Root research for drought tolerance in legumes: Quo vadis?

Among many factors that are associated with drought tolerance in legume crops, root traits have been considered to be the most important attributes enabling the plant to mine water efficiently from deeper soil layer under dry environments. Most of the methods used to evaluate roots are time consuming that provide valuable information about the root morphology but they do not reflect the dynamic characteristics of roots and root systems. Considerable amount of genetic variability with respect to root traits involving length, dry weight and root length density (RLD) has been observed. Apart from the observed morphological variation in roots which has specific significance of adaptation, their functional aspects involving direct water uptake and their related kinetics are equally important. A large lysimetric system has been developed at ICRISAT to make progress in this direction. Compared to other legumes, root traits in chickpea have been thoroughly investigated and consequently, preliminary breeding works have been initiated using root traits. Root measurements in plants grown in cylinders showed almost similar relationship with depth and RLD determined in the field and thus have been used to explore diversity for these traits in chickpea. The possible diversity for root traits has been analysed in wild relatives and transgenics as well. Using this method, root depth and RLD are being phenotyped in several recombinant inbred line populations. A major putative quantitative trait loci (QTL) for RLD was identified in a population involving a profuse rooting parent ICC 4958 and the contrasting Annigeri. These QTLs will be a faster and easier replacement technique against time-consuming phenotyping of roots and will be the way forward to introgress superior root characteristics. Efforts are also being made to use wild relatives of Cicer for inheriting some adaptive traits. In chickpea, a preliminary study is being carried out to test the response to drought stress of DREB1A and P5CSF transgenic events using a lysimetric system. The study demonstrated a modest increased in water uptake but not yield under water stress in one transgenic event of DREB1A and one P5CSF event. In groundnut, DREB1A triggers native genes that might be involved in root development. In this review, the progress made so far on roots in legume crops has been elucidated which might explore possibilities of breeding genotypes to inherit efficient root system in legumes.