Posts Tagged ‘maize’

Maize streak virus: the early history

30 March, 2015

The history of maize streak virus research is generally taken as starting in 1901, with the publication of the

The cover of the "Fuller Report"

The cover of the “Fuller Report”

by “Claude Fuller, Entomologist”. However, in the Report he does make reference to articles in the “Agricultural Journal” for August 3rd and 31st, 1900, and quotes personal sources as having noticed the disease of “mealie variegation” as early as the 1870s.  He comments that:

“…mealie growers…have been acquainted with variegated mealies…for at least 20 years…”, and “…Thomas Kirkman…has known the disorder for 30 years past…”.

His conclusions, although carefully arrived at, were very wrong. Fuller claimed the disease was due to soil deficiency or a “chemical enzyme” in soils, and could be combatted by intensive cultivation and “chemical manures”. However, his carefully-written account is still of great historical interest, and the observations are valuable as they are objective accounts of a skilled scientist.  The records of streaked grasses in particular are useful, as we still collect such samples to this day.  Fuller was later sadly a victim of one the first traffic accidents in what was then Lourenco Marques in Mozambique.

Streak symptoms in a maize leaf

The disease – now known as maize streak disease (MSD) – occurs only in Africa and adjacent Indian Ocean islands, where it is one of the worst occurring in maize.  The causal agent was discovered to be a virus by HH Storey in 1932, who termed it maize streak virus (MSV). The virus was found to be obligately transmitted by the leafhopper Cicadulina mbila, also by Storey, in 1928. In 1978, MSV was designated the type virus of the newly described group taxon Geminivirus.

Early studies indicated that there were several distinctly different African streak viruses adapted to different host ranges (Storey & McClean, 1930; McClean, 1947). These studies were based on the transmission of virus isolates between different host species and symptomatology.

In a subsequent study of streak virus transmission between maize, sugarcane, and Panicum maximum, the relatively new technique of immunodiffusion was employed, using antiserum to the maize isolate. From the results it was concluded that the maize, sugarcane, and Panicum isolates were strains of the same virus, MSV (Bock et al., 1974). The maize isolate was given as the type strain. The virus was only properly physically characterised in 1974, when the characteristic geminate or doubled particles were first seen by electron microscopy, and only found to be a single-stranded circular DNA virus in 1977 (Harrison et al., 1977).

Maize streak virus: photo from Robert G Milne in 1978

Maize streak virus: photo from Robert G Milne in 1978

The first isolates of MSV were sequenced in 1984 (Kenya, S Howell, 1984; Nigeria, P Mullineaux et al., 1984), and the virus was found to have a single component of single-stranded circular DNA (sscDNA), and to be about 2700 bases in size. The two isolates were about 98% identical in sequence. The second team took delight in noting that the first sequence was in fact of the complementary and not the virion strand.

A major advance in the field occurred in 1987, when Nigel Grimsley et al. showed that a tandem dimer clone of MSV-N in an Agrobacterium tumefaciens Ti plasmid-derived cloning vector, was infectious when the bacterium was injected into maize seedlings. Subsequently, Sondra Lazarowitz (1988) obtained the sequence of an infectious clone of a South African isolate (from Potchefstroom) – MSV-SA – and showed that it also shared about 98% identity with the first two sequences.

Since the early days other transmission tests and more sophisticated serological assays were performed on a wide range of streak isolates from different hosts and locales, and it was claimed that all forms of streak disease in the Gramineae in Africa were caused by strains of the same virus, MSV. This view changed as more and more viruses were characterised, however, and it became obvious that there were distinctly separate groupings of viruses that constituted different species: these were sugarcane streak viruses (SSV, see Hughes et al., 1993), the panicum streak viruses (PanSV, see Briddon et al., 1992), and the maize streak viruses. Together these viruses constituted an African streak virus group (see Hughes et al., 1992; Rybicki and Hughes, 1990), distinct from an Australasian striate mosaic virus group, and other more distantly related viruses (see here for the state of the art in 1997).  These studies together with a later one by Rybicki et al. in 1998 also pointed up the utility of the polymerase chain reaction (PCR) for amplification, detection and subsequent sequencing of DNA from diverse mastreviruses.

A more modern and comprehensive account can also be found here, in a recent review written for Molecular Plant Pathology.

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GMOs: poisons that will kill our children, or harmless foods?

