Towards identifying the full set of genes expressed during cassava post-harvest physiological deterioration

Expression pattern conferred by a glutamic acid-rich protein gene promoter in field-grown transgenic cassava (Manihot esculenta Crantz)

A major constraint for incorporating new traits into cassava using biotechnology is the limited list of known/tested promoters that encourage the expression of transgenes in the cassava's starchy roots. Based on a previous report on the glutamic-acid-rich protein Pt2L4, indicating a preferential expression in roots, we cloned the corresponding gene including promoter sequence. A promoter fragment (CP2; 731 bp) was evaluated for its potential to regulate the expression of the reporter gene GUSPlus in transgenic cassava plants grown in the field. Intense GUS staining was observed in storage roots and vascular stem tissues; less intense staining in leaves; and none in the pith. Consistent with determined mRNA levels of the GUSPlus gene, fluorometric analyses revealed equal activities in root pulp and stems, but 3.5 times less in leaves. In a second approach, the activity of a longer promoter fragment (CP1) including an intrinsic intron was evaluated in carrot plants. CP1 exhibited a pronounced tissue preference, conferring high expression in the secondary phloem and vascular cambium of roots, but six times lower expression levels in leaf vascular tissues. Thus, CP1 and CP2 may be useful tools to improve nutritional and agronomical traits of cassava by genetic engineering. To date, this is the first study presenting field data on the specificity and potential of promoters for transgenic cassava.

Constituents and secondary metabolite natural products in fresh and deteriorated cassava roots

A phytochemical analysis of cassava (Manihot esculenta Crantz) fresh roots and roots suffering from post-harvest physiological deterioration (PPD) has been carried out. The first isolation and identification of galactosyl diacylglycerides from fresh cassava roots is reported, as well as beta-carotene, linamarin, and beta-sitosterol glucopyranoside. The hydroxycoumarin scopoletin and its glucoside scopolin were identified from cassava roots during PPD, as well as trace quantities of esculetin and its glucoside esculin. There is no isoscopoletin in cassava roots during PPD.

Investigation of biosynthetic pathways to hydroxycoumarins during post-harvest physiological deterioration in Cassava roots by using stable isotope labelling

Cassava (Manihot esculenta Crantz) is an important starch-rich crop, but the storage roots only have a short shelf-life due to post-harvest physiological deterioration (PPD), which includes the over-production and polymerisation of hydroxycoumarins. Key aspects of coumarin secondary-metabolite biosynthesis remain unresolved. Here we exploit the accumulation of hydroxycoumarins to test alternative pathways for their biosynthesis. Using isotopically labelled intermediates (p-coumarate-2-(13)C, caffeate-2-(13)C, ferulate-2-(13)C, umbelliferone-2-(18)O and esculetin-2-(18)O), we show that the major biosynthetic pathway to scopoletin and its glucoside, scopolin, in cassava roots during PPD is through p-coumaric, caffeic and then ferulic acids. An alternate pathway through 2',4'-dihydroxycinnamate and umbelliferone leads to esculetin and esculin. We have used C(18)O(2)-carboxylate-labelled cinnamic and ferulic acids, and feeding experiments under an atmosphere of (18)O(2), to investigate the o-hydroxylation and cyclisation steps. We demonstrate that the major pathway is through o-hydroxylation and not via a proposed spirolactone-dienone intermediate.

The role of Rab GTPases in cell wall metabolism

The synthesis and modification of the cell wall must involve the production of new cell wall polymers and enzymes. Their targeted secretion to the apoplast is one of many potential control points. Since Rab GTPases have been strongly implicated in the regulation of vesicle trafficking, a review of their involvement in cell wall metabolism should throw light on this possibility. Cell wall polymer biosynthesis occurs mainly in the Golgi apparatus, except for cellulose and callose, which are made at the plasma membrane by an enzyme complex that cycles through the endomembrane system and which may be regulated by this cycling. Several systems, including the growth of root hairs and pollen tubes, cell wall softening in fruit, and the development of root nodules, are now being dissected. In these systems, secretion of wall polymers and modifying enzymes has been documented, and Rab GTPases are highly expressed. Reverse genetic experiments have been used to interfere with these GTPases and this is revealing their importance in regulation of trafficking to the wall. The role of the RabA (or Rab11) GTPases is particularly exciting in this respect.