Search

In a recent paper, we described for the first time the effects of fruit on the expression of putative homologues of genes involved in flowering pathways. It was our aim to provide insight into the molecular mechanisms underlying alternate bearing in citrus. However, a bioinformatics-based critique of our and other related papers has been given by Samach in the preceding Viewpoint article in this issue of Annals of Botany. The use of certain bioinformatic tools in a context of structural rather than functional genomics can cast doubts about the veracity of a large amount of data published in recent years. In this response, the contentions raised by Samach are analysed, and rebuttals of his criticisms are presented.

Hydraulic conductance exerts a strong influence on many aspects of plant physiology, namely: transpiration, CO2 assimilation, growth, productivity or stress response. However we lack full understanding of the contribution of root or shoot water transport capacity to the total water balance, something which is difficult to study in trees. Here we tested the hypothesis that whole plant hydraulic conductance modulates plant transpiration using two different seedlings of citrus rootstocks, Poncirus trifoliata (L.) Raf. and Cleopatra mandarin (Citrus reshni Hort ex Tan.). The two genotypes presented important differences in their root or shoot hydraulic conductance contribution to whole plant hydraulic conductance but, even so, water balance proved highly dependent on whole plant conductance. Further, we propose there is a possible equilibrium between root and shoot hydraulic conductance, similar to that between shoot and root biomass production, which could be related with xylem anatomy. [ABSTRACT FROM AUTHOR]

The effects of iron (Fe) deficiency on the low-molecular-weight organic acid (LMWOA) metabolism have been investigated in Carrizo citrange (CC) [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.] roots. Major LMWOAs found in roots, xylem sap and root exudates were citrate and malate and their concentrations increased with Fe deficiency. The activities of several enzymes involved in the LMWOA metabolism were also assessed in roots. In the cytosolic fraction, the activities of malate dehydrogenase (cMDH) and phosphoenolpyruvate carboxylase (PEPC) enzymes were 132 and 100% higher in Fe-deficient conditions, whereas the activity of pyruvate kinase was 31% lower and the activity of malic enzyme (ME) did not change. In the mitochondrial fraction, the activities of fumarase, MDH and citrate synthase enzymes were 158, 117 and 53% higher, respectively, in Fe-deficient extracts when compared with Fe-sufficient controls, whereas no significant differences between treatments were found for aconitase (ACO) activity. The expression of their corresponding genes in roots of Fe-deficient plants was higher than that measured in Fe-sufficient controls, except for ACO and ME. Also, dicarboxylate–tricarboxylate carrier (DTC) expression was significantly increased in Fe-deficient roots. In conclusion, Fe deficiency in CC seedlings causes a reprogramming of the carbon metabolism that involves an increase of anaplerotic fixation of carbon via PEPC and MDH activities in the cytosol and a shift of the Krebs cycle in the mitochondria towards a non-cyclic mode, as previously described in herbaceous species. In this scheme, DTC could play an important role shuttling both malate and reducing equivalents between the cytosol and the mitochondria. As a result of this metabolic switch malate and citrate concentrations in roots, xylem sap and root exudates increase. [ABSTRACT FROM AUTHOR]

This paper describes the physiological effects of abscisic acid (ABA) and 100 mM NaCl on citrus plants. Water potential, leaf abscission, ethylene production, photosynthetic rate, stomatal conductance, and chloride accumulation in roots and leaves were measured in plants of Salustiana scion [Citrus sinensis (L) Osbeck] grafted onto Carrizo citrange (Citrus sinensis [L.] Osbeck × Poncirus trifoliata [L.] Raf) rootstock. Plants under salt stress accumulated high amounts of chloride, increased ethylene production, and induced leaf abscission. Stomatal conductance and photosynthetic rates rapidly dropped after salinization. The addition of 10 mM ABA to the nutrient solution 10 days before the exposure to salt stress reduced ethylene release and leaf abscission. These effects were probably due to a decrease in the accumulation of toxic Cl- ions in leaves. In non-salinized plants, ABA reduced stomatal conductance and CO2 assimilation, whereas in salinized plants the treatment slightly increased these two parameters. The results suggest a protective role for ABA in citrus under salinity. [ABSTRACT FROM AUTHOR]

Citrus is a wide genus in which most of the cultivated species and cultivars are natural parthenocarpic mutants or hybrids (i.e. orange, mandarin, tangerine, grapefruit). The autonomous increase in GA 1 ovary concentration during anthesis was suggested as being the stimulus responsible for parthenocarpy in Citrus regardless of the species. To determine the exact GA-role in parthenocarpic fruit set, the following hypothesis was tested: GA triggers and maintains cell division in ovary walls causing fruit set. Obligate and facultative parthenocarpic Citrus species were used as a model system because obligate parthenocarpic Citrus sp (i.e. Citrus unshiu ) have higher GA levels and better natural parthenocarpic fruit set compared to other facultative parthenocarpic Citrus (i.e. Citrus clementina ). The autonomous activation of GA synthesis in C. unshiu ovary preceded cell division and CYCA1.1 up-regulation (a G2-stage cell cycle regulator) at anthesis setting a high proportion of fruits, whereas C. clementina lacked this GA-biosynthesis and CYCA1.1 up-regulation failing in fruit set. In situ hybridization experiments revealed a tissue-specific expression of GA20ox2 only in the dividing tissues of the pericarp. Furthermore, CYCA1.1 expression correlated endogenous GA 1 content with GA 3 treatment, which stimulated cell division and ovary growth, mostly in C. clementina . Instead, paclobutrazol (GA biosynthesis inhibitor) negated cell division and reduced fruit set. Results suggest that in parthenocarpic citrus the specific GA synthesis in the ovary walls at anthesis triggers cell division and, thus, the necessary ovary growth rate to set fruit. [ABSTRACT FROM AUTHOR]