Francisco Pérez-Alfocea

Hormonal changes in relation to biomass partitioning and shoot growth impairment in salinized tomato (Solanum lycopersicum L.) plants

Following exposure to salinity, the root/shoot ratio is increased (an important adaptive response) due to the rapid inhibition of shoot growth (which limits plant productivity) while root growth is maintained. Both processes may be regulated by changes in plant hormone concentrations. Tomato plants (Solanum lycopersicum L. cv Moneymaker) were cultivated hydroponically for 3 weeks under high salinity (100 mM NaCl) and five major plant hormones (abscisic acid, ABA; the cytokinins zeatin, Z, and zeatin-riboside, ZR; the auxin indole-3-acetic acid, IAA; and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, ACC) were determined weekly in roots, xylem sap, and leaves. Salinity reduced shoot biomass by 50–60% and photosynthetic area by 20–25% both by decreasing leaf expansion and delaying leaf appearance, while root growth was less affected, thus increasing the root/shoot ratio. ABA and ACC concentrations strongly increased in roots, xylem sap, and leaves after 1 d (ABA) and 15 d (ACC) of salinization. By contrast, cytokinins and IAA were differentially affected in roots and shoots. Salinity dramatically decreased the Z+ZR content of the plant, and induced the conversion of ZR into Z, especially in the roots, which accounted for the relative increase of cytokinins in the roots compared to the leaf. IAA concentration was also strongly decreased in the leaves while it accumulated in the roots. Decreased cytokinin content and its transport from the root to the shoot were probably induced by the basipetal transport of auxin from the shoot to the root. The auxin/cytokinin ratio in the leaves and roots may explain both the salinity-induced decrease in shoot vigour (leaf growth and leaf number) and the shift in biomass allocation to the roots, in agreement with changes in the activity of the sink-related enzyme cell wall invertase.

Hormonal changes during salinity-induced leaf senescence in tomato (Solanum lycopersicum L.)

Leaf senescence is one of the most limiting factors to plant productivity under salinity. Both the accumulation of specific toxic ions (e.g. Na+) and changes in leaf hormone relations are involved in the regulation of this process. Tomato plants (Solanum lycopersicum L. cv Moneymaker) were cultivated for 3 weeks under high salinity (100 mM NaCl) and leaf senescence-related parameters were studied during leaf development in relation to Na+ and K+ contents and changes in abscisic acid (ABA), cytokinins, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), and the auxin indole-3-acetic acid (IAA). Na+ accumulated to a similar extent in both leaves 4 and 5 (numbering from the base of the plant) and more quickly during the third week, while concurrently K+ contents sharply decreased. However, photosystem II efficiency, measured as the Fv/Fm ratio, decreased from the second week of salinization in leaf 4 but only at the end of the third week in the younger leaf 5. In the prematurely senescent leaf 4, ABA content increased linearly while IAA strongly decreased with salinization time. Although zeatin (Z) levels were scarcely affected by salinity, zeatin-riboside (ZR) and the total cytokinin content (Z+ZR) progressively decreased by 50% from the imposition of the stress. ACC was the only hormonal compound that increased in leaf tissue coincident with the onset of oxidative damage and the decline in chlorophyll fluorescence, and prior to massive Na+ accumulation. Indeed, (Z+ZR) and ACC contents and their ratio (Z+ZR/ACC) were the hormonal parameters best correlated with the onset and progression of leaf senescence. The influence of different hormonal changes on salt-induced leaf senescence is discussed.

The rootstock effect on the tomato salinity response depends on the shoot genotype

Abstract With the aim of determining whether the rootstock effect depends on the shoot genotype, two distinctly different tomato ( Lycopersicon esculentum Mill) genotypes, Moneymaker with excluder character and UC-82B with includer character, were grafted onto a commercial tomato hybrid (cv. Kyndia). Self-grafted plants were used as controls. The rootstock effect was first assessed by growing plants at different NaCl concentrations (0, 50 and 100 mM) under controlled conditions, and by determining the growth and physiological responses of the grafted plants after 35 days of salt treatment. The rootstock did not affect the shoot growth when Moneymaker was used as scion. However, when UC-82B was used as scion, the shoot biomass reduction induced by salinity was lower in plants with rootstock Kyndia than in the self-grafted plants. The effectiveness of the UC-82B grafted plants onto Kyndia was also showed on the basis of fruit yield. The rootstock also induced significant physiological changes in the leaves of these plants, with the most important changes being found in the ion saline accumulation and Na + /K + ratio. The Na + and Cl − accumulation induced by salinity in leaves of UC-82B was substantially lower when the root of this cultivar had been substituted by that of the cv. Kyndia, whereas the K + concentration was less reduced by salinity, and consequently the leaf Na + /K + ratio values were much lower. These results suggest that the saline ion accumulation in leaves was controlled predominantly by the genotype of the rootstock. In addition, the characteristics of the rootstock able to induce salt tolerance to the shoot depend on the salt tolerance mechanism of the shoot genotype.

