The genus Citrus, comprising some of the most widely cultivated fruit crops worldwide, includes an uncertain number of species. Here we describe ten natural citrus species, using genomic, phylogenetic and biogeographic analyses of 60 accessions representing diverse citrus germ plasms, and propose that citrus diversified during the late Miocene epoch through a rapid southeast Asian radiation that correlates with a marked weakening of the monsoons. A second radiation enabled by migration across the Wallace line gave rise to the Australian limes in the early Pliocene epoch. Further identification and analyses of hybrids and admixed genomes provides insights into the genealogy of major commercial cultivars of citrus. Among mandarins and sweet orange, we find an extensive network of relatedness that illuminates the domestication of these groups. Widespread pummelo admixture among these mandarins and its correlation with fruit size and acidity suggests a plausible role of pummelo introgression in the selection of palatable mandarins. This work provides a new evolutionary framework for the genus Citrus.

GH3 genes are main components of the hormonal mechanism regulating growth and development and, hence, are deeply involved in a broad range of physiological processes. They are implicated in hormonal homeostasis through the conjugation to amino acids of the free form of essential plant growth regulators such as indoleacetic and jasmonic acids. In this work, we showed that OsGH3.1 overexpression in rice caused dwarfism and significantly reduced both free auxin content and cell elongation. Functional classification of the transcriptomic profiling revealed that most genes involved in auxin biosynthesis and auxin signaling inhibition were induced and repressed, respectively. Many genes related to cell organization and biogenesis were also significantly downregulated. The survey also showed that, although the response to abiotic stresses was not clearly stimulated, OsGH3.1 overexpression did activate a significant number of defense-related genes. In successive bioassays, it was demonstrated that the resistance of rice plants to pathogen infection, evaluated with two different Magnaporthe grisea strains, was higher in the transformants overexpressing OsGH3.1. Taken together, these results indicate that OsGH3.1 overexpression reduces auxin content, inhibits cell growth and cell wall loosening, and enhances resistance to a fungal pathogen. Our results provide evidence that auxin homeostasis can regulate the activation of the defense response in rice.

Summary: We have investigated OsHKT2;1 natural variation in a collection of 49 cultivars with different levels of salt tolerance and geographical origins. The effect of identified polymorphism on OsHKT2;1 activity was analysed through heterologous expression of variants in Xenopus oocytes. OsHKT2;1 appeared to be a highly conserved protein with only five possible amino acid substitutions that have no substantial effect on functional properties. Our study, however, also identified a new HKT isoform, No‐OsHKT2;2/1 in Nona Bokra, a highly salt‐tolerant cultivar. No‐OsHKT2;2/1 probably originated from a deletion in chromosome 6, producing a chimeric gene. Its 5′ region corresponds to that of OsHKT2;2, whose full‐length sequence is not present in Nipponbare but has been identified in Pokkali, a salt‐tolerant rice cultivar. Its 3′ region corresponds to that of OsHKT2;1. No‐OsHKT2;2/1 is essentially expressed in roots and displays a significant level of expression at high Na⁺ concentrations, in contrast to OsHKT2;1. Expressed in Xenopus oocytes or in Saccharomyces cerevisiae, No‐OsHKT2;2/1 exhibited a strong permeability to Na⁺ and K⁺, even at high external Na⁺ concentrations, like OsHKT2;2, and in contrast to OsHKT2;1. Our results suggest that No‐OsHKT2;2/1 can contribute to Nona Bokra salt tolerance by enabling root K⁺ uptake under saline conditions.

Summary: The Zinnia mesophyll cell system consists of isolated leaf mesophyll cells in culture that can be induced, by auxin and cytokinin, to reproducibly trans‐differentiate into tracheary elements (TE) after 96 h, while in the presence of auxin alone the cells simply elongate. In a search for genes involved in modifications to cell‐wall architecture before any overt signs of cell differentiation, a differential hybridization of a 72‐h cDNA library with probes from mRNA at time‐points of 24 h and 72 h was done revealing a number of transcripts up‐regulated between these times. One of these cDNAs shows homology to pectate lyase, a pectin‐degrading enzyme. The complete cDNA sequence (ZePel) corresponds to a translated protein of 44 kDa with an N‐terminal signal peptide of about 2 kDa, and one potential N‐glycosylation site. Northern analysis confirms that the strong expression of this gene during TE induction occurs at a very early stage of the process and is due solely to the presence of auxin in the induction medium. In situ hybridization studies in young Zinnia stems show that ZePel expression is associated with vascular bundles and shoot primordia. Recombinant protein made in Escherichia coli possesses calcium‐dependent pectate lyase activity. Pectate lyase activity is detected in elongating and differentiating in vitro cell populations. The role of this enzyme in remodelling the cell wall during cell elongation and differentiation is discussed.