The major focus of the Johal lab is to investigate how crop plants cope with stresses imposed by an ever-changing environment. This is pursued in the context of a couple of bona fide diseases of maize, as well as by taking advantage of spontaneous mutants that form disease-like symptoms in the absence of pathogens. Another area of research is gene discovery, especially those genes that have an agronomic potential and which evolved naturally in maize.
Johal's research over the years has accomplished some major milestones in plant pathology. His graduate research showed that the key reason why Roundup is such an effective herbicide is because it impairs plants' ability to defend against soil-borne pathogens. Roundup brings this immune deficiency by inhibiting the formation of antimicrobials derived through the shikimate pathway.
Johal's postdoctoral research resulted in the cloning of a first ever disease resistance gene in plants (Science 258:985), something that was considered a holy grail at that time. This, in turn, led to the elucidation of the biochemical mechanism resulting in Hm1-conferred resistance. Turns out that Hm1 encodes an NADPH dependent reductase, whose function is to inactivate a virulence factor (HC-toxin) produced by Cochliobolus carbonum (CC), a fungal pathogen of maize. Considered as a host specific toxin, HC-toxin is a small cyclic tetrapeptide molecule with an epoxide group, and it transforms CC from a benign pathogen of maize to one that is perhaps the most destructive. An additional impact of this research was that it settled a long and bitter dispute among plant pathologists about the concept and significance of host specific toxins in plant disease.
Recent research on this disease has allowed Johal to show that Hm1 is a grass-specific disease resistance gene that evolved in a common ancestor of grasses some 50 million years ago. It probably happened as an adaptive strategy to contend with CC or CC-like pathogen capable of producing HC-toxin. When Hm1 breaks down, as it happened naturally in maize that led to the discovery of the disease and CC as a pathogen (PNAS 95:1686), or was accomplished experimentally in barley (PNAS 105:1762), plants of the grass family become susceptible to disease by CC. These findings indicate that Hm1 has served a guardian of grasses since antiquity.
Current research on the maize - CC interaction
The Johal lab is presently elucidating the mode of action of HC-toxin whereby it enables CC to colonize maize. He is also addressing why some other alleles of Hm1 confer protection against CC only during adult stages of the plant, another issue of longstanding interest and curiosity in plant pathology.
Maize disease lesion-mimic mutants
The Johal lab was one of the first to recognize the potential of disease lesion-mimic (Les/les) mutants and the opportunity they provided to understand how plants cope with diverse stresses. It was shown that while some of them have defects in disease resistance genes and pathways, most others result from errors and impairments in diverse metabolic pathways, including those involved in the biosynthesis and degradation of chlorophyll (Cell 89:25; Plant Cell 10:1095).
The Johal group continues to make high impact contributions in this area. While working with the Les mutants, they noticed that the severity of these mutants was easily influenced by the genetic background of the host plant. These observations led Johal to conceptualize that the easily visible and quantifiable phenotype of the Les mutants could be used as a reporter or assay to dissect the genetic basis of the age-old question of the 'genetic background' phenomenon. This quest resulted in the identification of a major QTL that suppresses cell damage associated with multiple mutants and stresses in maize.
Work with Slm1 allowed Johal to conceive a novel idea that is likely to have major implications for gene discovery and plant breeding. Dubbed 'mutant-assisted gene identification and characterization' (MAGIC), it relies on using the mutant phenotype as a reporter (indicator) for quickly discovering and harnessing genes that underlie basically any trait in plants. A key benefit of this idea is that it provides an excellent approach for unveiling the molecular basis of natural variation, which represents a huge but largely untapped source of genetic repertoire in plants. A testament to this idea is the recent funding by NSF of a Plant Genome Research Program project ($3.96 million) in which MAGIC is used effectively to uncover natural variation underlying immune responses in maize.
A new kind of dwarfing trait
Another major accomplishment of the Johal lab is the isolation and characterization of the maize brachytic-2 (br2) gene, which is characterized by semi-dwarf mutations reminiscent of 'green revolution' genes in rice and wheat. In collaboration with Angus Murphy (Department of Horticulture and Landscape), Johal demonstrated that the br2 gene controls plant height by encoding a multi-drug resistance transporter whose function is to facilitate polar movement of auxins across lower stalk internodes. An applied implication of this discovery was the demonstration that the sorghum equivalent of br2 is dw3, which confers a highly desirable but unstable dwarfing trait of long-standing interest and concern among sorghum breeders. This finding allowed Johal to address what was wrong with the gene in dw3, why it exhibited an unstable phenotype and how it could be corrected to confer a stably dwarf sorghum (Science 302:81).
Maize - Western corn rootworm (WCR) interaction
The most recent discovery by the Johal lab is that of crw1, a maize mutant whose leaves are almost devoured by the WCR beetle. The larvae of this beetle are the number one pest of corn in the U.S. Because of the damage it inflicts annually in terms of cost of control and yield loss, WCR has earned the nickname the "billion-dollar bug." Although the WCR beetle feeds almost exclusively on maize, its diet is normally restricted to pollen and silks, and all other plant parts are left largely unscathed. This however is not the case with crw1, whose leaves are especially favored by the beetle as a food source. crw1 inherits in a recessive manner, suggesting that a mechanism that normally renders maize leaves unpalatable to the WCR beetle is compromised in this mutant. The significance of crw1 lies in the fact that virtually nothing is known about the nature of genes and mechanisms that deter maize from being chewed by insect herbivores. A second crw2 mutant has now been identified that exhibits susceptibility not only to WCR but also a number of other insects.