29 October, 2013

I think I hung my colours out long ago in this “controversy”, but let us just be clear:

I DON’T BELIEVE ANY OF THE GMOs CURRENTLY BEING FARMED WORLDWIDE POSE ANY THREAT TO HUMANS OR STOCK ANIMALS AT ALL.  NONE!  NOR DO MOST BIOLOGICAL SCIENTISTS WHO ACTUALLY UNDERSTAND WHAT GENETIC MANIPULATION OF PLANTS ENTAILS.

Is that clear enough?  No ambiguity there?  Good!  Because the people who have taken poor Fair Lady magazine to task recently, mainly on their Facebook page, for daring to publish an article saying the same thing, would have you believe otherwise.  By relentless recycling of discredited animal feeding studies, reiteration of untruths, canards and plain lies, and by personal attacks on anyone expressing an alternate view.

Title page of the article

Title page of the article

I don’t think that Fair Lady will complain if I reproduce the title page, because I think their article is a reasoned, well written and factual exploration of the topic – which is a LOT more than I can say for most of the comments about the article.  Which includes gems like this:

“Oh dear Fairlady Magazine has made a BIG mistake!!!! Writing an article like this could put them out of business. I will never buy a Fairlady Magazine again and neither will any of my family. Stick to fashion Fairlady. Let Farmers Weekly publish an articles on GMO’s!!!! GMO’s are killing people. It’s not an exaggeration. It is proven, published, peer-reviewed fact. How many people do you know with cancer? Can you count on one hand or two. Ask yourself why. Maybe you could ask well-Informed People who are Fully Aware of the Irreversible Damage unleashed by Toxic GMOs on Earth.”

Now the problems that I have with the kinds of attacks on GMOs that are exemplified by these responses, are the following: these are the assertions that

  1. EVERYTHING is Monsanto’s fault
  2. ALL GMOs are toxic / poisonous
  3. There is ample evidence of harm to both animals and humans

All three of these straw men are, of course, rich in taurine excreta.  In the first place, while Monsanto may well have started the ball rolling on a big scale, and owns patents and seed rights on much of the early and simple one-trait GMOs, they do NOT own everything, and are NOT responsible for many of the recent developments still coming down the developmental pipeline – which are considerably more sophisticated than the ubiquitous herbicide-resistant or Bt-producing maize or cotton.  These would include plants resistant to various viruses, bacteria and fungi, plants engineered for higher nutrient / vitamin content (eg: Golden Rice and golden bananas), and drought- and salt-tolerant plants.

As for toxicity, NO GMO can be released if there is convincing evidence of toxicity in animal feeding trials, which HAVE to be conducted for each new “event”, or novel GMO.  I have sat on panels in SA which have assessed applications by seed companies to grow / produce GM crops, and I can tell you that this is a major feature of any application.  Where non-expert people often get confused is the fact that certain crop plants have been engineered to make insect-specific toxins normally produced by the bacterium Bacillus thuringiensis.  These are collectively known as “Bt toxins”, and the ones used as insecticides are specific for narrow ranges of related insects, and most often for lepidopterans – which include moths and butterflies.  Now the larvae of particularly certain species of moths are major agricultural pests, and include agents such as maize stalk borer and the cotton bollworm – and from Wikipedia:

“Spores and crystalline insecticidal proteins produced by B. thuringiensis have been used to control insect pests since the 1920s and are often applied as liquid sprays”.

That’s right: crystalline protein masses extracted from live bacteria and live spores of bacteria used to be sprayed around as pest-control agents.  Everywhere!  Moreover, from Wikipedia again,

“Because of their specificity, these pesticides are regarded as environmentally friendly, with little or no effect on humanswildlifepollinators, and most other beneficial insects and are used in Organic farming“.

Yes, really: actual Bt toxin, and actual spores that can develop into live bacteria, can be used in organic farming.  Now why would anyone have a problem with a technology that LIMITS exposure of the environment to a bacterial toxin, and most especially, to live bacteria, by containing the protein within the plant tissues?  Moreover, the amount of Bt in the edible seeds of maize is minimal, and people don’t eat cotton – so we are left with possible effects on wildlife, and cattle which eat the green parts of the plants.  And no-one has ever  shown any deleterious effects of Bt in GM plants on non-target organisms.  Oh, there was the Pusztai report, which claimed to have shown that snowdrop lectin-containing transgenic potatoes were toxic to mice – but this elicited the following comment:

“The [British] government’s Advisory Committee on Novel Foods and Processes(ACNFP) has dismissed Dr Pusztai’s findings as inconclusive and irrelevant due to serious doubts concerning the design of the study. The particular type of potatoes on which Dr Pusztai conducted his experiments would never have been approved for food use. Indeed, the ACNFP stated that had an application been submitted on the basis of the data collated from this flawed study, it would have undoubtedly been rejected”