Agronomical and physiological characterization of salinity tolerance in a commercial tomato hybrid

The salt tolerance of the commercial F1 tomato hybrid (Lycopersicon esculentum Mill) Radja (GC-793) has been agronomically and physiologically evaluated under greenhouse conditions, using a control (nutrient solution), a moderate (70 mM NaCl added to the nutrient solution) and a high salt level (140 mM NaCl), applied for 130 days. The results show that Radja is a Na+-excluder genotype, tolerant to moderate salinity. Fruit yield was reduced by 16% and 60% and the shoot biomass by 30% and more than 75% under moderate and high salinities, respectively. At 90 days of salt treatment (DST), the mature leaves feeding the 4th truss at fruiting accumulated little Na+ (178 mmol kg-1 DW). At this time, the sucrose concentration in these leaves even increased with moderate salinity and the amino acid proline was not accumulated under salt conditions as compared to control. At 130 DST, Na+ was accumulated mainly by the roots in proportion to the salt level applied, while in leaves appreciable amounts were found only at high salinity (452 mmol kg-1 DW). In the leaves, Cl- was always accumulated in proportion to the salt level and in a very much greater amounts than Na+ (until 1640 mmol kg-1 DW). The sucrose content was reduced in all plants by salinity, and was distributed preferentially toward the distal stem and peduncle of a truss at fruiting under moderate salinity, and toward the basal stem and root at high salinity. Moreover, proline was accumulated in different organs of the plant only at high salinity, coinciding with Na+ accumulation in leaves. Attempts are made to find a clear relationship between physiological behaviour triggered by stress and the agronomical behaviour, in order to assess the validity of physiological traits used for salt-tolerance selection and breeding in tomato.

Microbial amelioration of crop salinity stress

The use of soil and irrigation water with a high content of soluble salts is a major limiting factor for crop productivity in the semi-arid areas of the world. While important physiological insights about the mechanisms of salt tolerance in plants have been gained, the transfer of such knowledge into crop improvement has been limited. The identification and exploitation of soil microorganisms (especially rhizosphere bacteria and mycorrhizal fungi) that interact with plants by alleviating stress opens new alternatives for a pyramiding strategy against salinity, as well as new approaches to discover new mechanisms involved in stress tolerance. Although these mechanisms are not always well understood, beneficial physiological effects include improved nutrient and water uptake, growth promotion, and alteration of plant hormonal status and metabolism. This review aims to evaluate the beneficial effects of soil biota on the plant response to saline stress, with special reference to phytohormonal signalling mechanisms that interact with key physiological processes to improve plant tolerance to the osmotic and toxic components of salinity. Improved plant nutrition is a quite general beneficial effect and may contribute to the maintenance of homeostasis of toxic ions under saline stress. Furthermore, alteration of crop hormonal status to decrease evolution of the growth-retarding and senescence-inducing hormone ethylene (or its precursor 1-aminocyclopropane-1-carboxylic acid), or to maintain source-sink relations, photosynthesis, and biomass production and allocation (by altering indole-3-acetic acid and cytokinin biosynthesis) seem to be promising target processes for soil biota-improved crop salt tolerance.

Root-synthesized cytokinins improve shoot growth and fruit yield in salinized tomato (Solanum lycopersicum L.) plants

Salinity limits crop productivity, in part by decreasing shoot concentrations of the growth-promoting and senescence-delaying hormones cytokinins. Since constitutive cytokinin overproduction may have pleiotropic effects on plant development, two approaches assessed whether specific root-localized transgenic IPT (a key enzyme for cytokinin biosynthesis) gene expression could substantially improve tomato plant growth and yield under salinity: transient root IPT induction (HSP70::IPT) and grafting wild-type (WT) shoots onto a constitutive IPT-expressing rootstock (WT/35S::IPT). Transient root IPT induction increased root, xylem sap, and leaf bioactive cytokinin concentrations 2- to 3-fold without shoot IPT gene expression. Although IPT induction reduced root biomass (by 15%) in control (non-salinized) plants, in salinized plants (100 mM NaCl for 22 d), increased cytokinin concentrations delayed stomatal closure and leaf senescence and almost doubled shoot growth (compared with WT plants), with concomitant increases in the essential nutrient K+ (20%) and decreases in the toxic ion Na+ (by 30%) and abscisic acid (by 20–40%) concentrations in transpiring mature leaves. Similarly, WT/35S::IPT plants (scion/rootstock) grown with 75 mM NaCl for 90 d had higher fruit trans-zeatin concentrations (1.5- to 2-fold) and yielded 30% more than WT/non-transformed plants. Enhancing root cytokinin synthesis modified both shoot hormonal and ionic status, thus ameliorating salinity-induced decreases in growth and yield.

