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The Arp2/3 complex is activated at different cellular locations by different nucleation promoting factors (NPFs), WAVE at lamellipo dia, neural WASP (N-WASP) at clathrin-coated pits, and WASP and SCAR homologue [...], Cell migration requires the generation of branched actin networks that power the protrusion of the plasma membrane in lamellipodia (1,2). The actin-related proteins 2 and 3 (Arp2/3) complex is the molecular machine that nucleates these branched actin networks (3). This machine is activated at the leading edge of migrating cells by Wiskott-Aldrich syndrome protein (WASP)-family verprolin-homologous protein (WAVE, also known as SCAR). The WAVE complex is itself directly activated by the small GTPase Rac, which induces lamellipodia (4-6). However, how cells regulate the directionality of migration is poorly understood. Here we identify a new protein, Arpin, that inhibits the Arp2/3 complex invitro, and show that Rac signalling recruits and activates Arpin at the lamellipodial tip, like WAVE. Consistently, after depletion of the inhibitory Arpin, lamellipodia protrude faster and cells migrate faster. A major role of this inhibitory circuit, however, is to control directional persistence of migration. Indeed, Arpin depletion in both mammalian cells and Dicttyostelium discoideum amoeba resulted in straighter trajectories, whereas Arpin microinjection in fish keratocytes, one of the most persistent systems of cell migration, induced these cells to turn. The coexistence of the Rac-Arpin-Arp2/3 inhibitory circuit with the Rac-WAVE-Arp2/3 activatory circuit can account for this conserved role of Arpin in steering cell migration.

The distribution of dryland trees and their density, cover, size, mass and carbon content are not well known at sub-continental to continental scales 1-14 . This information is important for ecological protection, carbon accounting, climate mitigation and restoration efforts of dryland ecosystems 15-18 . We assessed more than 9.9 billion trees derived from more than 300,000 satellite images, covering semi-arid sub-Saharan Africa north of the Equator. We attributed wood, foliage and root carbon to every tree in the 0-1,000 mm year -1 rainfall zone by coupling field data 19 , machine learning 20-22 , satellite data and high-performance computing. Average carbon stocks of individual trees ranged from 0.54 Mg C ha -1 and 63 kg C tree -1 in the arid zone to 3.7 Mg C ha -1 and 98 kg tree -1 in the sub-humid zone. Overall, we estimated the total carbon for our study area to be 0.84 (±19.8%) Pg C. Comparisons with 14 previous TRENDY numerical simulation studies 23 for our area found that the density and carbon stocks of scattered trees have been underestimated by three models and overestimated by 11 models, respectively. This benchmarking can help understand the carbon cycle and address concerns about land degradation 24-29 . We make available a linked database of wood mass, foliage mass, root mass and carbon stock of each tree for scientists, policymakers, dryland-restoration practitioners and farmers, who can use it to estimate farmland tree carbon stocks from tablets or laptops., (© 2023. This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.)

A large proportion of dryland trees and shrubs (hereafter referred to collectively as trees) grow in isolation, without canopy closure. These non-forest trees have a crucial role in biodiversity, and provide ecosystem services such as carbon storage, food resources and shelter for humans and animals 1,2 . However, most public interest relating to trees is devoted to forests, and trees outside of forests are not well-documented 3 . Here we map the crown size of each tree more than 3 m 2 in size over a land area that spans 1.3 million km 2 in the West African Sahara, Sahel and sub-humid zone, using submetre-resolution satellite imagery and deep learning 4 . We detected over 1.8 billion individual trees (13.4 trees per hectare), with a median crown size of 12 m 2 , along a rainfall gradient from 0 to 1,000 mm per year. The canopy cover increases from 0.1% (0.7 trees per hectare) in hyper-arid areas, through 1.6% (9.9 trees per hectare) in arid and 5.6% (30.1 trees per hectare) in semi-arid zones, to 13.3% (47 trees per hectare) in sub-humid areas. Although the overall canopy cover is low, the relatively high density of isolated trees challenges prevailing narratives about dryland desertification 5-7 , and even the desert shows a surprisingly high tree density. Our assessment suggests a way to monitor trees outside of forests globally, and to explore their role in mitigating degradation, climate change and poverty.