Drought has been a major cause of agricultural disaster, yet how it affects the vulnerability of maize and wheat production in combination with several co-varying factors (i.e., phenological phases, agro-climatic regions, soil texture) remains unclear. Using a data synthesis approach, this study aims to better characterize the effects of those co-varying factors with drought and to provide critical information on minimizing yield loss. We collected data from peer-reviewed publications between 1980 and 2015 which examined maize and wheat yield responses to drought using field experiments. We performed unweighted analysis using the log response ratio to calculate the bootstrapped confidence limits of yield responses and calculated drought sensitivities with regards to those co-varying factors. Our results showed that yield reduction varied with species, with wheat having lower yield reduction (20.6%) compared to maize (39.3%) at approximately 40% water reduction. Maize was also more sensitive to drought than wheat, particularly during reproductive phase and equally sensitive in the dryland and non-dryland regions. While no yield difference was observed among regions or different soil texture, wheat cultivation in the dryland was more prone to yield loss than in the non-dryland region. Informed by these results, we discuss potential causes and possible approaches that may minimize drought impacts.

Global Synthesis of Drought Effects on Maize and Wheat Production

A revision of the 45 species of the pantropical genus Xylopia in Tropical Africa includes descriptions of six new species and a new section of the genus. The fruits and seeds of Xylopia show specializations that promote vertebrate dispersal, primarily by hornbills and monkeys. Over half of the African species have an Area of Occupancy (AOO) less than 80 km2, suggesting that they are in need of protection. African species are classified into five sections. Section Neoxylopia, with four species, is centered in the Guineo-Congolian Region and includes X. globosa sp. nov. Section Ancistropetala, with three species, occurs in the same region. Both of these sections are endemic to Africa. Section Xylopia, which extends to Madagascar and the American tropics, has only a single species in Africa, X. aethiopica. The three species of section Verdcourtia sect. nov. are restricted to the East African coast and Madagascar. The largest number of African species, (34) belong to section Stenoxylopia, in which the seeds lack the arils found in the other sections and instead have a fleshy sarcotesta. Section Stenoxylopia is divided into two informal groups, one centered in eastern and southern Africa (X. odoratissima group) and the other centered in the wetter forests of western and central Africa (X. acutiflora group). Five new species are described in section Stenoxylopia: Xylopia nilotica sp. nov. from Sudan, South Sudan, and Uganda, Xylopia calva sp. nov. from Nigeria and Cameroon, which is allied to X. phloiodora, and Xylopia monticola sp. nov. from Nigeria and Cameroon, X. piratae sp. nov. from Ivory Coast and Ghana, and X. unguiculata sp. nov. from Gabon. The latter three species are segregates of the former Xylopia acutiflora s. l. One new combination is made at the species level, X. shirensis comb. nov. Keys, descriptions, illustrations, distribution maps, and an index to numbered collections document diversity and assist with species identification. The name Unona oliveriana Baill. was found to pre-date the name Unona lepidota Oliv., requiring the combination Meiocarpidium oliverianum comb. nov. PhytoKeys 97: 1–252 (2018) doi: 10.3897/phytokeys.97.20975 http://phytokeys.pensoft.net Copyright David M. Johnson, Nancy A. Murray. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. MONOGRAPH Launched to accelerate biodiversity research A peer-reviewed open-access journal