Rootstock-mediated changes in xylem ionic and hormonal status are correlated with delayed leaf senescence, and increased leaf area and crop productivity in salinized tomato

Tomato crop productivity under salinity can be improved by grafting cultivars onto salt-tolerant wild relatives, thus mediating the supply of root-derived ionic and hormonal factors that regulate leaf area and senescence. A tomato cultivar was grafted onto rootstocks from a population of recombinant inbred lines (RILs) derived from a Solanum lycopersicum x Solanum cheesmaniae cross and cultivated under moderate salinity (75 mM NaCl). Concentrations of Na(+), K(+) and several phytohormones [abscisic acid (ABA); the cytokinins (CKs) zeatin, Z; zeatin riboside, ZR; and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC)] were analysed in leaf xylem sap in graft combinations of contrasting vigour. Scion leaf area correlated with photosystem II (PSII) efficiency (F(v)/F(m)) and determined fruit productivity. Xylem K(+) (but not Na(+)), K(+)/Na(+), the active CK Z, the ratio with its storage form Z/ZR and especially the ratio between CKs and ACC (Z/ACC and Z + ZR/ACC) were positively loaded into the first principal component (PC) determining both leaf growth and PSII efficiency. In contrast, the ratio ACC/ABA was negatively correlated with leaf biomass. Although the underlying physiological mechanisms by which rootstocks mediate leaf area or chlorophyll fluorescence (and thus influence tomato salt tolerance) seem complex, a putative potassium-CK interaction involved in regulating both processes merits further attention.

Interaction between hydrogen peroxide and plant hormones during germination and the early growth of pea seedlings

Hydrogen peroxide (H(2)O(2)) increased the germination percentage of pea seeds, as well as the growth of seedlings in a concentration-dependent manner. The effect of H(2)O(2) on seedling growth was removed by incubation with 10 microm ABA. The H(2)O(2)-pretreatment produced an increase in ascorbate peroxidase (APX), peroxidase (POX) and ascorbate oxidase (AAO). The increases in these ascorbate-oxidizing enzymes correlated with the increase in the growth of the pea seedlings as well as with the decrease in the redox state of ascorbate. Moreover, the increase in APX activity was due to increases in the transcript levels of cytosolic and stromal APX (cytAPX, stAPX). The proteomic analysis showed that H(2)O(2) induced proteins related to plant signalling and development, cell elongation and division, and cell cycle control. A strong correlation between the effect of H(2)O(2) on plant growth and the decreases in ABA and zeatin riboside (ZR) was observed. The results suggest an interaction among the redox state and plant hormones, orchestrated by H(2)O(2), in the induction of proteins related to plant signalling and development during the early growth of pea seedlings.

Hormonal regulation of source-sink relations to maintain crop productivity under salinity: a case study of root-to-shoot signalling in tomato

Salinity decreases crop yield first by reducing growth of assimilate-consuming sink organs and, second, by decreasing assimilate production in photosynthetically active source tissues. Although much work has focussed on controlling the accumulation of toxic ions (mainly Na+ and Cl-), the search for primary growth limiting factor(s) continues. The root, by sensing environmental constraints of the soil, may influence root-to-shoot signalling to control shoot growth and physiology, and ultimately agricultural productivity. Hormonal signals, such as cytokinins, ABA, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid and the auxin indole-3-acetic acid may coordinate assimilate production and usage in competing sinks (biomass partitioning). Hormonal regulation of source-sink relations during the osmotic phase of salinity (independent of specific ions) affects whole-plant energy availability to prolong the maintenance of growth, root function and ion homeostasis, and could be critical to delay the accumulation of Na+ or any other ion to toxic levels. This viewpoint emphasises that simultaneously maintaining growth and delaying early leaf senescence is necessary to increase crop yield in salt-affected soils.

Hormonal and metabolic regulation of source-sink relations under salinity and drought: From plant survival to crop yield stability

Securing food production for the growing population will require closing the gap between potential crop productivity under optimal conditions and the yield captured by farmers under a changing environment, which is termed agronomical stability. Drought and salinity are major environmental factors contributing to the yield gap ultimately by inducing premature senescence in the photosynthetic source tissues of the plant and by reducing the number and growth of the harvestable sink organs by affecting the transport and use of assimilates between and within them. However, the changes in source-sink relations induced by stress also include adaptive changes in the reallocation of photoassimilates that influence crop productivity, ranging from plant survival to yield stability. While the massive utilization of -omic technologies in model plants is discovering hundreds of genes with potential impacts in alleviating short-term applied drought and salinity stress (usually measured as plant survival), only in relatively few cases has an effect on crop yield stability been proven. However, achieving the former does not necessarily imply the latter. Plant survival only requires water status conservation and delayed leaf senescence (thus maintaining source activity) that is usually accompanied by growth inhibition. However, yield stability will additionally require the maintenance or increase in sink activity in the reproductive structures, thus contributing to the transport of assimilates from the source leaves and to delayed stress-induced leaf senescence. This review emphasizes the role of several metabolic and hormonal factors influencing not only the source strength, but especially the sink activity and their inter-relations, and their potential to improve yield stability under drought and salinity stresses.