A tomato LATERAL ORGAN BOUNDARIES transcription factor, SlLOB1 , predominantly regulates cell wall and softening components of ripening

Abstract

Fruit softening is a key component of the irreversible ripening program, contributing to the palatability necessary for frugivore-mediated seed dispersal. The underlying textural changes are complex and result from cell wall remodeling and changes in both cell adhesion and turgor. While a number of transcription factors (TFs) that regulate ripening have been identified, these affect most canonical ripening-related physiological processes. Here, we show that a tomato fruit ripening–specific LATERAL ORGAN BOUNDRIES (LOB) TF, SlLOB1, up-regulates a suite of cell wall–associated genes during late maturation and ripening of locule and pericarp tissues. SlLOB1 repression in transgenic fruit impedes softening, while overexpression throughout the plant under the direction of the 35s promoter confers precocious induction of cell wall gene expression and premature softening. Transcript and protein levels of the wall-loosening protein EXPANSIN1 (EXP1) are strongly suppressed in SlLOB1 RNA interference lines, while EXP1 is induced in SlLOB1-overexpressing transgenic leaves and fruit. In contrast to the role of ethylene and previously characterized ripening TFs, which are comprehensive facilitators of ripening phenomena including softening, SlLOB1 participates in a regulatory subcircuit predominant to cell wall dynamics and softening.

Interaction of two MADS-box genes leads to growth phenotype divergence of all-flesh type of tomatoes

“…Another insight brought by our study is related to the process of determining fruit firmness and texture. Although the deciphering of the components underlying the softening process has been mostly addressed by focusing on late stages of fruit development 3 – 5 , our data are consistent with the notion that a large component of texture and firmness of ripe fruit is determined at early pre-ripening stages, concomitant with the initiation of inner tissues differentiation. In this regard, the outcome of the study brings about a paradigm shift in thinking.…”

“…It is also a major criterion in determining whether the crop is better suited for fresh consumption or the processing industry. The dynamics of fleshy fruit tissue texture is complex, and softness has always been addressed as a ripening associated process 3 – 5 . Therefore, the control of fruit softening has been mainly achieved, so far, through slowing down the ripening process which leads to loss of sensory quality.…”

Regulation of fleshy fruit ripening: from transcription factors to epigenetic modifications

“…Silencing of SlGRAS38 reduced carotenoid and ethylene production in ripening fruits and caused downregulation of many M6 module genes, suggesting that SlGRAS38 acts as a central regulator downstream of RIN to promote fruit carotenoid and ethylene metabolism [ 84 ]. The tomato softening-specific TF LATERAL ORGAN BOUNDRIES (SlLOB1), which specifically regulates cell wall remodeling-associated genes, was identified from transcriptome data of liquefied locular tissue and pericarp at the mature green stage [ 85 ]. Fruits with repressed SlLOB1 expression exhibit small amounts of liquefied locular gel, delayed softening, and severely downregulated expression of the cell wall-loosening gene EXPANSIN1 ( EXP1 ), a direct target of SlLOB1 [ 85 ].…”

“…The tomato softening-specific TF LATERAL ORGAN BOUNDRIES (SlLOB1), which specifically regulates cell wall remodeling-associated genes, was identified from transcriptome data of liquefied locular tissue and pericarp at the mature green stage [ 85 ]. Fruits with repressed SlLOB1 expression exhibit small amounts of liquefied locular gel, delayed softening, and severely downregulated expression of the cell wall-loosening gene EXPANSIN1 ( EXP1 ), a direct target of SlLOB1 [ 85 ]. SlLOB1 RNAi lines display no difference from control fruits in ethylene production from ripening initiation until the medium ripening stage but accumulate more ethylene in the late ripening stage, indicating that SlLOB1 RNAi fruits might accumulate more carotenoids through ethylene regulation in the late ripening stage.…”

“…SlLOB1 RNAi lines display no difference from control fruits in ethylene production from ripening initiation until the medium ripening stage but accumulate more ethylene in the late ripening stage, indicating that SlLOB1 RNAi fruits might accumulate more carotenoids through ethylene regulation in the late ripening stage. In the early ripening stage, RIN and NOR may transactivate SlLOB1 in locular tissue to regulate fruit pericarp softening in an ethylene-independent manner [ 85 ].…”