A revision of Xylopia L. (Annonaceae): the species of Tropical Africa

AIM: Long‐distance dispersal has contributed to the disjunct biogeographical distribution of rain forest plants—something that has fascinated biogeographers since Humboldt's time. However, the dispersal agent for these tropical plant lineages remains puzzling. Here, we investigate which frugivory‐related traits may have facilitated past intercontinental long‐distance dispersal in the custard apple family (Annonaceae), a major vertebrate‐dispersed tropical plant family. We hypothesize that long‐distance dispersal was associated with the evolution of traits related to dispersal by large‐bodied mammals (e.g., large, dull‐coloured, “megafaunal” fruits) and strong‐flying, ocean‐crossing birds and bats (e.g., dehiscent, moniliform or cauliflorous fruits). LOCATION: Global. TAXON: Annonaceae. METHODS: We used a fossil‐calibrated phylogenetic framework to infer the biogeographic history of 234 Annonaceae species (10%, covering nearly all genera) in relation to the evolution of 15 frugivory‐related traits, using maximum likelihood and Bayesian inferences. Furthermore, we used linear and generalized linear models and phylogenetic simulations to test whether ancestral fruit traits during intercontinental dispersal were different from those of other lineages not involved in long‐distance dispersal. RESULTS: We inferred the ancestral Annonaceae fruits to be small with a single or few small seeds and a small number of fruitlets. These fruits were most probably apocarpous, indehiscent and/or moniliform (i.e., long beads of fruitlets). Furthermore, most of the long‐distance dispersal events in Annonaceae occurred via the expanded tropical forests in the Early Cenozoic (“geodispersal”), and were significantly associated with large (c. 3 cm long), dull‐coloured fruits and short stipes. Additionally, long‐distance dispersal was also facilitated by dehiscent, moniliform and non‐cauliflorous fruits. MAIN CONCLUSIONS: We suggest that the evolution of frugivory‐related traits associated with dispersal by frugivores that frequently move across large distances and/or barriers, such as large‐bodied mammals and strong‐flying birds, has contributed to the disjunct tropical biogeographical distribution of Annonaceae, and probably of tropical rain forest plants more generally.

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Which frugivory‐related traits facilitated historical long‐distance dispersal in the custard apple family (Annonaceae)?

Aim
The historical biogeography of the species-rich flowering plant genus Goniothalamus (c. 130 species) and the wider Annonaceae tribe Annoneae is investigated to evaluate hypotheses regarding the processes underlying tropical intercontinental disjunctions (‘long-distance dispersal [LDD]’ versus ‘boreotropics’) and floristic exchange between biodiversity hotspots in tropical Asia.

Location
Tropics, tropical Asia.

Methods
Divergence time estimation was based on plastid DNA sequence data (c. 10 kb; 164 Annonaceae accessions, including 65 Goniothalamus species) using an uncorrelated lognormal relaxed clock model and two fossil calibrations. Likelihood ancestral range estimation was performed using dispersal-extinction-cladogenesis models with and without time-stratified dispersal constraints.

Results
The boreotropics model incorporating high chances for intercontinental dispersal during 68–34 Ma fitted the data significantly better than LDD models enforcing low intercontinental dispersal rates. Multiple vicariance events were identified in Annoneae including at the Annoneae crown node (57–49 Ma, Africa/S+N America), and the Disepalum-Asimina split (27–24 Ma, Asia/N America). A wide ancestral range in continental SE Asia, W and E Malesia was inferred at the Goniothalamus crown node (24–21 Ma). Several dispersal events from continental SE Asia/W Malesia were identified in the Miocene: two north-eastwards to the Philippines, one eastwards to the Sahul Shelf and two westwards to India.

Main conclusions
The boreotropics hypothesis provides a well-supported, plausible explanation for the disjunctions and dispersal–vicariance patterns in tribe Annoneae. The wide distribution inferred at the Goniothalamus crown node indicates that its ancestors already bridged the marine gap between the Sunda and Sahul Shelves by LDD or rafting on tectonic plate microfragments. Dispersal across long-standing water bodies in Malesia occurred multiple times from the early Miocene as marine gaps decreased following tectonic plate convergence and land emergence. Substantially older crown group divergence estimates compared with previous studies indicate that hypotheses of rapid recent radiation in Goniothalamus must be rejected.

Historical biogeography of Goniothalamus and Annonaceae tribe Annoneae: dispersal–vicariance patterns in tropical Asia and intercontinental tropical disjunctions revisited

This paper provides a linear sequence of four subfamilies, 15 tribes and 106 genera of the magnoliid family Annonaceae, based on state-of-the-art and stable phylogenetic relationships. The linear sequence facilitates the organisation of Annonaceae herbarium specimens.