A nice exploration of the pervasive effects of bad publicity following publication of bad science was published recently: this was “When bad science makes good headlines: Bt maize and regulatory bans“, in Nature Biotechnology.  These authors state that

“Numerous laboratory toxicity studies and field experiments, as well as years of field observations in countries where Bt maize is cultivated, have provided evidence that the Cry1Ab protein expressed in Bt maize does not cause adverse effects on arthropods outside the order Lepidoptera (butterflies and moths), the group that contains the target pests. Supporting data have been analyzed in reviews and meta-analyses”

Another point of contention is herbicide-resistant plants, which again, have not convincingly been shown to be toxic.  I say convincingly, because anti-GMO activist will immediately quote “the Seralini study” which purportedly showed deleterious effects on lab rats fed transgenic maize producing a protein which detoxifies the herbicide glyphosate as well as the herbicide itself – to which I reply by inviting you to read this rather damning report by the European Food Safety Authority, which opens with the following statement:

“Serious defects in the design and methodology of a paper by Séralini et al. mean it does not meet acceptable scientific standards and there is no need to re-examine previous safety evaluations of genetically modified maize NK603″

Now I will remind everyone that this is an agency which is NOT in Monsanto’s pocket – or anyone else’s – and which upholds high standards in safeguarding the general public.  As do the US Department of Agriculture (USDA) and the Food & Drug Administration of the USA, which also have no problems with GMOs (FDA statement; USDA information).  Here is a another comprehensive refutation of the “evidence of toxicity” of glyphosate-resistant soybeans, an unpublished study that is widely quoted by anti-GM lobby.

As for “ample evidence of harm” – I can only refer you to what we biotechnologists would regard as an authoritative source, which is the journal Nature Biotechnology.  In a recent article on GMOs entitled “How safe does transgenic food need to be?” by Laura DeFrancesco, the author asks the question:

“Why, after transgenic products have been in the human food chain for more than a decade without overt ill effects, do these doubts persist? And will it ever be possible to gather sufficient evidence to ameliorate the concerns of skeptics and the public at large that these products are as safe as any other foodstuff?”

Further on, she says:

“Critics and proponents of genetically modified organisms (GMOs) alike agree that genetically modified foods have failed to produce any untoward health effects, and that the risk to human health from foods contaminated with pathogens is far greater than from GMOs” [my emphasis]

I don’t think I need to belabour the point further: I am hopelessly compromised, in the eyes of some of the more rabid activists, by being a biotechnologist at all, and especially – Gasp!! – BECAUSE MY LAB MAKES GMOs!!!  However, if that makes me more amenable to believe actual evidence-based findings, rather than unsubstantiated media hype, then so be it.

The REAL Top 10 for Plant Viruses

12 January, 2012

NOTE: this has now been published, and is citable, at:

Rybicki, EP (2014).  A Top Ten list for economically important plant viruses.  Archives of Virology January 2015, Volume 160, Issue 1, pp 17-20
http://dx.doi.org/10.1007/s00705-014-2295-9

The new version can also be downloaded here

This blog post has been read over 9 000 times since it was written (in 2012).  One of my most popular B-)

A recent MicrobiologyBytes post reported a slightly older Molecular Plant Pathology paper as giving a “Top Ten” ranking for plant viruses – at least, those of “…perceived importance, scientifically or economically, from the views of the contributors to the journal”.  Specifically, the article authors “…survey[ed] all plant virologists with an association with Molecular Plant Pathology and ask[ed] them to nominate which plant viruses they would place in a ‘Top 10’ based on scientific/economic importance”.  They got “…more than 250 votes from the international community”, and came up with the following list:

(1) Tobacco mosaic virus (TMV),
(2) Tomato spotted wilt virus (TSWV),
(3) Tomato yellow leaf curl virus (TYLCV),
(4) Cucumber mosaic virus (CMV),
(5) Potato virus Y (PVY),
(6) Cauliflower mosaic virus (CaMV),
(7) African cassava mosaic virus (ACMV),
(8) Plum pox virus (PPV),
(9) Brome mosaic virus (BMV) and
(10) Potato virus X (PVX),
with honourable mentions for viruses just missing out on the Top 10, including Citrus tristeza virus (CTV), Barley yellow dwarf virus (BYDV), Potato leafroll virus (PLRV) and Tomato bushy stunt virus (TBSV).

