Drought or/and Heat-Stress Effects on Seed Filling in Food Crops: Impacts on Functional Biochemistry, Seed Yields, and Nutritional Quality

Abstract

Drought (water deficits) and heat (high temperatures) stress are the prime abiotic constraints, under the current and climate change scenario in future. Any further increase in the occurrence, and extremity of these stresses, either individually or in combination, would severely reduce the crop productivity and food security, globally. Although, they obstruct productivity at all crop growth stages, the extent of damage at reproductive phase of crop growth, mainly the seed filling phase, is critical and causes considerable yield losses. Drought and heat stress substantially affect the seed yields by reducing seed size and number, eventually affecting the commercial trait '100 seed weight' and seed quality. Seed filling is influenced by various metabolic processes occurring in the leaves, especially production and translocation of photoassimilates, importing precursors for biosynthesis of seed reserves, minerals and other functional constituents. These processes are highly sensitive to drought and heat, due to involvement of array of diverse enzymes and transporters, located in the leaves and seeds. We highlight here the findings in various food crops showing how their seed composition is drastically impacted at various cellular levels due to drought and heat stresses, applied separately, or in combination. The combined stresses are extremely detrimental for seed yield and its quality, and thus need more attention. Understanding the precise target sites regulating seed filling events in leaves and seeds, and how they are affected by abiotic stresses, is imperative to enhance the seed quality. It is vital to know the physiological, biochemical and genetic mechanisms, which govern the various seed filling events under stress environments, to devise strategies to improve stress tolerance. Converging modern advances in physiology, biochemistry and biotechnology, especially the "omics" technologies might provide a strong impetus to research on this aspect. Such application, along with effective agronomic management system would pave the way in developing crop genotypes/varieties with improved productivity under drought and/or heat stresses.

Keywords: drought stress; heat stress; omics; photosynthates; seed filling; transcriptional regulation.

FIGURE 1

A generalized view of seed development depicting various growth stages of embryo (embryogenesis), commencing post fertilization, leading to formation of a mature seed.

FIGURE 2

Schematic representation of various processes during seed filling stage in both monocot (top left) as well as dicot (top right) plants. The import and metabolism of sucrose is depicted in the figure. Sucrose enters the seed coat via the chalazal vein. During the pre-storage phase, sucrose is degraded through the catalytic action of cell-wall bound invertase, when the invertase activity is high leading to high ratio of hexoses to sucrose promoting growth via cell division. During storage phase of development, the invertase activity is low, so sucrose is taken up directly by the cotyledons. A low ratio of hexoses: sucrose promotes differentiation and storage product synthesis. Sucrose metabolism in the cotyledons is catalyzed by a cycle of synthesis and breakdown via sucrose-phosphate synthase and sucrose synthase. The figure also explains the translocation of sucrose and other nutrients from source (endosperm/cotyledon) to sink (embryo) during developmental stages of seed. Carbon, nitrogen, phosphorus, and other minerals, produced from the hydrolysis of stored nutrients in endosperm/cotyledon are transported to the embryo for its growth, and mobilized toward assimilate-transport pathway into developing seeds, and toward the starch and sucrose synthesis pathway. C, Carbon; N, nitrogen; P, phosphorus; K, potassium; SUT, sucrose transporter; AAP, amino acids protein; PEPC, AK, PEP carboxylase and/or aspartate kinase; AGP, ADP glucose pyro-phosphorylase; GPT, plastidic glucose-6-P translocator; pPGM, plastidic phospho-glucomutase; FAT, fatty acid transporter. Stresses such as drought and heat may affect the seed filling by influencing any of these or multiple processes to accelerate (as in heat) or disrupt (as in heat, drought) the seed filling.

FIGURE 3

(A) Nutrient translocation from source to sink and a road map highlighting events associated with seed filling in monocot (cereals). (A) Plant takes up essential nutrients from the soil including N (nitrogen), P (phosphorus), K (potassium), Fe (iron), Zn (zinc), etc., and assimilates carbohydrate (sucrose) through fixing atmospheric CO₂ via photosynthesis. During seed filling stage the matured leaves translocate assimilates to the developing seed (sink), whereas, nutrients especially N and other minerals are remobilized from the senescing leaves to the sink organ (developing grain). The role of hormones and cross talk between source and sink during seed filling; at seed filling stage, stress hormones serve as key factors, which control the autophagy and senescence, thus translocating the N-pool and the minerals from the senescing leaves to the grain/seed. Auxins and cytokinins are important and regulate the seed cell numbers and size, this controlling the sink strength. (B) In dicots, the seed development and filling is controlled through transcriptional regulation. Several transcription factors interact/overlap with each other and also involve hormonal control during this event, as indicated in the figure. ABA, abscisic acid; GA, gibberellic acid; T-6-P, trehalose-6-phosphate; LEC1, leafy cotyledon 1; ABI3, abscisic acid-insensitive 3; EEL, enhanced em level; fusca3, FUS3; SnRK1, SNF1-related protein kinase; SUS, sucrose synthase; SUTs, sucrose transporters; IVT, invertase; RSR1, rice starch regulator 1; SSP, seed storage protein.

FIGURE 4

Schematic representation highlighting consequences of drought and/or heat stress on seed filling and ‘omics’ approach for crop improvement.