Fruit illumination stimulates cell division but has no detectable effect on fruit size in tomato (Solanum lycopersicum)

Heat shock exposure during early wheat grain development can reduce maximum endosperm cell number but not necessarily final grain dry mass

Post-anthesis heat shocks, which are expected to increase in frequency under climate change, may affect wheat grain development and lead to significant decreases in grain yield. Grain development occurs in three phases, the lag-phase, the filling-phase, and maturation. The growth of the three main compartments of the grain (outer layers (OLs), endosperm, embryo) is staggered, so that heat shocks affect time- and tissue-specific growth processes differentially depending on their timing. We hypothesized that heat shocks during the lag-phase may reduce final grain size, resulting from a reduction in endosperm cell number and/or a restricted OLs growth. Plants were heated for four consecutive days during the lag-phase or the filling-phase or both phases (lag- and filling-). Heat shocks consisted in four hours a day at 38°C and 21°C for the rest of the day. Controlled plants were maintained at 21/14°C (day/night). For each temperature treatment, kinetics of whole grain and compartment masses and dimensions were measured as well as the endosperm cell number. An early heat shock reduced endosperm cell proliferation. However, the growth patterns neither of endosperm nor of OLs were modified compared to controls, resulting in no differences in final grain size. Furthermore, compared to controls, a single heat shock during the filling-phase reduced both the duration and rate of dry mass accumulation into grains, whereas two consecutive shocks reduced the duration but enhanced the rate of dry mass of accumulation, even when endosperm cell number was reduced. The mean endosperm cell size was shown to be larger after early heat shocks. All together, these results suggest a compensatory mechanism exists to regulate endosperm cell size and number. This process might be a new mechanistic target for molecular studies and would improve our understanding of post-anthesis wheat tolerance to heat-shocks.

The conserved brassinosteroid-related transcription factor BIM1a negatively regulates fruit growth in tomato

Brassinosteroids (BRs) are steroid hormones that play key roles in plant development and defense. Our goal is to harness the extensive knowledge of the Arabidopsis BR signaling network to improve productivity in crop species. This first requires identifying components of the conserved network and their function in the target species. Here, we investigated the function of SlBIM1a, the closest tomato homolog of AtBIM1, which is highly expressed in fruit. SlBIM1a-overexpressing lines displayed severe plant and fruit dwarfism, and histological characterization of different transgenic lines revealed that SlBIM1a expression negatively correlated with fruit pericarp cell size, resulting in fruit size modifications. These growth phenotypes were in contrast to those found in Arabidopsis, and this was confirmed by the reciprocal ectopic expression of SlBIM1a/b in Arabidopsis and of AtBIM1 in tomato. These results determined that BIM1 function depends more on the recipient species than on its primary sequence. Yeast two-hybrid interaction studies and transcriptomic analyses of SlBIM1a-overexpressing fruit further suggested that SlBIM1a acts through its interaction with SlBZH1 to govern the transcriptional regulation of growth-related BR target genes. Together, these results suggest that SlBIM1a is a negative regulator of pericarp cell expansion, possibly at the crossroads with auxin and light signaling.

Size development of tomatoes growing in trusses: linking time of fruit set to diameter

BACKGROUND

Size of fruit is an important issue in determining yield at harvest. Even under controlled conditions, variation between fruit and trusses can be considerable. As an easy to measure indication of size, the diameter of tomatoes growing in trusses was assessed in three experiments with different number of tomatoes per truss, as well as cultivars, and also by varying the level of ions in the recirculated drain water.

RESULTS

By applying the von Bertalanffy growth model, more than 99% of the variation present could be explained by the time of fruit set for all tomatoes growing anywhere in the trusses. A linear relationship between time of fruit set and the biological shift factor, an indication of developmental age, was observed. Integrating this linear relationship in the analysis of the diameter data removed one stochastic variable (biological shift factor), effectively halving the number of parameters to be estimated.

CONCLUSION

The results of the present study indicate that the major part of the variation present in the diameter of tomatoes growing in trusses is the result of variation in the time of fruit set of individual fruits. The position within the greenhouse (i.e. local differences in assimilates supply) exerted only a minor effect on diameter development. Accordingly, the time of fruit set largely determines fruit size. Likely, growing conditions before fruit set are crucial for final fruit size. The time of fruit set of each tomato in the truss and the local growing conditions within the greenhouse that affect assimilate supply need to be assessed accurately for a reliable size prediction.

PaCYP78A9, a Cytochrome P450, Regulates Fruit Size in Sweet Cherry (Prunus avium L.)

Sweet cherry (Prunus avium L.) is an important fruit crop in which fruit size is strongly associated with commercial value; few genes associated with fruit size have, however, been identified in sweet cherry. Members of the CYP78A subfamily, a group of important cytochrome P450s, have been found to be involved in controlling seed size and development in Arabidopsis thaliana, rice, soybean, and tomato. However, the influence of CYP78A members in controlling organ size and the underlying molecular mechanisms in sweet cherry and other fruit trees remains unclear. Here, we characterized a P. avium CYP78A gene PaCYP78A9 that is thought to be involved in the regulation of fruit size and organ development using overexpression and silencing approaches. PaCYP78A9 was significantly expressed in the flowers and fruit of sweet cherry. RNAi silencing of PaCYP78A9 produced small cherry fruits and PaCYP78A9 was found to affect fruit size by mediating mesocarp cell proliferation and expansion during fruit growth and development. Overexpression of PaCYP78A9 in Arabidopsis resulted in increased silique and seed size and PaCYP78A9 was found to be highly expressed in the inflorescences and siliques of transgenic plants. Genes related to cell cycling and proliferation were downregulated in fruit from sweet cherry TRV::PaCYP78A9-silencing lines, suggesting that PaCYP78A9 is likely to be an important upstream regulator of cell cycle processes. Together, our findings indicate that PaCYP78A9 plays an essential role in the regulation of cherry fruit size and provide insights into the molecular basis of the mechanisms regulating traits such as fruit size in P. avium.

What drives fruit growth?

Cell division, endoreduplication (an increase in nuclear DNA content without cell division) and cell expansion are important processes for growth. It is debatable whether organ growth is driven by all three cellular processes. Alternatively, all could be part of a dominant extracellular growth regulatory mechanism. Cell level processes have been studied extensively and a positive correlation between cell number and fruit size is commonly reported, although few positive correlations between cell size or ploidy level and fruit size have been found. Here, we discuss cell-level growth dynamics in fruits and ask what drives fruit growth and during which development stages. We argue that (1) the widely accepted positive correlation between cell number and fruit size does not imply a causal relationship; (2) fruit growth is regulated by both cell autonomous and noncell autonomous mechanisms as well as a global coordinator, the target of rapamycin; and (3) increases in fruit size follow the neocellular theory of growth.