Although plastid developmental biology using Arabidopsis has been a great success, there is a limitation to the  type of plastids that can be studied.  An important developmental process in which plastids are central is that of fruit ripening and tomato fruit development has been a well researched system at the biochemical and molecular level.  Surprisingly little has been done to address the cell biology of how tomato fruit develop and how chloroplasts dedifferentiate into red chromoplasts.

The sequence of tomato fruit ripening from mature green (MG) past breaker (B) when red colouration is first apparent to 1, 3 and 7 days post breaker when fruit is fully ripe.  This sequence results in green chloroplasts being converted into red chromoplasts and chlorophyll and starch are replaced with red carotenoids.

We have initiated several studies to address different aspects of how this differentiation process occurs and how different aspects of it are controlled.

The fleshy tissue under the tomato skin is called the pericarp and in ripe fruit these pericarp cells are huge and contain up to 2000 chromoplasts.  These replicate from smaller populations of green chloroplasts in immature green fruit.  The first proper study of how the population of plastids changes during fruit ripening  (P. J.  Cookson, J. Kiano, P. D. Fraser, S. Romer, C. A. Shipton, W. Schuch, P. M. Bramley, and K. A. Pyke  (2003). Increases in cell elongation, plastid compartment size and translational control of carotenoid gene expression underly the phenotype of the High Pigment-1 mutant of Tomato.  Planta in press)  showed how the majority of plastid division occurs during the latter stages of green fruit development and at the breaker stage, just prior to the major onset of red pigment development.

In the high pigment1 mutant of tomato, the increased red colouration of the  fruit is caused by both an increase in chromoplast number and a small increase in chromoplast size.

Isolated pericarp cells from mature ripe tomato fruit viewed with Nomarski optics shwoing large populations of samll pigmented chromoplasts.  (a) wild type tomato cv. Ailsa craig (b) high pigment-1 (hp-1) mutant of tomato.  Images (c) wild type and (d) hp-1 show high magnification detail of chromoplasts


The real nature of chromoplast morphology was shown by the targeting of green fluorescent protein to the plastid compartment in transgenic tomato.   This revealed complex morphologies with stromules connecting some chromoplasts and extensive architecture with vesicles associated with these stromules (K. Pyke and C. Howells (2002) Plastid and stromule morphogenesis in tomato. Annals of Botany  90: 559-566).

Brightfield image of a chromoplast (left) and the  same image viewing green fluorescent protein fluorescence (right).  This reveals a long vesiculated stromule emenating from the red chromoplast body.

A brightfield image of two chromoplasts (left) and the GFP image (right) shows a complex stromule joining the chromoplast bodies together.

We are currently trying to figure out what these stromules do and how they relate to the ripening process.  In particular it is noticeable that green chloroplasts in immature fruit have very few stromules and red chromoplasts have significantly more.

Kevin Pyke HOME