Molecular Breeding in Wheat, Maize and Sorghum

Strategies for Improving Abiotic Stress Tolerance and Yield

Mohammad Anwar Hossain, Mobashwer Alam, Saman Seneweera, Sujay Rakshit, Robert Henry, Mohammad Anwar Hossain, Mobashwer Alam, Saman Seneweera, Sujay Rakshit, Robert Henry

• Publisher: CAB International
• Year: 2021
• ISBN:
  9781789245455
• Topic: Technology & Engineering
• Subtopic: Agronomy
• Index: Technology & Engineering

The global population is projected to reach almost 10 billion by 2050, and food and feed production will need to increase by 70%. Wheat, maize and sorghum are three key cereals which provide nutrition for the majority of the world's population. Their production is affected by various abiotic stresses which cause significant yield losses. The effects of climate change also increase the frequency and severity of such abiotic stresses. Molecular breeding technologies offer real hope for improving crop yields. Although significant progress has been made over the last few years, there is still a need to bridge the large gap between yields in the most favorable and most stressful conditions. This book: - Provides a valuable resource for wheat, maize and sorghum scientists working on breeding and molecular biology, physiology and biotechnology.- Presents the latest in-depth research in the area of abiotic stress tolerance and yield improvements.- Contains the necessary information to allow plant breeders to apply this research to effectively breed new varieties of these crops.It provides a consolidated reference for plant breeders and crop scientists working on the challenges of enhanced crop productivity and climate change adaptability.

1 Recent Understanding on Molecular Mechanisms of Plant Abiotic Stress Response and Tolerance

Geoffrey Onaga¹ and Kerstin Wydra²*

¹National Crops Resources Research Institute (NaCRRI), Namulonge, Uganda;

²Erfurt University of Applied Sciences, Erfurt, Germany

1.1 Introduction

Abiotic stresses are growth- and yield-limiting non-biological factors that constrain crop production. The major abiotic stresses limiting crop productivity include drought, salinity, waterlogging, temperature extremes, nutrient imbalances, metal toxicities, ozone and UV-B irradiation. According to global estimates, over 90% of arable lands are prone to one or more of these stresses, and yield losses of up to 70% have been reported in major food crops (dos Reis et al., 2012; Mantri et al., 2012). While it is difficult to get accurate estimates of the effects of each abiotic stress on crop production, it is estimated that yield losses in agricultural crops are mostly caused by high temperature (40%), salinity (20%), drought (17%), low temperature (15%) and flooding (Ashraf et al., 2008). The extent of some of these stresses has, however, increased in recent years. For instance, salinity in irrigated lands has increased by 37%, and more than 50% of arable land could be salt affected by the year 2050 (Munns and Tester, 2008; Qadir et al., 2014). Changes in precipitation patterns have augmented the frequency and severity of drought stress (IPCC, 2018; Naumann et al., 2018). The concentrations of greenhouse gases have increased and subsequently air and ocean temperatures have warmed (Raftery et al., 2017). Heavy metals contamination of arable lands also increased in many parts of the world (Rehman et al., 2018); and the frequency and severity of flooding/waterlogging are presently at alarming rates as a result of erratic weather patterns and sea level rise (Onaga and Wydra, 2016; Manik et al., 2019). All these factors have significant influences on plant growth and crop yields and will be exacerbated by further direct and indirect impacts of climate change. Thus, improved understanding of the molecular, physiological and biochemical bases of plant stress response and tolerance is necessary to decipher promising, functionally relevant molecular mechanisms for accelerated development of abiotic stress-tolerant cultigens.

