New observations on gametogenic development and re

Authors: Rigel SalinasGamboa Susan D Johnson Nidia SánchezLeón Anna M G Koltunow JeanPhilippe VielleCalzada

Publish Date: 2016/01/04

Volume: 29, Issue: 1-2, Pages: 165-177

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Abstract

Vigna unguiculata L Walp cowpea is recognized as a major legume food crop in Africa but seed yields remain low in most varieties adapted to local conditions The development of hybrid cowpea seed that could be saved after each generation enabling significant yield increases will require manipulation of reproductive development from a sexual to an asexual mode To develop new technologies that could support the biotechnological manipulation of reproductive development in cowpea we examined gametogenesis and seed formation in two transformable Africanadapted daylengthinsensitive varieties Here we show that these two varieties exhibit distinct morphological and phenological traits but share a common developmental sequence in terms of ovule formation and gametogenesis We present a reproductive calendar that allows prediction of male and female gametogenesis on the basis of sporophytic parameters related to floral bud size and reproductive organ development determining that gametogenesis occurs more rapidly in the anther than in the ovule We also show that the mode of megagametogenesis is of the Polygonumtype and not Oenotheratype as previously reported Finally we developed a wholemount immunolocalization protocol and applied it to detect meiotic proteins in the cowpea megaspore mother cell opening opportunities for comparing the dynamics of protein localization during male and female meiosis as well as other reproductive events in this emerging legume model systemMany of the poorest countries in the world derive 10–20  of their total dietary protein from grain legumes members of the Fabaceae Akibode and Maredia 2011 Cowpea Vigna unguiculata L Walp is one of the eight grain legumes currently being targeted for yield and agronomic improvement by the Consultative Group for International Agricultural Research CGIAR Cowpea originated in Africa which produces over 95  of the 54 million tons of dried cowpeas produced globally on an estimated 115 million hectares of land across a diverse range of farming systems in the semiarid regions of West and Central Africa Approximately 200 million individuals are estimated to consume the grain daily in West Africa as it provides a high source of protein 25  by weight and all plant organs are edible Cowpea is an important component in increasing sustainable farming production by being drought resistant and having the ability to fix atmospheric nitrogen contributing to reduced soil erosionConventional cowpea breeding is currently underpinned by the use of molecular markers Lucas et al 2011 2013 correlations between genetic and physical maps Pottorff et al 2014 and largescale genomic sequencing Andargie et al 2011 BarreraFigueroa et al 2011 Huang et al 2012 Transformation technologies have been established for cowpea and are being implemented to accelerate agronomic performance with strong indications of success in the development of seed borerresistant lines Kang et al 2014 Popelka et al 2006 Despite advances in increasing cowpea yields and stress tolerance over the last two decades many varieties are still low yielding and susceptible to a range of biotic and abiotic stresses Lush and Evans 1981 Singh 2014 Huynh et al 2015 Lucas et al 2015The development of cowpea hybrids has the potential to increase crop yields by 30  as suggested by published studies aimed at identifying levels of heterosis in progeny from crosses between inbred cowpea parents Bhaskaraiah et al 1980 Ortiz 1998 Ushakumari et al 2010 Kumari et al 2012 Hybrid seed production systems have not yet been developed in cowpea Impediments include selffertilizing hermaphrodite flowers low outcrossing rates 5  Timko and Singh 2008 and high flower drop rates Wien and Summerfield 1980 Ehlers and Hall 1996 Ojehomon and Samyaolu 1970 In all current hybrid seed production systems hybrid seeds need to be generated and purchased each growing season impeding their purchase by poor farmers The current inability to produce hybrid seeds relates to the plant’s sexual reproductive process Meiosis and recombination during gamete formation and gamete fusion at fertilization alleviate heterosis concurrently leading to trait segregation Development of cowpea hybrids from which seeds could be saved by farmers would offer the potential to generate higheryielding cowpea crops in an economical manner The generation of such “selfreproducing” hybrids would require changing reproduction in the sexually reproducing hybrid to an asexual seed forming mode mimicking the events of apomixis to generate clonal seed where vigor was preserved Recent studies have provided proofofconcept of clonal seed generation which might be applicable for the generation of selfreproducing cowpea hybrids Marimuthu et al 2011 Hand and Koltunow 2014 Gilbert 2015

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