In Arabidopsis (Arabidopsis thaliana), the carotenoid cleavage dioxygenases MORE AXILLARY GROWTH3 (MAX3) and MAX4 act together with MAX1 to produce a strigolactone signaling molecule required for the inhibition of axillary bud outgrowth. We show that both MAX3 and MAX4 transcripts are positively auxin regulated in a manner similar to the orthologous genes from pea (Pisum sativum) and rice (Oryza sativa), supporting evolutionary conservation of this regulation in plants. This regulation is important for branching control because large auxin-related reductions in these transcripts are associated with increased axillary branching. Both transcripts are up-regulated in max mutants, and consistent with max mutants having increased auxin in the polar auxin transport stream, this feedback regulation involves auxin signaling. We suggest that both auxin and strigolactone have the capacity to modulate each other's levels and distribution in a dynamic feedback loop required for the coordinated control of axillary branching.

The avocado, Persea americana , is a fruit crop of immense importance to Mexican agriculture with an increasing demand worldwide. Avocado lies in the anciently diverged magnoliid clade of angiosperms, which has a controversial phylogenetic position relative to eudicots and monocots. We sequenced the nuclear genomes of the Mexican avocado race, P. americana var. drymifolia , and the most commercially popular hybrid cultivar, Hass, and anchored the latter to chromosomes using a genetic map. Resequencing of Guatemalan and West Indian varieties revealed that ∼39% of the Hass genome represents Guatemalan source regions introgressed into a Mexican race background. Some introgressed blocks are extremely large, consistent with the recent origin of the cultivar. The avocado lineage experienced 2 lineage-specific polyploidy events during its evolutionary history. Although gene-tree/species-tree phylogenomic results are inconclusive, syntenic ortholog distances to other species place avocado as sister to the enormous monocot and eudicot lineages combined. Duplicate genes descending from polyploidy augmented the transcription factor diversity of avocado, while tandem duplicates enhanced the secondary metabolism of the species. Phenylpropanoid biosynthesis, known to be elicited by Colletotrichum (anthracnose) pathogen infection in avocado, is one enriched function among tandems. Furthermore, transcriptome data show that tandem duplicates are significantly up- and down-regulated in response to anthracnose infection, whereas polyploid duplicates are not, supporting the general view that collections of tandem duplicates contribute evolutionarily recent “tuning knobs” in the genome adaptive landscapes of given species.