The metabolic basis of pollen thermo-tolerance: perspectives for breeding

Abstract

Crop production is highly sensitive to elevated temperatures. A rise of a few degrees above the optimum growing temperature can lead to a dramatic yield loss. A predicted increase of 1-3 degrees in the twenty first century urges breeders to develop thermo-tolerant crops which are tolerant to high temperatures. Breeding for thermo-tolerance is a challenge due to the low heritability of this trait. A better understanding of heat stress tolerance and the development of reliable methods to phenotype thermo-tolerance are key factors for a successful breeding approach. Plant reproduction is the most temperature-sensitive process in the plant life cycle. More precisely, pollen quality is strongly affected by heat stress conditions. High temperature leads to a decrease of pollen viability which is directly correlated with a loss of fruit production. The reduction in pollen viability is associated with changes in the level and composition of several (groups of) metabolites, which play an important role in pollen development, for example by contributing to pollen nutrition or by providing protection to environmental stresses. This review aims to underline the importance of maintaining metabolite homeostasis during pollen development, in order to produce mature and fertile pollen under high temperature. The review will give an overview of the current state of the art on the role of various pollen metabolites in pollen homeostasis and thermo-tolerance. Their possible use as metabolic markers to assist breeding programs for plant thermo-tolerance will be discussed.

Figure 1

General effects of heat stress on plant physiology.

Figure 2

Effect of high temperature (34 °C/28 °C) on flowers of Solanum lycopersicum cv. Nagcarlang. Pictures represent mature flowers under control conditions (a) and high temperature (b–c). Under high temperature, anthers showed deformation, dark coloration of the anther tip and elongated pistils. Those flowers had a low percentage of pollen viability (<10%).

Figure 3

Pollen development from tetrad stage to mature pollen stage. Days before anthesis related to developmental stages are based on Lycopersicon esculentum Mill. “Trust” from [32] and [33]. A, anthesis, A-3, 3 days before anthesis, A-5, 5 days before anthesis, A-7, 7 days before anthesis, A-9, 9 days before anthesis. Nuclei are stained with DAPI.

Figure 4

Simplified metabolic pathways underlying the relationship between metabolites reported in this review such as carbohydrates, proline, lipids, glutathione, polyamines, flavonoids and hormones. PEP, phosphoenolpyruvate; TCA, tricarboxylic acid. Triose-P, triose-phosphate.

Figure 5

Effect of high temperature (32 °C/26 °C) on anthers of Solanum lycopersicum cv. microTom. Picture (a) shows an anther at mature stage of pollen development under control conditions. The opening of the locule is indicated with an arrow. Picture (b) shows anther at mature stage of pollen development under high temperature. The locule was not opened due to anther deformation and the presence of an extra layer of cells. Picture (c) shows a severe anther deformation under high temperature. The four distinct locules were no longer visible.

Figure 6

Metabolites affected by heat stress and their role in providing viable pollen.

Figure 7

Breeding approaches to improve crop thermo-tolerance and create new thermo-tolerant varieties. QTL, quantitative trait loci; RIL, recombinant inbred lines; SNP, single-nucleotide polymorphism; AS, anti-sense; OE, over-expression. * this approach is only used in the case of reverse genetics.