Yes, well.  Um.  Now I have an acquaintance with Molecular Plant Pathology – a recent review and an Editorship on the short-lived MPP Online – as well as knowing 6 of the 12 authors personally and being electronically acquainted with another two, and I was never asked….  And Brome mosaic??  Sweet little virus, and I spent some 7 years working on it (a major part of my Hons, MSc and PhD theses, since you ask), but important??  Cauliflower mosaic, too: great virus; tough as an old boot, supplied one of the most used promoters (35S) for plant expression – but economically important??

Now I am in the position of having worked quite a lot with four of the Top Ten plus alternates (namely, TMV, CMV, BMV and BYDV), and maintain an affection borne of long acquaintance – yet I have a problem with this list, and it is rather fundamental.  You see, I see only ONE virus in the major list – African cassava mosaic begomovirus (ACMV) – that infects and causes severe losses in one of the four major food crops grown on this planet: all the rest, excepting viruses infecting the also-ran potato, are pathogens of fruits, vegetables or horticulturally-important plants.  Or hardly pathogenic at all, as in the case of BMV – and before anyone argues, I probably have the best collection of African (and other) isolates of the virus in the world, and a lot of experience of it in the field.

I wrote this as a response to the MicrobiologyBytes blog post:

Interesting list – but wrong, as many of these things often are. TYLCV more important than the various African cassava geminiviruses?? Nonsense! And where is Maize streak virus – the most important viral pathogen of the most important crop plant in Africa? Where too the rice viruses?? The world’s top food crops are rice, maize, wheat, cassava and bananas – so what about Maize rayado fino virus, Rice dwarf…? Banana bunchy top or banana streak? I can bet the majority of the plant virologists polled (I was not, nor was anyone I know from around these parts) were from the developed world, and the northern hemisphere.

Gary Foster – the last and communicating author – replied:

Not a case of ‘wrong’, more a case of many forms of ‘right’.

in the review we state….’we are very much aware that importance and priorities can vary locally across continents and disciplines.’ But in the review we took a global snapshot.

The idea was to promote discussion, and I knew you would take up the challenge 😉

And again:

“People could vote on either scientific or economic importance. BMV is in Top 10 because of scientific importance as it states in the article….NOT economic.”

So here we go in taking up the challenge…!  First off, I think having a list of viruses where the economic importance ranges from “Major” through “Minor” to “Beneath Notice” is a cop-out, because it elevates scientific curiosities and sentimental favourites to equal or greater perceived importance to plant viruses that can actually lead to people dying.  I wrote in 1999, with my friend and colleague Gerhard Pietersen, a paper entitled “Plant virus disease problems in the developing world” (Rybicki EP, Pietersen, G; Adv Virus Res. 1999;53:127-75).  We took the view that the most important plant viruses in the world were those affecting the major food crops in the developing world specifically, seeing as these would affect the greatest number of people, and would probably be the least well controlled.  Our list, therefore, looks nothing like the one above.

Mrs Pauline Ruiru, on her farm near Githungiri, Kenya, in 1997 – note the devastated maize infected with MSV

In 1999, we wrote the following:

“The Food and Agriculture Organisation (FAO) has defined the major primary food crops (in order of volume grown ) in the developing world to be: (1) rice, (2) wheat, (3) maize, (4) cassava, (5) fresh vegetables, and (6) sweet potatoes. Other crops of major importance are sugarcane, oil palm fruit and soybeans.  The most important crops in the developing world as far as local populations are concerned, however, are bulk foods such as rice, maize, cassava, bananas, and sweet potatoes; vegetables such as beans and pumpkins; and fruits such as mangoes and coconuts”.

So: no tobacco, precious few tomatoes or potatoes, definitely no wheat, precious few things that could be affected by PPV…and 8 of the Top Ten gone, at a stroke.  I’ll allow the ACMD (African cassava mosaic disease) complex [note: NOT ACMV], and CMV, seeing as it infects damn nearly anything, including maize and most vegetables.

Another problem with the list as given above is that “TYLCV” is in fact better represented by a complex of reasonably distantly related geminiviruses which do similar things to tomatoes, in very different geographic areas: thus, we have the original TYLCV, as well as TYLC Sardinia V, and TYLCCNV and TYLCTHV – all separate species.  The supposed “ACMV” is probably neither the best studied nor even the most interesting of the ACMD agents: the East African CMV – ACMV recombinant virus which caused an epiphytotic in Uganda was far more economically important than ACMV, and has been followed in the literature (and in the field) by a host of brethren, all distantly enough related to be separate species (eg: SACMV), but all causing what looks like ACMD.

So what is my Top Ten?  I would not go as far – without researching and writing another review – as ranking them; however, from the basis of considering only viruses with sufficient economic impact to kill people if crops are affected, it would be these – ordered by crop importance.

