Extrafloral-nectar based partner manipulation in plant-ant relationship

Plant–ant interactions are generally considered as mutualisms, with both parties gaining benefits from the association. It has recently emerged that some of these mutualistic associations have, however, evolved towards other forms of relationships and, in particular, that plants may manipulate their partner ants to make reciprocation more beneficial, thereby stabilizing the mutualism. Focusing on plants bearing extrafloral nectaries, we review recent studies and address three key questions: (i) how can plants attract potential partners and maintain their services; (ii) are there compounds in extrafloral nectar that could mediate partner manipulation; and (iii) are ants susceptible to such compounds? After reviewing the current knowledge on plant–ant associations, we propose a possible scenario where plant-derived chemicals, such as secondary metabolites, known to have an impact on animal brain, could have evolved in plants to attract and manipulate ant behaviour. This new viewpoint would place plant–animal interaction in a different ecological context, opening new ecological and neurobiological perspectives of drug seeking and use

Na+ extrusion from the cytosol and tissue-specific Na+ sequestration in roots confer differential salt stress tolerance between durum and bread wheat

The progress in plant breeding for salinity stress tolerance is handicapped by the lack of understanding of the specificity of salt stress signalling and adaptation at the cellular and tissue levels. In this study, we used electrophysiological, fluorescence imaging, and real-time quantitative PCR tools to elucidate the essentiality of the cytosolic Na+ extrusion in functionally different root zones (elongation, meristem, and mature) in a large number of bread and durum wheat accessions. We show that the difference in the root’s ability for vacuolar Na+ sequestration in the mature zone may explain differential salinity stress tolerance between salt-sensitive durum and salt-tolerant bread wheat species. Bread wheat genotypes also had on average 30% higher capacity for net Na+ efflux from the root elongation zone, providing the first direct evidence for the essentiality of the root salt exclusion trait at the cellular level. At the same time, cytosolic Na+ accumulation in the root meristem was significantly higher in bread wheat, leading to the suggestion that this tissue may harbour a putative salt sensor. This hypothesis was then tested by investigating patterns of Na+ distribution and the relative expression level of several key genes related to Na+ transport in leaves in plants with intact roots and in those in which the root meristems were removed. We show that tampering with this sensing mechanism has resulted in a salt-sensitive phenotype, largely due to compromising the plant’s ability to sequester Na+ in mesophyll cell vacuoles. The implications of these findings for plant breeding for salinity stress tolerance are discussed

Plant responses to heterogeneous salinity: Growth of the halophyte Atriplex nummularia is determined by the root-weighted mean salinity of the root zone

Soil salinity is generally spatially heterogeneous, but our understanding of halophyte physiology under such conditions is limited. The growth and physiology of the dicotyledonous halophyte Atriplex nummularia was evaluated in split-root experiments to test whether growth is determined by: (i) the lowest; (ii) the highest; or (iii) the mean salinity of the root zone. In two experiments, plants were grown with uniform salinities or horizontally heterogeneous salinities (10–450mM NaCl in the low-salt side and 670mM in the high-salt side, or 10mM NaCl in the low-salt side and 500–1500mM in the high-salt side). The combined data showed that growth and gas exchange parameters responded most closely to the root-weighted mean salinity rather than to the lowest, mean, or highest salinity in the root zone. In contrast, midday shoot water potentials were determined by the lowest salinity in the root zone, consistent with most water being taken from the least negative water potential source. With uniform salinity, maximum shoot growth was at 120–230mM NaCl; ~90% of maximum growth occurred at 10mM and 450mM NaCl. Exposure of part of the roots to 1500mM NaCl resulted in an enhanced (+40%) root growth on the low-salt side, which lowered root-weighted mean salinity and enabled the maintenance of shoot growth. Atriplex nummularia grew even with extreme salinity in part of the roots, as long as the root-weighted mean salinity of the root zone was within the 10–450mM range

