed a corollary mechanism of bisexual flower regulation during stamen primordium development in which the inhibition of stamen primordia development was prevented by the malfunctioning CmCPR5, resulting in bisexual flowers.

The Effect of Macronutrient Availability on Pomegranate Reproductive Development

Pomegranate cultivation has expanded significantly in the last two decades. However, there is limited information on its fertilization requirements and the effect of macronutrient availability on its reproductive development. Two commercial pomegranate cultivars—“Wonderful” and “Emek”—were grown in 500-L containers for 3 years, using a fertigation system. Development and reproduction indices were measured to explore the trees’ responses to elevated levels of nitrogen (N), phosphorus (P) and potassium (K) in the irrigation solution. Andromonoecy rate was affected by nutrient levels only in the first year of the experiment, with higher levels of N and P leading to a greater proportion of hermaphrodites out of total flowers. P level had a positive effect on the total number of hermaphrodites per tree in both varieties. Differences recorded between hermaphroditic and staminate flowers included nutrient concentrations and dry weight. Fruit set and aril number were positively affected by N concentration in the irrigation solution. We conclude that only a severe deficiency of N and P affects the andromonoecy trait, and that at the levels examined in this study, K hardly influences pomegranate reproduction.

Flower-Specific Overproduction of Cytokinins Altered Flower Development and Sex Expression in the Perennial Woody Plant Jatropha curcas L

Jatropha curcas L. is monoecious with a low female-to-male ratio, which is one of the factors restricting its seed yield. Because the phytohormone cytokinins play an essential role in flower development, particularly pistil development, in this study, we elevated the cytokinin levels in J. curcas flowers through transgenic expression of a cytokinin biosynthetic gene (AtIPT4) from Arabidopsis under the control of a J. curcas orthologue of TOMATO MADS BOX GENE 6 (JcTM6) promoter that is predominantly active in flowers. As expected, the levels of six cytokinin species in the inflorescences were elevated, and flower development was modified without any alterations in vegetative growth. In the transgenic J. curcas plants, the flower number per inflorescence was significantly increased, and most flowers were pistil-predominantly bisexual, i.e., the flowers had a huge pistil surrounded with small stamens. Unfortunately, both the male and the bisexual flowers of transgenic J. curcas were infertile, which might have resulted from the continuously high expression of the transgene during flower development. However, the number and position of floral organs in the transgenic flowers were well defined, which suggested that the determinacy of the floral meristem was not affected. These results suggest that fine-tuning the endogenous cytokinins can increase the flower number and the female-to-male ratio in J. curcas.

Developmental changes and organelle biogenesis in the reproductive organs of thermogenic skunk cabbage (Symplocarpus renifolius)

Sex-dependent thermogenesis during reproductive organ development in the inflorescence is a characteristic feature of some of the protogynous arum species. One such plant, skunk cabbage (Symplocarpus renifolius), can produce massive heat during the female stage but not during the subsequent male stage in which the stamen completes development, the anthers dehisce, and pollen is released. Unlike other thermogenic species, skunk cabbage belongs to the bisexual flower group. Although recent studies have identified the spadix as the thermogenic organ, it remains unclear how individual tissues or intracellular structures are involved in thermogenesis. In this study, reproductive organ development and organelle biogenesis were examined during the transition from the female to the male stage. During the female stage, the stamens exhibit extensive structural changes including changes in organelle structure and density. They accumulate high levels of mitochondrial proteins, including possible thermogenic factors, alternative oxidase, and uncoupling protein. By contrast, the petals and pistils do not undergo extensive changes during the female stage. However, they contain a larger number of mitochondria than during the male stage in which they develop large cytoplasmic vacuoles. Comparison between female and male spadices suggests that mitochondrial number rather than their level of activity correlates with thermogenesis. Their spadices, even in the male, contain a larger amount of mitochondria that had greater oxygen consumption, compared with non-thermogenic plants. Taken together, our data suggest that the extensive maturation process in stamens produces massive heat through increased metabolic activities. The possible mechanisms by which petal and pistil metabolism may affect thermogenesis are also discussed.