A conserved module in the formation of moss midribs and seed plant axillary meristems.

Different evolutionary lineages have evolved distinct characteristic body plans and anatomical structures, but their origins are largely elusive. For example, seed plants evolve axillary meristems to enable lateral branching. In moss, the phyllid (leaf) midrib containing specialized cells is responsible for water conduction and support. Midribs function like vascular tissues in flowering plants but may have risen from a different evolutionary path. Here, we demonstrate that midrib formation in the model moss Physcomitrium patens is regulated by orthologs of Arabidopsis LATERAL SUPPRESSOR (LAS), a key regulator of axillary meristem initiation. Midribs are missing in loss-of-function mutants, and ectopic formation of midrib-like structures is induced in overexpression lines. Furthermore, the PpLAS/AtLAS genes have conserved functions in the promotion of cell division in both lineages, which alleviates phenotypes in both Physcomitrium and Arabidopsis las mutants. Our results show that a conserved regulatory module is reused in divergent developmental programs, water-conducting and supporting tissues in moss, and axillary meristem initiation in seed plants.

SAV4 is required for ethylene-induced root hair growth through stabilizing PIN2 auxin transporter in Arabidopsis.

Root hair development is regulated by hormonal and environmental cues, such as ethylene and low phosphate. Auxin efflux carrier PIN2 (PIN-FORMED 2) plays an important role in establishing a proper auxin gradient in root tips, which is required for root hair development. Ethylene promotes root hair development through increasing PIN2 abundance in root tips, which subsequently leads to enhanced expression of auxin reporter genes. However, how PIN2 is regulated remains obscure. Here, we report that Arabidopsis thaliana sav4 (shade avoidance 4) mutant exhibits defects in ethylene-induced root hair development and in establishing a proper auxin gradient in root tips. Ethylene treatment increased SAV4 abundance in root tips. SAV4 and PIN2 co-localize to the shootward plasma membrane (PM) of root tip epidermal cells. SAV4 directly interacts with the PIN2 hydrophilic region (PIN2HL) and regulates PIN2 abundance on the PM. Vacuolar degradation of PIN2 is suppressed by ethylene, which was weakened in sav4 mutant. Furthermore, SAV4 affects the formation of PIN2 clusters and its lateral diffusion on the PM. In summary, we identified SAV4 as a novel regulator of PIN2 that enhances PIN2 membrane clustering and stability through direct protein-protein interactions. Our study revealed a new layer of regulation on PIN2 dynamics.

SHADE AVOIDANCE 4 Is Required for Proper Auxin Distribution in the Hypocotyl.

The phytohormone auxin is involved in virtually every aspect of plant growth and development. Through polar auxin transport, auxin gradients can be established, which then direct plant differentiation and growth. Shade avoidance responses are well-known processes that require polar auxin transport. In this study, we have identified a mutant, shade avoidance 4 (sav4), defective in shade-induced hypocotyl elongation and basipetal auxin transport. SAV4 encodes an unknown protein with armadillo repeat- and tetratricopeptide repeat-like domains known to provide protein-protein interaction surfaces. C terminally yellow fluorescent protein-tagged SAV4 localizes to both the plasma membrane and the nucleus. Membrane-localized SAV4 displays a polar association with the shootward plasma membrane domain in hypocotyl and root cells, which appears to be necessary for its function in hypocotyl elongation. Cotransfection of SAV4 and ATP-binding cassette B1 (ABCB1) auxin transporter in tobacco (Nicotiana benthamiana) revealed that SAV4 blocks ABCB1-mediated auxin efflux. We thus propose that polarly localized SAV4 acts to inhibit ABCB-mediated auxin efflux toward shoots and facilitates the establishment of proper auxin gradients.

Complete Nucleotide Sequence of Canine Distemper Virus HLJ1-06, Isolated from Foxes in China.

A new strain of canine distemper virus, HLJ1-06, has been isolated from foxes in China, and its complete genome has been sequenced and analyzed. The phylogenetic analysis suggests that HLJ1-06 belongs to the Asia-1 cluster and has low identity to the vaccine strain.