Plant growth and physiology under heterogeneous salinity

Background Soil salinity is heterogeneous, and within the root-zone of single plants the salinity of the soil solution can vary widely. Scope This review shows that water uptake by roots from the least saline part of the soil is the key factor driving shoot growth; plants with part of the root at low salinity (0–10 mM NaCl) had 3- to 10-fold higher shoot dry mass than plants with roots in uniformly saline (50–800 mM NaCl) media. Plants in heterogeneous salinity had shoot water potentials similar to those of plants growing in uniform low-salt media, and this was likely a result of uptake of low salinity water and reduced stomatal conductance. Under heterogeneous conditions, roots in saline media took up ions, resulting in higher shoot Na+ and Cl- concentrations compared with plants growing in low-salt media. Conclusions Results from split-root experiments complement knowledge of plant responses to uniform salinities; the next challenge is to develop new protocols so that this understanding can be extrapolated to more complex soil- and field-based systems. More work is also required to understand the physiological mechanisms underlying changes in stomatal conductance and shoot ion regulation in plants under heterogeneous salinities and how these are linked to the saline parts of the root-zone

Friend or Foe? Chloride patterning in halophytes

In this opinion article, we challenge the traditional view that breeding for reduced Cl− uptake would benefit plant salinity tolerance. A negative correlation between shoot Cl− concentration and plant biomass does not hold for halophytes – naturally salt tolerant species. We argue that, under physiologically relevant conditions, Cl− uptake requires plants to invest metabolic energy, and that the poor selectivity of Cl−-transporting proteins may explain the reported negative correlation between Cl− accumulation and crop salinity tolerance. We propose a new paradigm: salinity tolerance could be achieved by improving the selectivity of some of the broadly selective anion-transporting proteins (e.g., for NO3− > Cl−), alongside tight control of Cl− uptake, rather than targeting traits mediating its efflux from the root

Differential tolerance to combined salinity and O2 deficiency in the halophytic grasses Puccinellia ciliata and Thinopyrum ponticum: The importance of K+ retention in roots

Saline environments of terrestrial halophytes are often prone to waterlogging, yet the effects on halophytes of combined salinity and waterlogging have rarely been studied. Either salinity or hypoxia (low O2) alone can interfere with K+ homeostasis, therefore the combination of salinity or hypoxia is expected to impact significantly on K+ retention in roots. We studied mechanisms of tolerance to the interaction of salinity with hypoxia in Puccinellia ciliata and Thinopyrum ponticum, halophytic grasses that differ in waterlogging tolerance. Plants were exposed to aerated and stagnant saline (250mM NaCl) treatments with low (0.25mM) and high (4mM) K+ levels; growth, net ion fluxes and tissue ion concentrations were determined. P. ciliata was more tolerant than T. ponticum to stagnant-saline treatment, producing twice the biomass of adventitious roots, which accumulated high levels of Na+, and had lower shoot Na+. After 24h of saline hypoxic treatment, MIFE measurements revealed a net uptake of K+ (∼40nmolm-2s-1) for P. ciliata, but a net loss of K+ (∼20nmolm-2s-1) for the more waterlogging sensitive T. ponticum. NaCl alone induced K+ efflux from roots of both species, with channel blocker tests implicating GORK-like channels. P. ciliata had constitutively a more negative root cell membrane potential than T. ponticum (-150 versus -115mV). Tolerance to salinity and hypoxia in P. ciliata is related to increased production of adventitious roots, regulation of shoot K+/Na+, and a superior ability to maintain negative membrane potential in root cells, resulting in greater retention of K+

Soil volatile analysis by proton transfer reaction-time of flight mass spectrometry (PTR-TOF-MS)

We analyzed the volatile organic compounds (VOCs) emitted from different soils by using the PTR-MS-TOF technique under laboratory conditions and compared them with soil chemical biochemical activities. The emitted VOCs were related to soil microbial biomass, soil respiration and some soil enzyme activities so as to evaluate if size and activity of soil microbial communities influenced the soil VOCs profiles. Our results showed that the emitted VOCs discriminated between soils with different properties and management, and differences in the VOCs emission profiles were likely related to the active metabolic pathways in the microbial communities of the three studied soil. Our results also showed that some soil enzyme activities such as \u3b2-glucosidase and arylsulfatase were possibly involved in the release of compounds fueling microbial metabolic pathways leading to the production of specific VOCs. It was concluded that the PTR-MS-TOF technique is suitable for analyze VOCs emission from soil and that studies comparing soil enzyme activities and soil volatile profiles can reveal the origin of VOCs and give further insights on microbial activity and soil functionality