As the result of intensive research and breeding efforts over the last 20 years, the yield potential and yield quality of cereals have been greatly improved. Nowadays, yield safety has gained more importance because of the forecasted climatic changes. Drought and high temperature are especially considered as key stress factors with high potential impact on crop yield. Yield safety can only be improved if future breeding attempts will be based on the valuable new knowledge acquired on the processes determining plant development and its responses to stress. Plant stress responses are very complex. Interactions between plant structure, function and the environment need to be investigated at various phases of plant development at the organismal, cellular as well as molecular levels in order to obtain a full picture. The results achieved so far in this field indicate that various plant organs, in a definite hierarchy and in interaction with each other, are involved in determining crop yield under stress. Here we attempt to summarize the currently available information on cereal reproduction under drought and heat stress and to give an outlook towards potential strategies to improve yield safety in cereals.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2007.01727.x

The effect of drought and heat stress on reproductive processes in cereals.

Male reproductive processes in flowering plants take place in the stamen (Esau, 1977). This sporophytic organ system contains diploid cells that undergo meiosis and produce haploid male spores, or microspores. Microspores divide mitotically and differentiate into multicellular male gametophytes, or pollen grains, that contain the sperm cells. Figures 1 and 2 show that the stamen consists of two morphologically distinct partsthe anther and the filament. The filament is a tube of vascular tissue that anchors the stamen to the flower and serves as a conduit for water and nutrients. By contrast, the anther contains the reproductive and nonreproductive tissues that are responsible for producing and releasing pollen grains so that pollination and fertilization processes can occur within the flower. Figure 1 shows that anther development can be divided into two general phases. During phase 1, the morphology of the anther is established, cell and tissue differentiation occur, and microspore mother cells undergo meiosis. At the end of phase 1, the anther contains most of its specialized cells and tissues, and tetrads of microspores are present within the pollen sacs (Figure 1). During phase 2, pollen grains differentiate, the anther enlarges and is pushed upward in the flower by filament extension, and tissue degeneration, dehiscence, and pollen grain release occur (Figure 1). The cellular processes that regulate anther cell differentiation, establish anther tissue patterns, and cause the anther to switch from a histospecification program (phase l) to a cell degeneration and dehiscence program (phase 2) are not known. The developmental events leading to anther formation and pollen release are exquisitely timed and choreographed (Koltunow et al., 1990; Scott et al., 1991). Cell differentiation and dehiscence events occur in a precise chronological order that correlates with floral bud size (Koltunow et al., 1990; Scott et al., 1991). This permits the mechanisms responsible for cell-type differentiation, tissue degeneration, and cellspecific gene activation within the anther to be explored with relative Base. During the past few years, there has been an explosive burst of interest in anther biology, both as a system to dissect plant developmental processes at the molecular and genetic levels (Koltunow et al., 1990; Gasser, 1991; McCormick,

Anther development: basic principles and practical applications.

The great taxonomic diversity of the Orchidaceae is often attributed to adaptive radiation for specific pollinators driven by selection for outcrossing. However, when one looks beyond the product to the process, the evidence for selection is less than overwhelming. We explore this problem by discussing relevant aspects of orchid biology and asking which aspects of reproduction explain the intricate pollination mechanisms and diversification of this family. We reaffirm that orchids are primarily pollination limited, the severity of which is affected by resource constraints. Fruit set is higher in temperate than in tropical species, and in species which offer pollinator rewards than those that do not. Reproductive success is skewed towards few individuals in a population and effective population sizes are often small. Population structure, reproductive success and gene flow among populations suggest that in many situations genetic drift may be as important as selection in fostering genetic and morphological variation in this family. Although there is some evidence for a gradualist model of evolutionary change, we believe that the great diversity in this family is largely a consequence of sequential and rapid interplay between drift and natural selection. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84, 1–54.

Variation in sexual reproduction in orchids and its evolutionary consequences: a spasmodic journey to diversification

https://www.agroparistech.fr/IMG/pdf/wilcocket_al.pdf

Pollination failure in plants: why it happens and when it matters.

This paper reviews the literature concerning digestion and nutrient content of pollen. Four topics are addressed in detail: 1) The mechanism of pollen digestion by animals; 2) The efficiency of mechanical and digestive removal of pollen content by various animals; 3) Range and taxonomic distribution of pollen nutrients, and 4) Adaptive hypotheses proposed to associate pollen chemistry with pollinator reward. Studies on the mechanism(s) of pollen digestion remain inconclusive, but suggest that differences in digestibility among pollen types may reflect differences in pollen wall porosity, thickness, and composition. Although hummingbirds reportedly digest pollen very poorly, most animals studied, including those that do not regularly consume pollen, can digest 50–100% of ingested grains. Overlooked and recent research of pollen protein content shows that pollen grains may contain over 60% protein, double the amount cited in some studies of pollen-feeding animals. Adaptive hypotheses that associate pollen starch and pollen caloric content with pollinator reward remain unsubstantiated when critically viewed through the lens of phylogeny.

