Is flower/corolla closure linked to decrease in viability of desiccation-sensitive pollen? Facts and hypotheses: a review of current literature with the support of some new experimental data

Pollen hydration status at dispersal depends on many parameters and on this basis two main groups of grains may be distinguished: those with a low water content are usually named orthodox or desiccation-resistant, while those with a higher water content at dispersal are known as recalcitrant or desiccation-sensitive due to their reduced mechanisms to keep water constant. On exposure, the latter lose water and die quickly. Although the end of flower receptivity may occur in different ways, in species possessing recalcitrant pollen it often consists in corolla closure. In addition to data available from current literature, also a new set of experiments was performed: the viability of pollen of eight entomophilous species having nectar as pollinator reward, desiccation-sensitive pollen and corolla closure at the end of receptivity, irrespective of pollination, was tested from the onset of anthesis until flower closure using a fluorochromatic reaction. Pollen viability fell sharply in all species, albeit at different rates, depending on initial water content, and may be also on types of carbohydrate reserves and inherent enzymes interconverting them. On the basis of experimental data and the literature, the authors speculate on the effects of corolla closure: it avoids dispersal of pollen with reduced viability, it avoids collection of unconsumed nectar while facilitating its reabsorption by the nectary parenchyma and its utilization for other purposes, and it avoids contamination by moulds and bacterial spores that could penetrate the plant via the nectar and nectary parenchyma

Is the pollination efficiency of long-lived orchid flowers affected by age?

The long-lived flowers of orchids increase the chances of pollination and thus the reproductive success of the species. However, a question arises: does the efficiency of pollination, expressed by fruit set, vary with the flower age? The objective of this study was to verify whether the flower age of Corymborkis flava(Sw.) Kuntze affects pollination efficiency. The following hypotheses were tested: 1) the fruit set of older flowers is lower than that of younger ones; 2) morphological observations (perianth and stigmatic area), stigma receptivity test by using a solution of hydrogen peroxide and hand-pollination tests are equally effective in defining the period of stigmatic receptivity. Flowers were found to be receptive from the first to the fourth day of anthesis. Fruit set of older flowers (third and fourth day) was lower than that of younger flowers. Morphological observations, the stigma receptivity test and hand-pollinations were equally effective in defining the period of stigmatic receptivity. However, to evaluate the maximum degree of stigma receptivity of orchid species with long-lived flowers, we recommend hand-pollinations, beyond the period of receptivity

Phytoplasmas: An Introduction

Phytoplasmas are among the most recently discovered plant pathogens. They are wall-less prokaryotes restricted to phloem tissue, associated with diseases affecting several hundred plant species. The impact of phytoplasma diseases on agriculture is impressive and, at the present day, no effective curative strategy has been developed. The availability of rapid and sensitive techniques for phytoplasma detection as well as the possibility to study their relationship with the host plants is a prerequisite for the management of phytoplasma-associated diseases

Heterologous expression and processing of the flavescence dorée phytoplasma variable membrane protein VmpA in Spiroplasma citri

BACKGROUND: Flavescence dorée (FD) of grapevine is a phloem bacterial disease that threatens European vineyards. The disease is associated with a non-cultivable mollicute, a phytoplasma that is transmitted by the grapevine leafhopper Scaphoideus titanus in a persistent, propagative manner. The specificity of insect transmission is presumably mediated through interactions between the host tissues and phytoplasma surface proteins comprising the so-called variable membrane proteins (Vmps). Plant spiroplasmas and phytoplasmas share the same ecological niches, the phloem sieve elements of host plants and the hemocoel of insect vectors. Unlike phytoplasmas, however, spiroplasmas, and Spiroplasma citri in particular, can be grown in cell-free media and genetically engineered. As a new approach for studying phytoplasmas-insect cell interactions, we sought to mimic phytoplasmas through the construction of recombinant spiroplasmas exhibiting FD phytoplasma Vmps at the cell surface.RESULTS: Here, we report the expression of the FD phytoplasma VmpA in S. citri. Transformation of S. citri with plasmid vectors in which the vmpA coding sequence was under the control of the S. citri tuf gene promoter resulted in higher accumulation of VmpA than with the native promoter. Expression of VmpA at the spiroplasma surface was achieved by fusing the vmpA coding sequence to the signal peptide sequence of the S. citri adhesin ScARP3d, as revealed by direct colony immunoblotting and immunogold labelling electron microscopy. Anchoring of VmpA to the spiroplasma membrane was further demonstrated by Triton X-114 protein partitioning and Western immunoblotting. Using the same strategy, the secretion of free, functionally active β-lactamase (used as a model protein) into the culture medium by recombinant spiroplasmas was achieved.CONCLUSIONS: Construction of recombinant spiroplasmas harbouring the FD phytoplasma variable membrane protein VmpA at their surface was achieved, which provides a new biological approach for studying interactions of phytoplasma surface proteins with host cells. Likewise, the secretion of functional β-lactamase by recombinant spiroplasmas established the considerable promise of the S. citri expression system for delivering phytoplasma effector proteins into host cells