Regulación hormonal y metabólica de las relaciones fuente-sumidero en respuesta al estrés salino en tomate (Solanum lycopersicum L.)

Resumo

The aim of this study was to analyze how salinity limits tomato plant productivity by affecting the source-sink relationships through changes in major hormonal concentrations and some sink-specific enzyme activities. We studied how those changes are related to both the limiting (fruit and shoot growth impairment and induced leaf senescence) and adaptive (biomass reallocation) processes imposed by the stress, in respect to the classical view of ionic relations. Accordingly, seedlings of Solanum lycopersicum L. were maintained in nutrient solution in the presence of moderate salt stress varying from 75 to 100 mM NaCl. Temporal correlations between PSII efficiency decline and changes in different hormone and ion concentrations were established in developing leaves. Changes in bioactive cytokinins (zeatin+zeatin-riboside, Z+ZR) and the ethylene precursor ACC concentrations and their ratios were the best hormonal parameters explaining the onset and progression of leaf senescence, prior to massive Na+ accumulation. Moreover, the differential auxin (indole acetic acid, IAA) and cytokinin (Z) responses in leaves and roots were well related to both the salinity-induced decrease in shoot vigour and the shift in biomass allocation to the roots, in agreement with changes in the activity of the sink-related enzyme cell wall invertase. Direct or indirect manipulation of hormonal concentrations and distribution were also investigated either through different strategies: exogenous hormonal application, grafting on wild tomato relatives, or the use of transgenics. An indirect but genetic evidence for the hormonal implication in salt tolerance was provided by a grafting approach. Accordingly, a tomato cultivar was grafted onto rootstocks from a population of recombinant inbred lines derived from a Solanum lycopersicum x S. cheesmaniae cross and cultivated under moderate salinity. The rootstock-mediated leaf xylem K+ (but not Na+), K+/Na+, the active cytokinin Z, the ratio Z/ZR, and especially the ratio between cytokinins and ACC (Z+ZR/ACC) were positively loaded into the first principal component determining both leaf growth and PSII efficiency. In contrast, the ratio ACC/ABA was negatively correlated with leaf biomass. These results highlight a pivotal role for CKs, probably in interaction with K+, in tomato salt tolerance. Alterations of source-sink relationships, rather than Na+, seem to be also a major cause for the decreased pollen viability and induced flower abortion under salinity, and thus negatively affecting fruit yield. Hence, the influence of both metabolic (sucrose metabolism) and hormonal factors on the fruit sink activity and strength, and hence on the overall fruit yield, in tomato plants growing under salinity was also studied. Different classical and functional physiological approaches have been integrated: from the spray and pray of the exogenous hormonal application, to the clone and play of the CIN1 (cell wall invertase) and IPT (de novo cytokinins biosynthesis) gene expression regulation. Similar metabolic and hormonal changes were observed in both transgenic CIN1 and grafted onto rootstock-IPT plants that increased not only fruit weight but also fruit number. Those positive effects were due to a lower flower abortion and higher fruit strength, which was related to (i) a higher sucrolytic activities or lower inhibition by salinity (sink activity); and/or (ii) a higher activity of the mature leaves (source activity).