Delivering resistance to a major constraint for rain-fed rice production

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At a time when we are considering new ways to increase food security in developing countries, and not least sub-Saharan Africa, we have important research that will be of immense value, specifically for small-scale farmer production of upland or rain-fed nonirrigated rice. These studies will lead to increased sustainability by reducing external inputs because the traits being developed are for crop protection against parasitic weeds (Scholes & Press, 2008) and will be delivered via the seed. For the first time, we see a rigorous investigation of upland rice cultivars in terms of post-attachment resistance to the parasitic weeds, Striga hermonthica and S. asiatica (Cissoko et al., pp. 952–963) and pre-attachment resistance to the parasitic weed, S. hermonthica (Jamil et al., pp. 964–975) in this issue of New Phytologist. The New Rice for Africa (NERICA) rice cultivars and some of their ancestry have been chosen because of their importance in Africa for upland cultivation, and it is the major constraints on yield, the parasitic weeds known also as witchweeds, S. hermonthica and S. asiatica, to which the work is directed (Fig. 1).

Figure 1.

Rain-fed rice (NERICA) in Kenya highly infested with the parasitic weed Striga hermonthica. Courtesy of Professor Zeyaur Khan, International Centre of Insect Physiology and Ecology (ICIPE).

‘… because this new type of weed resistance will be delivered via the seed, the approach is sustainable and could be managed even via farmer collected seed …’

Understanding and exploiting differences in post-attachment to rice by parasitic weeds

Cissoko et al. demonstrate pronounced differences in post-attachment resistance to the two parasite species in the NERICA cultivars and their ancestors, some (CG14, NERICA 1 and 10) showing substantial resistance effects. This is the primary discovery made in this paper and will serve further investigations into the molecular mechanisms underpinning this type of resistance and will, even while such studies take place, provide the basis for new breeding programmes. However, the study continues to investigate the phenotype of post-attachment Striga hermonthica and S. asiatica resistance in the NERICA cultivars. Thus necrosis in the host, at the site of attempted attachment, is observed when parasites fail to develop. This has been observed before with the Oryza sativa cultivar, Nipponbare infected by S. hermonthica and also with sorghum cultivars infected by S. asiatica. However, most interesting is the similarity with resistance in cowpea cultivars to S. gesnerioides reported by Timko in 2009 (Li & Timko, 2009), the significance being that in the cowpea–S. gesnerioides system, this phenotype was linked to the classical gene-for-gene resistance mechanism further elaborated in the Timko reference and relating to previous work by some of this team (Gurney et al., 2006). This will provide further opportunities and impetus for determining the specific mechanisms for the resistance traits discovered here. By studying the two parasitic species S. hermonthica and S. asiatica, there was an indication that the resistance discovered here is a relatively broad spectrum and effective against a number of ecotypes of S. hermonthica and S. asiatica. The final section of the work deals with the issue of damage caused by those parasites that develop even on the resistant plants, it having been demonstrated clearly at the outset that although highly resistant cultivars are identified here, none of the resistance was complete. However, these final studies show clearly that the biomass of the host improved in resistant compared with susceptible cultivars.

The authors refer variously to the need for the types of resistant traits discovered here being used in an integrated programme with other forms of resistance. In this respect they specifically recommend integration with pre-attachment resistance and refer to such a study: Jamil et al.

Low strigolactone production lowers pre-attachment to rice of parasitic weeds

The work by Jamil et al. demonstrates that levels, from root exudation, of the germination stimulants, strigolactones, by this cereal negatively correlate with resistance at this stage of parasite development, that is, the lower strigolactone levels exuded by rice roots the greater is the resistance because of the role of these compounds as germination stimulants for Striga hermonthica (Matusova et al., 2005). Although critics would immediately respond by pointing out that the strigolactones play an important role in the plants’ regulation of rhizosphere interactions (Lopez-Raez et al., 2008), the authors acknowledge this and clearly demonstrate that some structural types within the strigolactones are more stimulatory to S. hermonthica than others. Thus, they suggest selective reduction in the amount of, for example, the methoxy-5-deoxystrigol isomers 2 and 3 could raise resistance to S. hermonthica whereas other strigolactones such as 5-deoxystrigol and orobanchol could be allowed to remain for the benefit of rhizosphere interactions (Fig. 2). This would avoid creating NERICA cultivars resistant to S. hermonthica that would lack positive effects on arbuscular mycorrhizal fungi (AMF), which indeed is now well understood (Akiyama et al., 2010) as are other advantageous effects known as a consequence of pioneering work by the senior author of this paper, H. J. Bouwmeester, (Kohlen et al., 2011). In addition, they also report a negative correlation between tillering, and the strigolactone exudation level and S. hermonthica infestation, thereby suggesting that higher tillering cultivars will have greater pre-attachment resistance.

Figure 2.

The structure of strigolactones may provide selectivity between high Striga germination stimulation (left) and promotion of arbuscular mycorrhizal association (right).

The work is entirely laboratory based, but the authors prominently acknowledge the need for field testing and even extend to providing valuable advice for such work to be done in extension of the discoveries made here. The authors also explain that, where using a reduction in certain strigolactones for pre-attachment resistance to Striga hermonthica, this must be done in integration with other approaches. In this they specifically stipulate the potential value for integrating with the use of cultivars showing post-germination resistance and indeed Jamil et al. appears together with another paper dealing with post-attachment resistance and to which they refer: Cissoko et al.

In conclusion, as a consequence of the two studies cited initially here (Cissoko et al. and Jamil et al.), we have discoveries that present a real breakthrough in defining breeding opportunities for new rain-fed rice varieties, particularly for resource poor farming systems, to overcome constraints on yield by parasitic weeds. To date only cultural approaches to controlling these weeds are compatible with the low cost inputs available to the bulk of the affected farming systems (Pickett et al., 2010). However, because this new type of weed resistance will be delivered via the seed, the approach is sustainable and could be managed even via farmer collected seed thereby avoiding subsequent seasonal inputs – a lesson also for Northern agriculture?

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