An apple somatic mutation of delayed fruit maturation date is primarily caused by a retrotransposon insertion-associated large deletion.

Somatic mutations may alter important traits in tree fruits, such as fruit color, size and maturation date. Autumn Gala (AGala), a somatic mutation from apple cultivar Gala, matures 4 weeks later than Gala. To understand the mechanisms underlying the delayed maturation, RNA-seq analyses were conducted with fruit sampled at 13 (Gala) and 16 (AGala) time-points during their growth and development. Weighted gene co-expression network analysis (WGCNA) of 23 372 differentially expressed genes resulted in 25 WGCNA modules. Of these, modules 1 (r = -0.98, P = 2E-21) and 2 (r = -0.52, P = 0.004), which were suppressed in AGala, were correlated with fruit maturation date. Surprisingly, 77 of the 152 member genes in module 1 were harbored in a 2.8-Mb genomic region on chromosome 6 that was deleted and replaced by a 10.7-kb gypsy-like retrotransposon (Gy-36) from chromosome 7 in AGala. Among the 77 member genes, MdACT7 was the most suppressed (by 10.5-fold) in AGala due to a disruptive 2.5-kb insertion in coding sequence. Moreover, MdACT7 is the exclusive apple counterpart of Arabidopsis ACT7 known of essential roles in plant development, and the functional allele MdACT7, which was lost to the deletion in AGala, was associated with early fruit maturation in 268 apple accessions. Overexpressing alleles MdACT7 and Mdact7 in an Arabidopsis act7 line showed that MdACT7 largely rescued its stunted growth and delayed initial flowering while Mdact7 did not. Therefore, the 2.8-Mb hemizygous deletion is largely genetically causal for fruit maturation delay in AGala, and the total loss of MdACT7 might have contributed to the phenotype.

Actin isovariant ACT7 controls root meristem development in Arabidopsis through modulating auxin and ethylene responses.

The meristem is the most functionally dynamic part in a plant. The shaping of the meristem requires constant cell division and elongation, which are influenced by hormones and the cytoskeletal component, actin. Although the roles of hormones in modulating meristem development have been extensively studied, the role of actin in this process is still elusive. Using the single and double mutants of the vegetative class actin, we demonstrate that actin isovariant ACT7 plays an important role in root meristem development. In the absence of ACT7, but not ACT8 and ACT2, depolymerization of actin was observed. Consistently, the act7 mutant showed reduced cell division, cell elongation, and meristem length. Intracellular distribution and trafficking of auxin transport proteins in the actin mutants revealed that ACT7 specifically functions in the root meristem to facilitate the trafficking of auxin efflux carriers PIN1 and PIN2, and consequently the transport of auxin. Compared with act7, the act7act8 double mutant exhibited slightly enhanced phenotypic response and altered intracellular trafficking. The altered distribution of auxin in act7 and act7act8 affects the response of the roots to ethylene, but not to cytokinin. Collectively, our results suggest that ACT7-dependent auxin-ethylene response plays a key role in controlling Arabidopsis root meristem development.

Lipid Body Dynamics in Shoot Meristems: Production, Enlargement, and Putative Organellar Interactions and Plasmodesmal Targeting.

Post-embryonic cells contain minute lipid bodies (LBs) that are transient, mobile, engage in organellar interactions, and target plasmodesmata (PD). While LBs can deliver γ-clade 1,3-ß-glucanases to PD, the nature of other cargo is elusive. To gain insight into the poorly understood role of LBs in meristems, we investigated their dynamics by microscopy, gene expression analyzes, and proteomics. In developing buds, meristems accumulated LBs, upregulated several LB-specific OLEOSIN genes and produced OLEOSINs. During bud maturation, the major gene OLE6 was strongly downregulated, OLEOSINs disappeared from bud extracts, whereas lipid biosynthesis genes were upregulated, and LBs were enlarged. Proteomic analyses of the LB fraction of dormant buds confirmed that OLEOSINs were no longer present. Instead, we identified the LB-associated proteins CALEOSIN (CLO1), Oil Body Lipase 1 (OBL1), Lipid Droplet Interacting Protein (LDIP), Lipid Droplet Associated Protein1a/b (LDAP1a/b) and LDAP3a/b, and crucial components of the OLEOSIN-deubiquitinating and degradation machinery, such as PUX10 and CDC48A. All mRFP-tagged LDAPs localized to LBs when transiently expressed in Nicotiana benthamiana. Together with gene expression analyzes, this suggests that during bud maturation, OLEOSINs were replaced by LDIP/LDAPs at enlarging LBs. The LB fraction contained the meristem-related actin7 (ACT7), "myosin XI tail-binding" RAB GTPase C2A, an LB/PD-associated γ-clade 1,3-ß-glucanase, and various organelle- and/or PD-localized proteins. The results are congruent with a model in which LBs, motorized by myosin XI-k/1/2, traffic on F-actin, transiently interact with other organelles, and deliver a diverse cargo to PD.

Actin Isovariant ACT7 Modulates Root Thermomorphogenesis by Altering Intracellular Auxin Homeostasis.

High temperature stress is one of the most threatening abiotic stresses for plants limiting the crop productivity world-wide. Altered developmental responses of plants to moderate-high temperature has been shown to be linked to the intracellular auxin homeostasis regulated by both auxin biosynthesis and transport. Trafficking of the auxin carrier proteins plays a major role in maintaining the cellular auxin homeostasis. The intracellular trafficking largely relies on the cytoskeletal component, actin, which provides track for vesicle movement. Different classes of actin and the isovariants function in regulating various stages of plant development. Although high temperature alters the intracellular trafficking, the role of actin in this process remains obscure. Using isovariant specific vegetative class actin mutants, here we demonstrate that ACTIN 7 (ACT7) isovariant plays an important role in regulating the moderate-high temperature response in Arabidopsis root. Loss of ACT7, but not ACT8 resulted in increased inhibition of root elongation under prolonged moderate-high temperature. Consistently, kinematic analysis revealed a drastic reduction in cell production rate and cell elongation in act7-4 mutant under high temperature. Quantification of actin dynamicity reveals that prolonged moderate-high temperature modulates bundling along with orientation and parallelness of filamentous actin in act7-4 mutant. The hypersensitive response of act7-4 mutant was found to be linked to the altered intracellular auxin distribution, resulted from the reduced abundance of PIN-FORMED PIN1 and PIN2 efflux carriers. Collectively, these results suggest that vegetative class actin isovariant, ACT7 modulates the long-term moderate-high temperature response in Arabidopsis root.