Pollen nutritional content and digestibility for animals

It appears that the tapetum is universally present in land plants, even though it is sometimes difficult to recognize, because it serves mostly as a tissue for meiocyte/spore nutrition. In addition to this main function, the tapetum has other functions, namely the production of the locular fluid, the production and release of callase, the conveying of P.A.S. positive material towards the loculus, the formation of exine precursors, viscin threads and orbicules (= Ubisch bodies), the production of sporophytic proteins and enzymes, and of pollenkitt/tryphine. Not all these functions are present in all land plants:Embryophyta. Two main tapetal types are usually distinguished in theSpermatophyta: the secretory or parietal type and the amoeboid or periplasmodial type; in lower groups, however, other types may be recognized, with greater or lesser differences. A hypothetical phylogenesis of the tapetum is proposed on the basis of its morphological appearance and of the nutritional relations with meiocytes/spores. The evolutionary trends of the tapeta tend towards a more and more intimate and increasingly greater contact with the spores/pollen grains. Three evolutionary trends can be recognized: 1) an intrusion of the tapetal cells between the spores, 2) a loss of tapetal cell walls, and 3) increasing nutrition through direct contact in narrow anthers.

The tapetum: its form, function and possible phylogeny in Embryophyta

In most species, arrest of growth and a decrease in water content occur in seeds and pollen before they are dispersed. However, in a few cases, pollen and seeds may continue to develop (germinate). Examples are cleistogamy and vivipary. In all other cases, seeds and pollen are dispersed with a variable water content (2–70%), and consequently they respond differently to environmental relative humidity that affects dispersal and maintenance of viability in time. Seeds with low moisture content shed by the parent plant after maturation drying can generally desiccate further to moisture contents in the range of 1–5% without damage and have been termed ‘orthodox’. Pollen that can withstand dehydration also was recently termed orthodox. Seeds and pollen that do not undergo maturation drying and are shed at relatively high moisture contents (30–70%) are termed ‘recalcitrant’. Since recalcitrant seeds and pollen are highly susceptible to desiccation damage, they cannot be stored under conditions suitable for orthodox seeds and pollen. Hence, there are four types of plants with regard to tolerance of pollen and seeds to desiccation. Orthodoxy allows for dispersal over greater distances, longer survival, and greater resistance to low relative humidity. The advantage of recalcitrance is fast germination. Orthodoxy and recalcitrance are often related to environment rather than to systematics. It has been postulated that certain types of genes are involved during presentation and dispersal of pollen and seeds, since molecules (sucrose, polyalcohols, late embryogenic abundant proteins, antioxidants, etc.) that protect different cell compartments during biologically programmed drying have been detected in both.

Pollen and seed desiccation tolerance in relation to degree of developmental arrest, dispersal, and survival

The aim of this paper is to draw attention to partially hydrated pollen, namely, pollen grains having a high water content (>30%); this type of pollen is more frequent than previously thought. Various cyto-physiological strategies are used to retain water during exposure and dispersal such as cytoplasm carbohydrates; in the absence of such strategies, fast pollination must be ensured, because uncontrolled loss of water leads to pollen death. On the other hand, a state of partial hydration allows a fast tube emission (even within 3–5 min). Several methods for determining the hydration status of pollen at anthesis are proposed.

Pollen hydration status at dispersal: cytophysiological features and strategies.

Types of carbohydrate reserves in pollen: localization, systematic distribution and ecophysiological significance

Group-specific functions linked to the dispersal of pollen and spores are reported in all the groups of land plants. Anthocerotopsida tapetal cells envelop developing meiocytes and spores; after exine formation their walls thicken, the cytoplasm starts to degenerate, and elaters from more than one tapetal cell are formed. Elaters aid spore dispersal from the sporangium. In some Marchantiopsida the tapetal cells form elaters but each originates from one cell. In Equisetophyta, the spores have two or four elaters composed of lignin and connected with the exine. They are formed with the aid of the tapetal cytoplasm once exine is complete. The elaters of this group (called also adpressors) differ from the above in that they adhere to and originate connected with the exine. In Polypodiophyta like Lecanopteris mirabilis, the spores have superficial strands which facilitate adhesion to dispersing ants. One of the main features of Spermatophyta parietal tapetum is to produce orbicules. In strictly anemophilous species pollenkitt is not produced and pollen grains are dispersed individually. Since both exine and orbicules consist of sporopollenin, they have the same electrostatic charges. This is believed to aid expulsion from the anther at anthesis. Pollenkitt is a degeneration product of amoeboid and periplasmodial tapetum in Magnoliophyta. It forms a layer on pollen grains and has at least four different functions related to dispersal: (1) it causes the grains to unite in clumps and (2) to adhere to pollinator bodies; (3) it protects the pollen grain cytoplasm from sunlight; and (4) the oily and perfumed components attract pollinators.

Gian Gabriele Franchi

Papers

The tapetum: its form, function and possible phylogeny in Embryophyta

Pollen and seed desiccation tolerance in relation to degree of developmental arrest, dispersal, and survival

Pollen hydration status at dispersal: cytophysiological features and strategies.

Types of carbohydrate reserves in pollen: localization, systematic distribution and ecophysiological significance

Role of the tapetum in pollen and spore dispersal