Elucidating the functional role of endoreduplication in tomato fruit development

Abstract

Background: Endoreduplication is the major source of endopolyploidy in higher plants. The process of endoreduplication results from the ability of cells to modify their classical cell cycle into a partial cell cycle where DNA synthesis occurs independently from mitosis. Despite the ubiquitous occurrence of the phenomenon in eukaryotic cells, the physiological meaning of endoreduplication remains vague, although several roles during plant development have been proposed, mostly related to cell differentiation and cell size determination.

Scope: Here recent advances in the knowledge of endoreduplication and fruit organogenesis are reviewed, focusing on tomato (Solanum lycopersicum) as a model, and the functional analyses of endoreduplication-associated regulatory genes in tomato fruit are described.

Conclusions: The cyclin-dependent kinase inhibitory kinase WEE1 and the anaphase promoting complex activator CCS52A both participate in the control of cell size and the endoreduplication process driving cell expansion during early fruit development in tomato. Moreover the fruit-specific functional analysis of the tomato CDK inhibitor KRP1 reveals that cell size and fruit size determination can be uncoupled from DNA ploidy levels, indicating that endoreduplication acts rather as a limiting factor for cell growth. The overall functional data contribute to unravelling the physiological role of endoreduplication in growth induction of fleshy fruits.

Fig. 1.

Occurrence of endoreduplication in fleshy fruits. The maximal number of endocycles determined in fully ripened fleshy fruits was plotted against the duration of fruit growth until ripening.

Fig. 2.

Schematic representation of endoreduplication cycle control in plants. The arrest of mitotic activity in endoreduplicating tissues originates from the inactivation of the mitotic CDKB1/CYCA2;3 complex (proposed as being the mitosis inducing factor, MIF). This inactivation may be achieved by fixation of a CDK inhibitor (KRP) and subsequent release of the cyclin moiety that is degraded by the APC^CCS52A/26S proteasome pathway. During the endocycle, an active CDKA/CYCD complex associates and phosphorylates the retinoblastoma protein (Rb). The G → S transition occurs due to the release of the E2F transcription factor which is necessary for the transcription of S-phase-related genes, in particular the A-type cyclins (CYCA). At the G → S transition, the CDKA/CYCD complex is likely to be inhibited by KRP; CYCD is then degraded and CDKA can now associate with S-phase cyclins. In early S, the induced CYCA3;1 associates with CDKA in a complex that is necessary for the progression throughout the S phase. At the end of the S phase, CYCA3;1 is degraded by the APC^CCS52A/26S proteasome pathway; CDKA is then released and available for a new round of the endoreduplication cycle. In early G, CDKA is inactivated by the inhibitory phosphorylation mediated by WEE1 as to allow a proper cell growth prior to commit to another round of DNA synthesis.

Fig. 3.

Functional analysis of SlKRP1 in tomato fruits with time (days post-anthesis). (A) Relative SlKRP1 mRNA abundance in pericarp from ProPEPC2:SlKRP1°^E-developing fruit compared with wild type. The period of expression of the PEPC2 promoter is indicated by the shading. (B) Evolution of endoreduplication index in pericarp from ProPEPC2:SlKRP1°^E-developing fruits compared with wild type. (C) Growth curve of ProPEPC2:SlKRP1°^E fruits compared with wild type. (D) Evolution of mean cell size in pericarp from ProPEPC2:SlKRP1°^E developing fruits compared with wild type.