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Vol. 21 No. 3 - July 2015

Crop Productivity, Environmental Stress and Global Climate Change:
the Next Green Revolution?

A major goal of agriculture has been to maximize the investment of plant vegetative biomass into seed yield.Since the 'Green Revolution' of the 1950s and 1960s, the seed yield of the major staple cereal crops such as rice, wheat and corn more than tripled globally. At the heart of the major successes of the Green Revolution was the introduction of dwarfing genes into the world's leading cereals, rice and wheat. These new semi-dwarf genotypes were characterized by a combination of improved lodging resistance, and responsiveness to fertilizer and irrigation inputs which resulted in enhanced harvest index, defined as the ratio of seed dry mass to plant dry weight. The yield of these major staple cereal crops increased  more than 3-fold since 1960 and despite a continuing increase in the human population by about  4 billion over this period, the per-capita availability of calories from these crops increased by  about 1.5-fold. However, despite this impressive success in agronomic yields over the past 60 years, world hunger remains a major and persistent issue. A major challenge for the future will be to feed the estimated 9-12 billion people on earth by 2050 when future climate change senarios predict decreases in crop production over much of the globe due to environmental stress.

Abiotic Stress and Crop Yield

A significant yield gap exists between the average attained crop yields and the maximal yields possible for the major cereal crops. As illustrated in Table 1, the yield gaps for the major cereal crops vary from a low of 34% for oat to a high of 89% for sorghum with an average yield gap of 63.3% for the 7 crops listed. Thus, the yield gaps indicate that most of the major cereals generated by classical plant breeding are underperforming with respect to actual attained grain yields in the field. Clearly, susceptibility to changes in the abiotic environment is a crucial limitation to crop yield. Such limitations will most likely be exacerbated as a consequence of future climate change. Consequently, the generation of improved genetic stocks must involve minimizing the limitations imposed on crop yield by the environment, which will vary not only on a daily as well as a seasonal basis, but also as a function of geography.Photoautotrophs must constantly adjust to fluctuations in ambient irradiance, temperature, nutrient and water availability, which result in an imbalance in cellular energy budget, quantified in vivo as excitation pressure. Any limitation in the ability of a plant species to respond appropriately to high excitation pressure conditions to re-establish an energy balance, that is, photostasis, leads to photoinhibitionof photosynthesis and decreased productivity.  Overwintering plants which are characterized by a dwarf phenotype exhibit an exceptional capacity to acclimate to high excitation pressure generated during the process of cold acclimation. Rather than exhibiting an inhibition of photosynthesis, cold-acclimated winter cereals exhibit light-saturated rates of CO2 assimilation that are 20 to 35% greater than those of non-acclimated controls, irrespective of measuring temperature. This is due to a co-ordinated feed-forward stimulation of photosynthetic CO2 fixation and readjustments in photosynthetic and respiratory carbon metabolism coupled to an increased capacity for long-distance translocation between source and sink which results in an increased resistance to photoinhibition , increased water-use efficiency (WUE) and minimal reliance on photoprotection through non-photochemical quenching (NPQ). This is translated into a 60% higher seed yield per plant in cold acclimated winter wheat under controlled environmental conditions. Furthermore, cold acclimated winter wheat not only exhibits enhanced photosynthetic performance but also the potential for increased resistance to biotic stress.However, the ability to exhibit such system-wide adjustments in plant phenotype, and photosynthetic performance is cultivar-specific. In contrast to winter cultivars, spring cereals do not exhibit a dwarf phenotype upon cold acclimation and are also not able to up-regulate photosynthetic capacity and carbon metabolism but rather exhibit a significant inhibition in photosynthetic capacity and a decrease in biomass compared to cold acclimated winter cereals.

Winter cereals exhibit the capacity for system-wide modifications in plant architecture, photosynthetic performance, enhanced WUE, and seed yield, coupled with increased resistance to abiotic and biotic stress when assessed under controlled environment conditions. Is this enhanced yield potential realized under natural field conditions? According to Stastistics Canada, the average yield of winter wheat in Canada was about 30% greater than for spring wheat over the 52 year period from 1961-2013. Thus, it appears that the enhanced photosynthetic capacity, WUE and resistance to photoinhibition assessed under controlled environment conditions is translated into increased seed yield under natural field conditions. Thus, the process of cold acclimation in cereals may be exploited to reduce the yield gap of major crops in response to environmental stress. Winter varieties of cereals appear to represent an untapped genetic potential to provide a significant increase in grain yield which could be translated into enhanced food production for a growing human population.

CBFs Govern Photosynthetic Response and Phenotypic Plasticity

How are the system-wide alterations in plant phenotype, photosynthetic performance, resistance to abiotic stress and seed yield in winter cultivars regulated? CBFs (C-repeat binding factors) are members of the Apetela2/Ethylene Response Binding Protein (AP2/EREBP) family of transcription factors in Arabidopsis thaliana   known to regulate the expression of COR genes necessary for the acquisition of freezing tolerance as well as the induction of the dwarf phenotype. However, in addition, growth at 25°C of a Brassica napusCBFoverexpressor, BnCBF17, induces increased light-saturated rates of photosynthesis, respiration, plant biomass, and WUE typically observed during coldacclimation. Remarkably, overexpression of BnCBF17 in Brassica napus circumvents the requirement for cold acclimation to co-ordinate the system-wide adjustments in plant phenotype, photosynthetic performance, and source-sink relationships.

Recently, global transcriptome analyses of Arabidopsis thaliana showed that AtCBF3 previously assumed to be regulated by low temperature is in fact governed through intracellular retrograde regulation by the chloroplast redox status measured as excitation pressure. As a consequence, chloroplast redox poise governs CBF expression through retrograde regulation, which initiates a system-wide cascade of molecular events that not only up-regulates photosynthesis, respiration, and carbon metabolism, but also the accumulation of growth-inactive gibberellic acid (GA) and the accumulation of DELLA proteins to suppress growth and generate a dwarf phenotype. In wheat, this is coupled to a 30-40% enhancement of seed yield under controlled environment conditions   as well as natural field conditions. This is reminiscent of the pleiotropic effects observed for semi-dwarf cereal cultivars that spearheaded the Green Revolution of the 1950s and 1960s. Thus, exploitation of winter cereal cultivars combined with standard plant biotechnological approaches to modulate the expression of CBFs may represent an important methodology for the generation of the next green revolution needed to maintain crop productivity at a level sufficient to feed the growing human population under projected increases in the severity and frequency of abiotic and biotic stress due to global climate change.

*Dept. of Biology and the Biotron Centre for Experimental Climate Change, University of Western Ontario, London, Canada N6A 5B7 <[email protected]> 

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.