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A linear sequence to facilitate curation of herbarium specimens of Annonaceae

The floristic treatment of Engler and Diels, published in 1901, provides the only infrageneric classification of the pantropical genus Xylopia (Annonaceae) Here we test and extend that classification using molecular and seed morphology characters Phylogenetic relationships were reconstructed using data from four plastid regions obtained from 44 of the approximately 165 species in the genus, recovering four well-supported major clades Seed characters were examined for these taxa, and six aril morphologies, three previously undocumented, were distinguished; we also document the presence of a sarcotesta on the seeds of many species Molecular and seed data support recognition of five sections within the genus; one, Xylopia sect Rugosperma, is proposed here as new Our phylogenetic results suggest an African origin for the genus and reveal complex biogeographic patterns, likely facilitated by long-distance dispersal

https://bioone.org/journals/systematic-botany/volume-42/issue-2/036364417X695484/Plastid-and-Seed-Morphology-Data-Support-a-Revised-Infrageneric-Classification/10.1600/036364417X695484.pdf

Plastid and Seed Morphology Data Support a Revised Infrageneric Classification and an African Origin of the Pantropical Genus Xylopia (Annonaceae)

Root metaxylem vessels are responsible for axial water transport and contribute to hydraulic architecture. Variation in metaxylem vessel size and number can impact drought tolerance in crop plants, including rice, a crop that is particularly sensitive to drought. Identifying and validating candidate genes for metaxylem development would aid breeding efforts for improved varieties for drought tolerance. We identified three transcription factor candidate genes that potentially regulate metaxylem vessel size and number in rice based on orthologous annotations, published expression data, and available root and drought-related QTL data. Single gene knockout mutants were generated for each candidate using CRISPR-Cas9 genome editing. Root metaxylem vessel area and number were analyzed in 6-week-old knockout mutants and wild-type plants under well-watered and drought conditions in the greenhouse. Compared with wild type, LONESOME HIGHWAY (OsLHW) mutants had fewer, smaller metaxylem vessels in shallow roots and more, larger vessels in deep roots in drought conditions, indicating that OsLHW may be a repressor of drought-induced metaxylem plasticity. The AUXIN RESPONSE FACTOR 15 mutants showed fewer but larger metaxylem vessel area in well-watered conditions, but phenotypes were inconsistent under drought treatment. ORYZA SATIVA HOMEBOX 6 (OSH6) mutants had fewer, smaller metaxylem vessels in well-watered conditions with greater effects on xylem number than size. OSH6 mutants had larger shoots and more, deeper roots than the wild type in well-watered conditions, but there were no differences in performance under drought between mutants and wild type. Though these candidate gene mutants did not exhibit large phenotypic effects, the identification and investigation of candidate genes related to metaxylem traits in rice deepen our understanding of metaxylem development and are needed to facilitate incorporation of favorable alleles into breeding populations to improve drought stress tolerance.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pld3.328

Targeted mutation of transcription factor genes alters metaxylem vessel size and number in rice roots.

Root metaxylem are phenotypically diverse structures whose function is related to their anatomy, particularly under drought stress. Much research has dissected the genetic machinery underlying metaxylem phenotypes in dicots, but monocots are relatively unexplored. In maize (Zea mays), a robust pipeline integrated a GWAS of root metaxylem phenes under well-watered and water stress conditions with a gene co-expression network to identify candidate genes most likely to impact metaxylem phenotypes. We identified several promising candidate genes in 14 gene co-expression modules inferred to be functionally relevant to xylem development. We also identified five gene candidates that co-localized in multiple root metaxylem phenes in both well-watered and water stress conditions. Using a rice GWAS conducted in parallel, we detected overlapping genetic architecture influencing root metaxylem phenotypes by identifying eight pairs of syntenic candidate genes significantly associated with metaxylem phenes. There is evidence that the genes of these syntenic pairs may be involved in biosynthetic processes related to the cell wall, hormone signaling, oxidative stress responses, and drought responses. Our study demonstrates a powerful new strategy for identifying promising gene candidates and suggests several gene candidates that may enhance our understanding of vascular development and responses to drought in cereals. One sentence summary Cross-species genome-wide association studies and a gene coexpression network identified genes associated with root metaxylem phenotypes in maize under water stress and non-stress and rice.

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Jenna E. Reeger

Papers

Plastid and Seed Morphology Data Support a Revised Infrageneric Classification and an African Origin of the Pantropical Genus Xylopia (Annonaceae)

Targeted mutation of transcription factor genes alters metaxylem vessel size and number in rice roots.

Shared genetic architecture underlying root metaxylem phenotypes under drought stress in cereals