Rice: the rice tungro disease agents RTBV, a dsDNA badnavirus, and rice tungro spherical virus RTSV, an ssRNA waikavirus, in Asia.  Rice yellow mottle (RYMV) ssRNA sobemovirus in Africa.  Rice hoja blanca virus (RHBV, ssRNA(-) Tenuivirus) in South America.

Wheat: Barley yellow dwarf luteoviruses (BYDV) – again, actually a complex of ssRNA viruses which in fact belong in different species – is almost certainly the worst viral pathogen of wheat worldwide.

A cryoEM image reconstruction of an MSV particle (Kyle Dent, EM Unit, UCT)

Maize: the ssDNA geminiviral pathogen Maize streak mastrevirus (MSV) is the worst viral pathogen of maize in the whole of Africa, where maize is the the most common staple food.  A recent review from our group – in Molecular Plant Pathology, I will note – details the very significant economic impact of the virus, as well as the considerable body of molecular virological research on it.  We wrote in 2009:

“Maize streak disease (MSD) was first recorded in South Africa by Claude Fuller (1901), the Government Entomologist of Natal. Fuller also quoted personal sources who noticed the disease of ‘mealie variegation’, as it was then described, as early as the 1870s. …Over 100 years later, MSD remains the most significant viral disease of Africa’s most important food crop (Bosque-Pérez, 2000), costing between US$120M and US$480M per year according to one conservative estimate based on average annual yield losses of only 6%–10%”.  As losses can be up to 100%, this is almost certainly an underestimate – Ed.

Staying with maize, Maize rayado fino virus (MRFV, ssRNA Marafivirus) is possibly the most important virus in North and especially South America.   The ssRNA potyviruses Maize dwarf mosaic and Sugarcane mosaic viruses are probably the most widespread viruses of maize, having essentially a worldwide distribution, and often being associated with severe disease.

Sweet potato: Sweet potato feathery mottle potyvirus (SPFMV) is probably the worst pathogen affecting this increasingly used crop worldwide, but pathology is exacerbated by co-infection with Sweet potato sunken vein closterovirus (SPSVV).

Main picture: cassava plant showing the effects of severe ACMD. Note lack of leaves, and of neighbouring plants. Insets, top: healthy leaves; middle, mild infection; bottom, severe infection. All photographs by EP Rybicki, taken in western Kenya, June, 1997

Cassava: the Africa-limited ACMD complex of ACMV, EACMV, SACMV and others together constitute a major threat to food security in the continent, especially given an increased use of cassava continent-wide.  As an object example of why I choose to go with the viruses mentioned, it is worth revisiting something I wrote in 1999:

“It is quite remarkable to pass within a few kilometers from areas with mild ACMD to areas where there are almost no cassava plants left growing. The inevitable lag in replacement of the crop by sweet potato, for example, results in severe hardship for farming families accustomed to using it as a staple in their diet. The wave of ACMD across Uganda may be a good example of the devastating effect of a plant virus on the human population.”

Twelve years on, I see no reason to revise the statement.

Bananas: the worst virus affecting bananas worldwide has to Banana bunchy top nanovirus (BBTV): this ssDNA pathogen has been identified in numerous developing countries in Oceania, Africa, and Asia and has caused devastating epidemics.  Also-rans include the dsDNA Banana streak badnavirus (BSV) – also found integrated into the genome of many Musa spp. – and the ssRNA Cucumber mosaic cucumovirus (CMV).

So, the Rybicki Top Ten (in alphabetical order):

  • African cassava mosaic disease begomovirus complex
  • Banana bunchy top nanovirus (BBTV)
  • Banana streak badnavirus (BSV)
  • Barley yellow dwarf disease luteovirus complex
  • Cucumber mosaic cucumovirus (CMV: OK, reluctantly, because it DOES infect damn nearly anything)
  • Maize streak mastrevirus (MSV)
  • Maize dwarf mosaic / Sugarcane mosaic potyviruses
  • Rice tungro disease complex
  • Rice yellow mottle sobemovirus (RYMV)
  • Sweet potato feathery mottle potyvirus (SPFMV)

Also-rans:

  • The legion of tomato begomoviruses worldwide, but especially in Asia
  • Tomato spotted wilt tospovirus, because it IS still an emerging virus
  • Various South American (mainly Brazilian) vegetable begomoviruses
  • Various potyviruses, mainly in vegetables, in Asia

So there it is – viruses causing severe hardship, affecting real people.  And my affectionate favourite would also be BMV…B-)