Tomato ACS4 is necessary for timely start of and progression through the climacteric phase of fruit ripening.

Suzanne W Hoogstrate,Ivo Rieu,Celestina Mariani,Lambertus J A Van Bussel,Simona M Cristescu,Wim H Vriezen,Eric Cator

doi:10.3389/fpls.2014.00466

Abstract

Climacteric fruit ripening, as it occurs in many fruit crops, depends on a rapid, autocatalytic increase in ethylene production. This agriculturally important process has been studied extensively, with tomato simultaneously acting both as a model species and target crop for modification. In tomato, the ethylene biosynthetic genes ACC SYNTHASE2 (ACS2) and ACS4 are highly expressed during fruit ripening, with a combined loss of both ACS2 and ACS4 activity preventing generation of the ethylene burst necessary for fruit ripening. However, the individual roles and importance of ACS2 and ACS4 have not been determined. In this study, we examined specifically the role of ACS4 by comparing the phenotype of an acs4 mutant firstly with that of the wild-type, and secondly with two novel ripening-inhibitor (rin) mutants. Ethylene production during ripening was significantly reduced in both acs4-1, and rin lines, with rin genotypes showing the weaker ethylene burst. Also i) the time between anthesis and the start of fruit ripening and ii) the time required to progress through ripening were significantly longer in acs4-1 than in the wild type, but shorter than in the strongest rin mutant. The delay in ripening was reflected in the lower expression of ripening-related transcripts during the mature green and light red ripening stages. Furthermore, expression of ACS2 and ACS4 was strongly dependent on a functional RIN gene, while ACS2 expression was largely independent of ACS4. Altogether, we show that ACS4 is necessary for normal progression of tomato fruit ripening and that mutation of this gene may provide a useful means for altering ripening traits.

Highlights

Climacteric fruit ripening is a mechanism by which fully grown fruits go through a final phase of changes in texture, color, smell, and taste
The truncated protein still contains the complete MADS-box domain (IPR002100) for DNA binding, but has lost most of the K-box, making it a likely null-allele. These two new rin mutant lines are unique in that they are solely mutated in the LeMADS-RIN locus and differ from the classical rin mutant, in which the mutation consists of a deletion of both LeMADS-RIN and the neighboring LeMADS-MACROCALYX (LeMADS-MC), the latter being is associated with sepal development (Vrebalov et al, 2002)
ACS2, ACS4, EXP1, PG, and phytoene synthase 1 (PSY1) expression is not or hardly up-regulated during ripening (DellaPenna et al, 1989; Rose et al, 1997; Barry et al, 2000; Fujisawa et al, 2011, 2012; Martel et al, 2011), ethylene production does not rise above the pre-climacteric levels, even after 120 days of development, and fruits do not color beyond yellow (Robinson and Tomes, 1968; Herner and Sink, 1973; Tigchelaar et al, 1978)

Summary

Introduction

Climacteric fruit ripening is a mechanism by which fully grown fruits go through a final phase of changes in texture, color, smell, and taste. It is accompanied by a burst in respiration and a rapid, autocatalytic increase in the production of the gaseous plant hormone ethylene (Alexander and Grierson, 2002). Exogenous ethylene can still activate ethylene response genes in a rin background, but less effectively than in the wild type (Lincoln and Fischer, 1988), suggesting that RIN has ethylene-independent functions and acts upstream of both ethylene- and non-ethylene-mediated ripening processes. RIN probably acts as part of a protein complex containing another ripening-associated www.frontiersin.org

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