Why are second-generation transgenic crops not yet available in the market?

Recent studies predict that global food demand of major grain crops will not be accompanied by the required increase in yield (Hall and Richards, 2013). Additionally, current figures estimate that the impact of climate change on agriculture will yield losses of 8-43%, mainly due to abiotic stresses. A second generation of transgenic crops (SGTC) was projected to mitigate these constraints worldwide. However, SGTC remain unavailable as market products. Here, we present our viewpoints about current limitations and future perspectives.

Co-ordination between leaf initiation and leaf appearance in field-grown maize (Zea mays): genotypic differences in response of rates to temperature.

BACKGROUND AND AIMS: In maize (Zea mays), early flowering date, which is a valuable trait for several cropping systems, is associated with the number of leaves per plant and the leaf appearance rate. Final leaf number depends upon the rate and duration of leaf initiation. The aims of this study were to analyse the genotypic variation in the response to temperature of leaf appearance rate and leaf initiation rate, and to investigate the co-ordination between these processes under field conditions. METHODS: Sixteen hybrids of different origins were grown under six contrasting environmental conditions. The number of appeared leaves was measured twice a week to estimate leaf appearance rate (leaves d(-1)). Plants were dissected at four sampling dates to determine the number of initiated leaves and estimate leaf initiation rate (leaves d(-1)). A co-ordination model was fitted between the number of initiated leaves and the number of appeared leaves. This model was validated using two independent data sets. KEY RESULTS: Significant (P < 0.05) differences were found among hybrids in the response to temperature of leaf initiation rate (plastochron) and leaf appearance rate (phyllochron). Plastochron ranged between 24.3 and 36.4 degree days (degrees Cd), with a base temperature (Tb) between 4.0 and 8.2 degrees C. Phyllochron ranged between 48.6 and 65.5 degrees Cd, with a Tb between 2.9 and 5.0 degrees C. A single co-ordination model was fitted between the two processes for all hybrids and environments (r2= 0.96, P < 0.0001), and was successfully validated (coefficient of variation < 9 %). CONCLUSIONS: This work has established the existence of genotypic variability in leaf initiation rate and leaf appearance rate in response to temperature, which is a promising result for maize breeding; and the interdependence between these processes from seedling emergence up to floral initiation.

Control of kernel weight and kernel water relations by post-flowering source-sink ratio in maize.

The maize (Zea mays L.) kernel undergoes large changes in water content during its development. Whether such changes regulate the pattern of kernel development or are simply a consequence of it has not yet been established because other factors, such as assimilate supply, can also affect the rate and duration of kernel growth. This study was conducted to determine whether variation in kernel weight (KW) in response to source-sink treatments is mediated by a change in kernel water relations. Two hybrids were sown at three stand densities (one, eight and 18 plants m-2), and kernel numbers were restricted to control the post-flowering source-sink ratio within each stand density. Kernel development and water relations [water content, water potential (psiw), osmotic potential (psis) and turgor] were monitored throughout grain filling. Final KW varied from 253 to 372 mg per kernel in response to source-sink treatments. For both genotypes, variation in KW was a result of a change in kernel growth rate (r2 = 0.91; P < 0.001) and not in the duration of kernel filling. Final KW was closely correlated with maximum kernel water content (r2 = 0.94; P < 0.001) achieved during rapid dry matter accumulation. However, variation in KW was not reflected in kernel water status parameters (psiw, psis or turgor), which remained fairly stable across treatments. These results indicate that maximum water content provides an easily quantifiable measure of kernel sink capacity in maize. Kernel water status parameters may affect the duration of grain filling, but have no discernible impact on kernel growth rate.