Molecular and Genetic Bases of Fruit Firmness Variation in Blueberry—A Review

Abstract

Blueberry (Vaccinium spp.) has been recognized worldwide as a valuable source of health-promoting compounds, becoming a crop with some of the fastest rising consumer demand trends. Fruit firmness is a key target for blueberry breeding as it directly affects fruit quality, consumer preference, transportability, shelf life, and the ability of cultivars to be machine harvested. Fruit softening naturally occurs during berry development, maturation, and postharvest ripening. However, some genotypes are better at retaining firmness than others, and some are crispy, which is a putatively extra-firmness phenotype that provides a distinct eating experience. In this review, we summarized important studies addressing the firmness trait in blueberry, focusing on physiological and molecular changes affecting this trait at the onset of ripening and also the genetic basis of firmness variation across individuals. New insights into these topics were also achieved by using previously available data and historical records from the blueberry breeding program at the University of Florida. The complex quantitative nature of firmness in an autopolyploid species such as blueberry imposes additional challenges for the implementation of molecular techniques in breeding. However, we highlighted some recent genomics-based studies and the potential of a QTL (Quantitative Trait Locus) mapping analysis and genome editing protocols such as CRISPR/Cas9 to further assist and accelerate the breeding process for this important trait.

High-Resolution Linkage Map and QTL Analyses for Machine Harvest Traits in Autotetraploid Blueberry

“…Overall, all traits considered in this study are tightly related to fruit ripening, senescence, and abscission. These processes are the result of complex interactions of plant hormones, signaling pathways, and transcriptional and cellular modifications (Seymour et al, 2002; Giovannoni, 2004; Estornell et al, 2013; Cappai et al, 2018).…”

“…Several genes involved in phytohormone pathways were detected at the QTL intervals for all traits. For firmness and firmness retention traits, ethylene and ABA are well-known for their role as ripening promoters and, subsequently, senescence and softening (Suzuki et al, 1997; Zhu et al, 2012; Sun et al, 2013; Cappai et al, 2018). Within the QTL interval for firmness, for example, there were two putative ‘ethylene-responsive transcription factor (ERFs)’ encoding genes.…”

“…The force-deformation response of compressed berries was measured in grams per 1 mm of deflection (g/mm) (Ehlenfeldt and Martin, 2002). A sample size of 25 berries per genotype was used in 2018 and a sample size of 50 berries per genotype was used in 2019, as this sample size was reported to increase accuracy (Cappai et al, 2018). Berry firmness measurements were averaged per genotype within each year.…”

“…Firmness is a genetically controlled trait that has long been among the focus of blueberry breeding programs (Edwards et al, 1974; Cappai et al, 2018; Cellon et al, 2018; Ferrão et al, 2018). It is also a desirable trait across the whole market chain: for growers, less fruit is discarded due to physical damage; for retailers, firmer berries have a longer shelf life; and for consumers, firm texture positively correlates to liking (Mehra et al, 2013; Olmstead and Finn, 2014; Yu et al, 2014; Gilbert et al, 2015).…”

“…It is also a desirable trait across the whole market chain: for growers, less fruit is discarded due to physical damage; for retailers, firmer berries have a longer shelf life; and for consumers, firm texture positively correlates to liking (Mehra et al, 2013; Olmstead and Finn, 2014; Yu et al, 2014; Gilbert et al, 2015). At a molecular level, fruit firmness is intertwined with fruit ripening, and it is the result of a complicated network of interactions between two main hormones, ethylene and abscisic acid (ABA), and the cell wall disassembly process (Brummell, 2006; Vicente et al, 2007; Chiabrando and Giacalone, 2011; Cappai et al, 2018). The exact cascade of events that leads to fruit softening has not been completely elucidated in blueberry, even though current evidence indicates that pectin solubilization in the cell wall could be one of the main outcomes (Lara et al, 2004; Vicente et al, 2007; Angeletti et al, 2010; Beaudry et al, 2016).…”