In response to abiotic stress challenge, plants have evolved intricate mechanisms allowing optimal perception and subsequent transduction of the stress signals. The first reaction to stress involves activation of extracellular and intra-cellular receptors/sensors localized at the cell membrane such as histidine kinases, phytochromes, receptor-like kinases and G-protein-coupled receptors. Second messengers such as reactive oxygen species (ROS), reactive nitrogen species (RNS), reactive sulfur species (RSS), reactive carboxyl species (RCS), inositol phosphatase, Ca²⁺ ions and abscisic acid (ABA) are subsequently triggered. Among the second messengers, ROS accumulation is depicted as the earliest signal in many plant abiotic stress responses which ultimately determine the cell fate. The response to these messengers can be either plastic or elastic (Cramer et al., 2011), depending on cellular reactive species accumulation and oxidant–antioxidant balance. Plastic responses mainly occur through unrestrained accumulation of reactive species which cause oxidative damage to cellular components and structure, inhibiting vital cellular processes such as protein synthesis and energy metabolism. When stringently controlled, ROS accumulation, together with other secondary messengers – such as H₂S, methylgly-oxal, NO and stored pools of Ca²⁺– and stress receptors, form the signalling machinery that employs numerous stress-responsive downstream transducing molecules, including phosphorylation cascades, such as calcium-dependent protein kinase (CDPK), mitogen-activated protein kinase (MAPK) and dephosphorylation phosphatases (e.g. ABI1 and ABI2). Transcription factors (TFs) are activated or suppressed by protein kinases or phosphatases, respectively, and the modified TFs form a complex regulatory network, mediated by various molecules including co-regulators and cross-regulators such as G-box and W-box binders. Subsequently, TFs directly regulate the expression of stress-responsive genes by interacting with the specific cis-elements in their promoter regions. Genes expressed in plants exposed to abiotic stress include genes encoding metabolic and structural proteins as well as regulatory proteins. Abiotic stresses also lead to altered DNA methylation/demethylation, histone post-translational modifications (PTMs), remodelling of chromatin, small RNAs and long non-coding RNAs.

While new molecular pathways are yet to be discovered in most crop species, significant achievements have been made to discover some of the genes involved in the aforementioned molecular responses to stress in plants. This chapter provides an overview of the recent significant perspectives on molecules involved in response and tolerance to drought and salinity, the two major abiotic stresses affecting crop production, and highlights major molecular components identified in major cereals. For some general aspects, readers can refer to previous work on a related topic (Onaga and Wydra, 2016).

1.2 Molecular Mechanisms of Abiotic Stress Response and Tolerance

1.2.1 Drought stress

Plant response to drought is brought about by various mechanisms and depends on crop species, genotype, the age and stage of plant development, and the duration and severity of water loss. Under drought plants generally lose leaf water leading to decreased leaf osmotic potential, stomatal conductance, transpiration rates and photosynthesis (Farooq et al., 2009). Drought stress can also impair mineral uptake by limiting nutrient supply through mineralization and reducing nutrient diffusion and mass flow in the soil (Samarah et al., 2004). The consequence of these changes is a reduction in crop growth and yields, and sometimes increased susceptibility to biotic stresses. The reproductive phase of development is the most sensitive stage to drought stress in several crops and investigating drought effects during reproductive development is of great relevance due to its direct negative impact on crop yields.

The drought stress signal in plants is first perceived at the root level through receptors from the cell membrane/cell wall which convert extracellular stress signals into intracellular secondary messengers. Although several primary sensing mechanisms have been proposed, the true primary sensing receptors have not been clearly figured out due to the complexity of plant responses to drought stress. Several hypotheses have suggested either a redox imbalance or changes in the cell-wall integrity as a trigger to molecular responses to drought. Redox imbalance causes the pH of xylem sap to increase, triggering the loading and transportation of second messengers, such as ABA, throughout the plant. Several ABA transporters have been reported, including multidrug and toxin efflux transporters (MATEs), ATP-binding cassette (ABC), NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER FAMILY (NPF) and more recently PLASMA MEMBRANE PROTEIN 1 (OsPM1) in rice (Takahashi et al., 2020). Change in cell-wall integrity or hydraulic pressure is believed to be perceived by Ca²⁺-permeable mechanosensitive channels (MCA1 and MCA2) (Yamanaka et al., 2010). Recently, a member of non-selective cation channels, reduced hyperosmolality-induced [Ca²⁺] increase 1 (OSCA1), which is one of the hyperosmolarity-gated calcium channels located at the plasma membrane, was suggested as the first potential osmosensor in plants (for review see Lamers et al., 2020). The authors suggest that membrane tension or high extracellular osmotic potential experienced b...