Citrus is the main fruit tree crop in the world and therefore has a tremendous economical, social and cultural impact in our society. In recent years, our knowledge on plant reproductive biology has increased considerably mostly because of the work developed in model plants. However, the information generated in these species cannot always be applied to citrus, predominantly because citrus is a perennial tree crop that exhibits a very peculiar and unusual reproductive biology. Regulation of fruit growth and development in citrus is an intricate phenomenon depending upon many internal and external factors that may operate both sequentially and simultaneously. The elements and mechanisms whereby endogenous and environmental stimuli affect fruit growth are being interpreted and this knowledge may help to provide tools that allow optimizing production and fruit with enhanced nutritional value, the ultimate goal of the Citrus Industry. This article will review the progress that has taken place in the physiology of citrus fruiting during recent years and present the current status of major research topics in this area. Key words: abiotic stresses, abscission, color break, flowering, fruit set, ripening Fisiologia da frutificação em citrus. Citrus é a principal fruteira no mundo, tendo, portanto, profundos impactos econômicos, sociais e culturais em nossa sociedade. Nos últimos anos, o conhecimento sobre a biologia reprodutiva de plantas tem aumentado consideravelmente, principalmente em função de trabalhos desenvolvidos com plantas-modelo. Todavia, a informação produzida nessas espécies nem sempre pode ser aplicada a citrus, fundamentalmente porque citrus é uma cultura arbórea perene com uma biologia reprodutiva muito peculiar e incomum. A regulação do crescimento e desenvolvimento do fruto em citrus é um fenômeno complexo e dependente de muitos fatores externos e internos que podem operar tanto seqüencialmente como simultaneamente. Os elementos e mecanismos pelos quais estímulos ambientes e endógenos afetam o crescimento do fruto vêm sendo interpretados, e esse conhecimento pode auxiliar a prover ferramentas que permitiriam otimizar a produção per se, além da obtenção de frutos com maior valor nutricional, o objetivo precípuo da Industria de Citrus. Neste artigo, revisam-se os avanços que vêm ocorrendo na fisiologia da frutificação de citrus durante os últimos anos; apresenta-se, também, o status atual de pesquisas mais relevantes nessa área. Palavras-chave: estresses abióticos, floração, maturação, vingamento de frutos

SummaryAfter-ripening (AR) is a time and environment regulated process occurring in the dry seed, which determines the germination potential of seeds. Both metabolism and perception of the phytohormone abscisic acid (ABA) are important in the initiation and maintenance of dormancy. However, molecular mechanisms that regulate the capacity for dormancy or germination through AR are unknown. To understand the relationship between ABA and AR, we analysed genome expression in Arabidopsis thaliana mutants defective in seed ABA synthesis (aba1-1) or perception (abi1-1). Even though imbibed mutant seeds showed no dormancy, they exhibited changes in global gene expression resulting from dry AR that were comparable with changes occurring in wildtype (WT) seeds. Core gene sets were identified that were positively or negatively regulated by dry seed storage. Each set included a gene encoding repression or activation of ABA function (LPP2 and ABA1, respectively), thereby suggesting a mechanism through which dry AR may modulate subsequent germination potential in WT seeds. Application of exogenous ABA to after-ripened WT seeds did not reimpose characteristics of freshly harvested seeds on imbibed seed gene expression patterns. It was shown that secondary dormancy states reinstate AR status-specific gene expression patterns. A model is presented that separates the action of ABA in seed dormancy from AR and dry storage regulated gene expression. These results have major implications for the study of genetic mechanisms altered in seeds as a result of crop domestication into agriculture, and for seed behaviour during dormancy cycling in natural ecosystems.

As well as being phytohormones, gibberellins (GAs) are present in some fungi and bacteria. Indeed, GAs were first discovered in the fungus Gibberella fujikuroi, from which gibberellic acid (GA3) and other GAs are produced commercially. Although higher plants and the fungus produce structurally identical GAs, there are important differences in the pathways and enzymes involved. This has become particularly apparent with the identification of almost all of the genes for GA-biosynthesis in Arabidopsis thaliana and G. fujikuroi, following the sequencing of the Arabidopsis genome and the detection of a GA-biosynthesis gene cluster in the fungus. For example, 3b-hydroxylation occurs early in the pathway in G. fujikuroi and is catalyzed by a cytochrome P450 monooxygenase, whereas it is usually the final step in plants and is catalyzed by 2-oxoglutarate-dependent dioxygenases. Similarly, 20-oxidation is catalyzed by dioxygenases in plants and a cytochrome P450 in the fungus. Even where cytochrome P450s have equivalent functions in plants and Gibberella, they are unrelated in terms of amino acid sequence. These profound differences indicate that higher plants and fungi have evolved their complex biosynthetic pathways to GAs independently and not by horizontal gene transfer.

Gene StGA20ox1 encoding potato GA 20-oxidase is expressed to relatively high levels in leaves and regulated by daylength. To investigate whether this gene is involved in photoperiodic regulation of tuber formation, we have obtained transgenic potato plants expressing sense and antisense copies of the StGA20ox1 cDNA. Over-expression of this cDNA resulted in taller plants that required a longer duration of a short day photoperiod (SD) to tuberize. Tubers from these plants had a decreased time of dormancy and developed sprouts with elongated internodes. Plants expressing antisense copies of the StGA20ox1 cDNA had shorter stems, a decreased length of the internodes and tuberized earlier than control plants, showing increased tuber yields. Antisense inhibition of this gene had no visible effect on the time of dormancy of the tubers, although at the end of dormancy these formed sprouts with shortened internodes. Decreased levels of endogenous GA20 and GA1 were detected in the apex and first leaves of the antisense lines. These results demonstrate the involvement of the GA 20-oxidase activity encoded by StGA20ox1 in the control of stem elongation and in tuber induction but not in tuber dormancy, indicating that the latter may be regulated by another member of the gene family.